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Unstable zone-boundary phonon modes drive atomic displacements linked to a rich array of properties. Yet, the electronic origin of the instability remains to be clearly explained. Here, we propose that bonding interaction between Bloch states belonging to different wavevectors leads to such instability via the pseudo- or second-order Jahn--Teller effect. Our first-principles calculations and representation theory-based analyses show that rotations of anion coordinated octahedra, an archetypal example of zone-boundary phonon condensations, are induced by this bonding mechanism. The proposed mechanism is universal to any non-zone-center phonon condensations and could offer a general approach to understand the origin of structural phase transitions in crystals., Comment: 6 pages, 4 figures; typo corrected

This work addresses the study of the high-temperature phase sequence of ${\mathrm{Bi}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{FeO}}_{3}$ by undertaking temperature-dependent high-resolution neutron powder diffraction (NPD) and Raman spectroscopy measurements. A determination of lattice parameters, phase fractions, and modulation wave vector was performed by Pawley refinement of the NPD data. The analysis revealed that ${\mathrm{Bi}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{FeO}}_{3}$ exhibits an incommensurate modulated orthorhombic $Pn{2}_{1}a(00\ensuremath{\gamma})000$ structure at room temperature, with a weak ferromagnetic behavior, likely arising from a canted antiferromagnetic ordering. Above ${T}_{1}=543\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the low-temperature modulated $Pn{2}_{1}a(00\ensuremath{\gamma})000$ evolves monotonically into a fractionally growing Pnma structure up to ${T}_{\mathrm{N}}=663\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. At 663 K, the low-temperature canted antiferromagnetic phase is suppressed concurrently with the switching of the former into a nonmodulated $Pn{2}_{1}a$ structure that continues to coexist with the Pnma one, until the latter is expected to reach the 100% fraction of the sample volume at high temperatures above 733 K. The $Pn{2}_{1}a$ space group is obtained from the Pnma one through the ${\mathrm{\ensuremath{\Gamma}}}_{4}^{\ensuremath{-}}$ polar distortion. Neutron diffraction and Raman spectroscopy results provide evidence for the emergence of noteworthy linear spin-phonon coupling. In this regard, magnetostructural coupling is observed below ${T}_{\mathrm{N}}$, revealed by the relation between the weak ferromagnetism of the canted iron spins and the ${\mathrm{FeO}}_{6}$ octahedra symmetric stretching mode. The correlation between magnetization and structural results from NPD provides definite evidence for the magnetic origin of the structural modulation. The analysis of the temperature-dependent magnetization and the magnetic peak intensity as well yields a critical exponent (\ensuremath{\beta}) value of 0.38. The lower limit of the phase coexistence temperature ${T}_{1}=543\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, marking the emergence of the Pnma phase, is also associated with the temperature whereupon the modulation magnitude starts to decrease.

Samarium nickelate ($\mathrm{Sm}\mathrm{Ni}{\mathrm{O}}_{3}$) is a representative quantum material, presenting ``exotic'' properties including strong correlations and metal-to-insulator transition. It was also shown that $\mathrm{Sm}\mathrm{Ni}{\mathrm{O}}_{3}$ presents the unusual antidoping effect, where the resistivity of the material increases with doping instead of decreasing. In this paper, we study intrinsic defects in this compound, and discuss the antidoping behavior in the presence of these defects. Our results show that the negatively charged oxygen vacancy (${V}_{\text{O}}^{\ensuremath{-}2}$) can be very stable, and the extra electrons move towards the small octahedra (${\mathrm{Ni}}^{4+}$ sites), similar to the antidoping effect in the pristine material. This indicates that antidoping should still be observed even in the presence of defects.

International audience; Magnetite is one of the most fascinating materials exhibiting the enigmatic first-order Verwey transition which is conventionally manipulated through chemical doping. Here we show that heating magnetite results in a spontaneous charge reordering and consequently a hole self-doping effect at the octahedral sublattice. Core level x-ray spectroscopy measurements combined with theory uncovers that there are three regimes of self-doping that map the temperature dependence of the electrical conductivity and magnetism up to the Curie temperature. Our results provide an elegant analogy between the effect of chemical doping and temperature-driven self-doping on trimerons in magnetite.

We report the crystal structures, initial magnetic and charge transport characterization, and calculated electronic structures of the hexagonal oxyhalide perovskites, ${\mathrm{Ba}}_{7}{\mathrm{Ru}}_{4}{\mathrm{O}}_{15}{\mathrm{Cl}}_{2}$ and ${\mathrm{Ba}}_{7}{\mathrm{Ru}}_{4}{\mathrm{O}}_{15}{\mathrm{Br}}_{2}$. The experimental information is obtained through the study of single crystals. Face-sharing $\mathrm{Ru}{\mathrm{O}}_{6}$ octahedra form ${\mathrm{Ru}}_{2}{\mathrm{O}}_{9}$ dimers in a layered triangular geometry in these materials; minor amounts of off-magnetic-site structural disorder are present in our crystals. Both are magnetically isotropic, with 2.5 \ensuremath{\mu}B/mol-Ru, Curie-Weiss theta \ensuremath{-}185 K, and 2.9 \ensuremath{\mu}B/mol-Ru, Curie-Weiss theta \ensuremath{-}168 K, respectively. Broad features in the magnetic susceptibility and heat capacity associated with magnetic ordering (at 35 and 37 K, respectively) are observed. The charge transport band gaps are 0.03 eV for the oxychloride and 0.006 eV for the oxybromide. There is no gap at the Fermi level for either of these compounds in density-functional theory electronic structure calculations.

The combination of reduced dimensionality and tunable structural distortions in layered perovskite oxides makes these materials ideal platforms for designing novel properties and functionalities. One example is hybrid improper ferroelectricity in $n=2$ Ruddlesden-Popper oxides, where the combination of a layered crystal structure and rotations of the metal-oxide octahedra break symmetry and induce a polarization. Precisely controlling the octahedral rotation distortions, for example by the application of hydrostatic pressure, provides a pathway to tune and optimize the properties of these materials. We combine group theoretic methods, density functional theory calculations, and Landau theory analysis to investigate how octahedral rotations respond to pressure in the hybrid improper ferroelectrics ${\mathrm{Sr}}_{3}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}, {\mathrm{Ca}}_{3}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, and ${\mathrm{Sr}}_{3}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$. We find that factors that are known to control the pressure response of $\mathrm{A}\mathrm{B}{\mathrm{O}}_{3}$ perovskites---the formal charge of the $A$- and $B$-site cations, tolerance factor, and $B$-site chemistry---also impact the pressure response of these layered perovskites. We also show that coupling between the octahedral rotation and strain order parameters plays a key role in determining the overall pressure response. Despite some similarities, we find that these layered perovskites display a distinct pressure response compared to their $\mathrm{A}\mathrm{B}{\mathrm{O}}_{3}$ perovskite analogs. By identifying trends and underlying mechanisms that control octahedral rotations in Ruddlesden-Popper oxides under pressure, this work lays the foundation for tailoring the structure and properties of these materials.

We describe a strategy to design ferroelectric heteroanionic materials based on the coupling of anion order and octahedral tilts in $n=1$ Ruddlesden-Popper structures, which allows noncentrosymmetric and polar compounds to arise from centrosymmetric anion-ordered structures. We investigate the relative phase stabilities of the polymorphs of ${\mathrm{Sr}}_{2}{\mathrm{ScO}}_{3}\mathrm{F}$ and ${\mathrm{Ca}}_{2}{\mathrm{ScO}}_{3}\mathrm{F}$ derived from this coupling using electronic structure calculations. We find that large degrees of octahedral tilting can stabilize different anion orders derived from assembly of $[{\mathrm{ScO}}_{5}{\mathrm{F}]}^{8\ensuremath{-}}$ octahedra. To further understand the link between octahedral tilting and anion order, we quantitatively separate the contributions of electrostatics and covalent interactions to the stability of tilts. We find that the tilts are driven primarily by covalent interactions and that local out-of-plane polar displacements, induced from the anion order, further stabilize the octahedral tilts through the pseudo-Jahn-Teller effect.

We report real space location of hydrogen in single crystalline $\mathrm{Fe}/\mathrm{V}$ superstructures. Anisotropic strain is quantified versus hydrogen concentration by using the yield of backscattered primary 2 MeV $^{4}\mathrm{H}\mathrm{e}$ ions for incidence in different crystallographic directions. From a comparison of ion channeling in combination with $^{1}\mathrm{H}(^{15}\mathrm{N},\ensuremath{\alpha}\ensuremath{\gamma})^{12}\mathrm{C}$ nuclear reaction analysis and Monte Carlo simulations we show that hydrogen is located in octahedral $z$ sites and quantify its vibrational amplitude of 0.2 \AA{}.

Transition metal compounds can feature different types of magnetic anisotropy. By studying a honeycomb-lattice material with disconnected transition-metal octahedra, the authors reveal the crucial role of the single-ion anisotropy term that arises despite the nominally quenched orbital moment of the ${d}^{3}$ magnetic ion. Their findings suggest that the effect of single-ion anisotropy should not be ignored when searching for signatures of Kitaev interactions in spin-3/2 honeycomb magnets.

Structural domains in ferroic materials can be manipulated to achieve novel properties and functionalities of devices with multiple tunabilities. Herein, we report a study of tunable domain structures in ferroelastic ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{3}$ (LSMO) films via interfacial engineering. Distinct domain structures are formed in rhombohedral LSMO films depending on the crystallographic orientations of the orthorhombic ${\mathrm{NdGaO}}_{3}$ (NGO) substrates. A unidirectional lattice modulation is observed in LSMO films grown on $\mathrm{NGO}{(110)}_{\mathrm{Or}}$ substrates, whereas a unidirectional twinned-domain structure is generated by $\mathrm{NGO}{(001)}_{\mathrm{Or}}$ substrates (where Or in the subscript denotes orthorhombic notation). The orientation-dependent domain structures in LSMO films are controlled by anisotropic strain as well as interfacial oxygen octahedral coupling at the heterointerface between LSMO and NGO. The orbital occupancy and in-plane magnetic anisotropy are markedly affected by strain relief induced by the unidirectional structural domains in LSMO films. In addition, the structural results observed in LSMO/$\mathrm{NGO}{(001)}_{\mathrm{Or}}$ films further demonstrate that anisotropic strain is capable to disturb octahedral connectivity in epitaxial films. Our findings provide future directions to understand and control the domain structures in ferroelastic heterostructures of perovskite materials and open a pathway towards tailoring the desirable physical properties.

First-principles investigations of the high-temperature superconducting system ${\mathrm{Ba}}_{2}\mathrm{Cu}{\mathrm{O}}_{3+\ensuremath{\delta}}$, recently discovered at $\ensuremath{\delta}\ensuremath{\approx}0.2$ at ${T}_{c}=70$ K, are applied to demonstrate the effects of oxygen ordering on the electronic and magnetic properties. The observed ``highly overdoped'' superconducting phase displays stretched Cu-planar oxygen ${\mathrm{O}}_{\mathrm{P}}$ distances and anomalously shortened Cu-apical ${\mathrm{O}}_{\mathrm{A}}$ separations compared with other cuprates. The stoichiometric system $\ensuremath{\delta}=0$, with its strongly one-dimensional (1D) Cu-${\mathrm{O}}_{\mathrm{P}}$ chain structure, when nonmagnetic shows 1D Fermi surfaces that lead, within density functional theory, to antiferromagnetic Cu-${\mathrm{O}}_{\mathrm{P}}$ chains (a spin-Peierls instability). Accounting for 1D fluctuations and small interchain coupling according to the theory of Schulz indicates this system, like ${\mathrm{Sr}}_{2}\mathrm{Cu}{\mathrm{O}}_{3}$, is near the 1D Luttinger-liquid quantum critical phase. The unusual Cu-O bond lengths per se have limited effects on other properties for $\ensuremath{\delta}=0$. We find that a ``doubled bilayer'' structure of alternating Cu-${\mathrm{O}}_{\mathrm{P}}$ chains and wide rung ${\mathrm{Cu}}_{3}{\mathrm{O}}_{4}$ ladders is the energetically preferred one of three possibilities where the additional oxygen ions bridge Cu-${\mathrm{O}}_{\mathrm{P}}$ chains in the superconducting phase $\ensuremath{\delta}=1/4$. Nominal formal valences of the three Cu sites are discussed. The sixfold (octahedral) site is the most highly oxidized, accepting somewhat more holes in the ${d}_{{z}^{2}}$ orbital than in the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital. The implication is that two-band physics is involved in the pairing mechanism and the superconducting carriers. The Fermi surfaces of this metallic bilayer structure show both 1D and 2D strong (incipient) nesting instabilities, possibly accounting for the lack of clean single-phase samples based on this structure and suggesting importance for the pairing mechanism.

Double halide perovskites have attracted increasing interest due to their great potential to eliminate the toxicity and instability issues in lead halide perovskites. Unfortunately, octahedral cation antisites easily form in common double halide perovskites and induce deep defect levels. Here, using first-principles calculation methods, we report unusual defect properties in the double halide perovskite ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{I}}_{6}$, including the coexistence of shallow and deep defect transition energy levels of iodine vacancies, absent of octahedral cation antisites, and different dominant donor defects such as interstitials $({\mathrm{Au}}_{\mathrm{i}})$ and I substituting Au $({\mathrm{I}}_{\mathrm{Au}})$. We attribute these unusual defect properties to the multivalence effect of Au and strong covalent bonding between Au and I. Based on the defect properties, we propose two possible applications of ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{I}}_{6}$. On one hand, ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{I}}_{6}$ could be a useful candidate for photodetectors under I-rich growth conditions due to its relatively low carrier densities because the Fermi level is pinned near the middle of the band gap due to the strong compensation of the dominant donors ${\mathrm{Au}}_{\mathrm{i}}$ and acceptor ${\mathrm{V}}_{\mathrm{Cs}}$. On the other hand, ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{I}}_{6}$ can be made $n$-type if it is grown under I-poor conditions, which can be further enhanced by growing the sample at 600 K and then quenching to 300 K. Consequently, ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{I}}_{6}$ has potential as an $n$-type photovoltaic absorber. Our studies not only enrich the defect physics in multivalent double halide perovskites but also provide useful information for advancing ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{I}}_{6}$ technologies.

The origin of the pronounced and composition-dependent band-gap bowing in Sn/Pb mixed perovskite alloys has been under debate for a long time. Previous studies reported conflicting results on whether the chemical or structural effect is the dominant mechanism. In this paper, the band-gap bowing effect and its possible origins in recently synthesized two-dimensional (2D) ${\mathrm{Cs}}_{2}{\mathrm{Pb}}_{x}{\mathrm{Sn}}_{1\text{\ensuremath{-}}x}{\mathrm{I}}_{2}{\mathrm{Cl}}_{2}$ alloys are investigated from first-principles calculations. In agreement with experiments, a large and composition-dependent bowing coefficient is observed. By analyzing the contribution from volume deformation, charge exchange, structural relaxation, and short-range order, it is found that the dominant mechanism causing the anomalous gap bowing is the structural relaxation-induced wave-function localization, forming isovalent-defect-like states, despite the negligible octahedral distortion and small lattice mismatch between the two end compounds. This is understood by the s-p repulsion-induced strong antibonding character of the valence-band maximum which leads to a large deformation potential, thus even a small atomic displacement can result in a large shift of the energy level. These results thus highlight the critical role of strong deformation potential and structural relaxation effect in unusual band evolution of 2D Sn/Pb perovskite alloys, and can be helpful to the modulation of their band gap for optoelectronic applications.

Many ferroelectric (FE) organic-inorganic hybrid perovskites (OIHPs) show great promise in the fields of photovoltaic cells and information memory devices. We systematically investigated the FE, optical, and electric properties of quasi-one-dimensional OIHPs $(\mathrm{MV})A{\mathrm{I}}_{3}{\mathrm{Cl}}_{2}$ ($\mathrm{MV}=\mathrm{methylviologen}, A=\mathrm{Bi},\mathrm{Sb}$) using density functional theory calculations. The FE polarization mechanism of $(\mathrm{MV})A{\mathrm{I}}_{3}{\mathrm{Cl}}_{2}$ mainly originates from octahedral distortion along the direction of octahedral chains with the $p\text{\ensuremath{-}}p$ coupling of ${A}^{3+}$ cations and ${\mathrm{I}}^{\ensuremath{-}}$ anions. Due to the loosely coupled FE chains, $(\mathrm{MV})\mathrm{Bi}{\mathrm{I}}_{3}{\mathrm{Cl}}_{2}$ and $(\mathrm{MV})\mathrm{Sb}{\mathrm{I}}_{3}{\mathrm{Cl}}_{2}$ possess a relatively low energy barrier of polarization switching at the 180\ifmmode^\circ\else\textdegree\fi{} reversal of the FE polarization. The estimated upper limit memory densities of 14.9 and $15.0\phantom{\rule{0.16em}{0ex}}\mathrm{Tb}/\mathrm{c}{\mathrm{m}}^{2}$ for $(\mathrm{MV})\mathrm{Bi}{\mathrm{I}}_{3}{\mathrm{Cl}}_{2}$ and $(\mathrm{MV})\mathrm{Sb}{\mathrm{I}}_{3}{\mathrm{Cl}}_{2}$, respectively, hold promise to be applied to high-density FE memory devices. The strong anisotropic optical absorption of $(\mathrm{MV})A{\mathrm{I}}_{3}{\mathrm{Cl}}_{2}$ is in the visible light region, and its estimated maximum power conversion efficiencies are g23%. The high anisotropic carrier mobility in $(\mathrm{MV})A{\mathrm{I}}_{3}{\mathrm{Cl}}_{2}$ enhances the separation of electron-hole pairs. Both $(\mathrm{MV})\mathrm{Bi}{\mathrm{I}}_{3}{\mathrm{Cl}}_{2}$ and $(\mathrm{MV})\mathrm{Sb}{\mathrm{I}}_{3}{\mathrm{Cl}}_{2}$ possess positive piezoelectric effect; therefore, strain design is an effective approach to enhance power conversion efficiency and carrier mobility of $(\mathrm{MV})A{\mathrm{I}}_{3}{\mathrm{Cl}}_{2}$, leading to experimental design of FE photovoltaic cell.

The dynamic structure factor $S(Q,E)$ of liquid ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ has been obtained by inelastic x-ray scattering, where $Q$ and $E$ are momentum and energy transfer, respectively. The dispersion curve of the longitudinal acoustic excitation energy exhibits flat-topped $Q$ dependence similarly to that in liquid GeTe and liquid Bi, where the local structure is modulated by Peierls distortion. The flat-topped energy in liquid ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ is split into low and high energy parts arising from Sb-Te and Ge-Te correlations, respectively. Furthermore, the high energy part depends on $Q$ like an optical mode with decreasing $Q$ to zero, and it approaches the vibrational energy of fourfold coordinated Ge atoms with octahedral configurations. The result indicates that a majority of Ge in the liquid stays in octahedral order as predicted by first-principles molecular dynamics simulations.

Author(s): Karki, B; Alfailakawi, A; Frandsen, BA; Everett, MS; Neuefeind, JC; Xu, B; Wang, H; Fang, M; Freelon, B | Abstract: We describe the local structural properties of the iron oxychalcogenides, La2O2Fe2OM2(M=S,Se), by using pair distribution function analysis applied to total scattering data. Our results from neutron powder diffraction show that M = S and Se possess similar nuclear structures at low and room temperatures. The local crystal structures were studied by investigating deviations in atomic positions and the extent of the formation of orthorhombicity. Analysis of the total scattering data suggests that buckling of the Fe2O plane occurs below 100 K. The buckling may occur concomitantly with a change in octahedral height. Furthermore, within a typical range of 1-2 nm, we observed a short-range orthorhombiclike structure suggestive of nematic fluctuations in both of these materials.

${\mathrm{GeCo}}_{2}{\mathrm{O}}_{4}$ is a unique system in the family of cobalt spinels $A{\mathrm{Co}}_{2}{\mathrm{O}}_{4}$ ($A$= Sn, Ti, Ru, Mn, Al, Zn, Fe, etc.) in which magnetic Co ions stabilize on the pyrochlore lattice exhibiting a large degree of orbital frustration. Due to the complexity of the low-temperature antiferromagnetic (AFM) ordering and long-range magnetic exchange interactions, the lattice dynamics and magnetic structure of a ${\mathrm{GeCo}}_{2}{\mathrm{O}}_{4}$ spinel have remained puzzling. To address this issue, here we present theoretical and experimental investigations of the highly frustrated magnetic structure, and the infrared (IR) and Raman-active phonon modes in the spinel ${\mathrm{GeCo}}_{2}{\mathrm{O}}_{4}$, which exhibits an AFM ordering below the N\'eel temperature ${T}_{N}\ensuremath{\sim}21$ K and an associated cubic ($Fd\overline{3}m$) to tetragonal ($I{4}_{1}/amd$) structural phase transition whose location at ${T}_{N}$ vs at a lower ${T}_{S}\ensuremath{\sim}16$ K is controversial. Our density functional theory ($\mathrm{DFT}+U$) calculations reveal that one needs to consider magnetic-exchange interactions up to the third-nearest neighbors to get an accurate description of the low-temperature AFM order in ${\mathrm{GeCo}}_{2}{\mathrm{O}}_{4}$. At room temperature, three distinct IR-active modes (${T}_{1\mathrm{u}}$) are observed at frequencies 680, 413, and 325 ${\mathrm{cm}}^{\ensuremath{-}1}$ along with four Raman-active modes ${A}_{1\mathrm{g}}, {T}_{2\mathrm{g}}$(1), ${T}_{2\mathrm{g}}$(2), and ${E}_{\mathrm{g}}$ at frequencies 760, 647, 550, and 308 ${\mathrm{cm}}^{\ensuremath{-}1}$, respectively, which match reasonably well with our $\mathrm{DFT}+U$ calculated values. All the IR-active and Raman-active phonon modes exhibit signatures of moderate spin-phonon coupling. The temperature dependence of various parameters, such as the shift, width, and intensity, of the Raman-active modes is also discussed. Noticeable changes around ${T}_{N}\ensuremath{\sim}21$ K and ${T}_{S}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}16$ K are observed in the Raman line parameters of the ${E}_{\mathrm{g}}$ and ${T}_{2\mathrm{g}}$(1) modes, which are associated with the modulation of the Co-O bonds in ${\mathrm{CoO}}_{6}$ octahedra during the excitations of these modes.

We report x-ray and optical absorption spectroscopy as well as hard x-ray photoemission spectroscopy on hexagonal ${\mathrm{Ba}}_{3}{\mathrm{CuSb}}_{2}{\mathrm{O}}_{9}$ in which a Cu $3d$ spin-orbital liquid state is suggested from absence of the Jahn-Teller distortion of ${\mathrm{Cu}}^{2+}{\mathrm{O}}_{6}$ octahedra and of the magnetic ordering. The experimental results and their cluster model analysis indicate that O $2p$ holes play a crucial role in realizing the spin-orbital liquid state of hexagonal ${\mathrm{Ba}}_{3}{\mathrm{CuSb}}_{2}{\mathrm{O}}_{9}$. These oxygen holes appear due to the ``reaction'' ${\mathrm{Sb}}^{5+}\ensuremath{\rightarrow}{\mathrm{Sb}}^{3+}\phantom{\rule{4pt}{0ex}}+$ two O $2p$ holes, with these holes being able to attach to Cu ions. The present work opens avenues towards spin-charge-orbital entangled liquid state in transition-metal oxides with small or negative charge transfer energy.

Empirical ${d}^{0}$ rule, i.e., magnetic unpairing $d$ occupation tends to be chemically incompatible for ferroelectrics, has for long inhibited the availability of room-temperature single-phase multiferroics. Here, we readdress the ferroelectricity of type-I multiferroic $\mathrm{BiFe}{\mathrm{O}}_{3}$, and conclusively clarify hybrid origins of lone-pair electrons' instability. Surprisingly, the sole lone-pair mechanism cannot account for overall polarization. Thus, beyond the popular belief, we propose that, with half-filling ${t}_{2g}^{3}{e}_{g}^{2}$ configuration and stereochemical activity of Bi's lone pair preparticipating, renormalized nonempty Fe $3d$ orbitals render $\mathrm{BiFe}{\mathrm{O}}_{3}$ a strong p-d charge-transfer insulator, and further induce non-${d}^{0}$ ferroelectricity through antiferromagnetic semicovalent superexchange. The magnetoelectric coupling scenario is numerically elaborated by mean-field p-d model, and first-principles generalized gradient approximation $\mathrm{GGA}+U$ calculations. The magnetically induced ferroelectricity also reconciles the competitivity of octahedral rotations and ferroelectricity, which turns into cooperative behavior as octahedral rotations straighten the $\mathrm{Fe}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Fe}$ bonds and strengthen superexchange interactions. Such mechanism is in principle ubiquitous in magnets with superexchange interaction and manipulatable to hybridization engineering.

Chromium (II) fluoroperovskites $A{\mathrm{CrF}}_{3}\phantom{\rule{4pt}{0ex}}(A={\mathrm{Na}}^{+},{\mathrm{K}}^{+})$ are strongly correlated Jahn-Teller active materials at low temperatures. In this paper, we examine the role that the $A$-site ion plays in this family of fluoroperovskites using both experimental methods (x-ray diffraction, optical absorption spectroscopy, and magnetic fields) and density functional theory simulations. Temperature-dependent optical absorption experiments show that the spin-allowed transitions ${E}_{2}$ and ${E}_{3}$ only merge completely for $A=\text{Na}$ at 2 K. Field-dependent optical absorption measurements at 2 K show that the oscillating strength of the spin-allowed transitions in ${\mathrm{NaCrF}}_{3}$ increases with increasing applied field. Direct magneto-optical correlations suppress the spin-flip transitions for ${\mathrm{KCrF}}_{3}$ below its N\'eel temperature. In ${\mathrm{NaCrF}}_{3}$ the spin-flip transitions vanish abruptly below 9 K revealing magneto-optical correlations possibly linked to crystal structure changes. This suggests that as the long range ordering is reduced, local Jahn-Teller effects in the individual ${\mathrm{CrF}}_{6}^{4\ensuremath{-}}$ octahedra take control of the observed behavior. Our results show clear deviation from the pattern found for the isoelectronic ${A}_{x}{\mathrm{MnF}}_{3+x}$ system. The size of the $A$-site cation is shown to be central in dictating the physical properties and phase transitions in $A{\mathrm{CrF}}_{3}$, opening up the possibility of varying the composition to create novel states of matter with tunable properties.

Strong spin-orbital-coupling magnetic systems with the honeycomb structure can provide bond-directional interactions which may result in Kitaev quantum spin liquids and exotic anyonic excitations. However, one of the key ingredients in real materials---disorders---has been much less studied in Kitaev systems. Here we synthesized a trigonal ${\mathrm{SrIr}}_{2}{\mathrm{O}}_{6\ensuremath{-}\ensuremath{\delta}}$ with $\ensuremath{\delta}\ensuremath{\approx}0.25$, which consists of two-dimensional honeycomb Ir planes with edge-sharing ${\mathrm{IrO}}_{6}$ octahedra. First-principles computation and experimental measurements suggest that the electronic system is gapped, and there should be no magnetic moment as the ${\mathrm{Ir}}^{5+}$ ion has no unpaired electrons. However, significant magnetism has been observed in the material, and it can be attributed to disorders that are most likely from oxygen vacancies. No magnetic order is found down to 0.05 K, and the low-temperature magnetic properties exhibit power-law behaviors in magnetic susceptibility and zero-field specific heat, and a single-parameter scaling of the specific heat under magnetic fields. These results provide strong evidence for the existence of the random singlet phase in ${\mathrm{SrIr}}_{2}{\mathrm{O}}_{6\ensuremath{-}\ensuremath{\delta}}$, which offers a different member to the family of spin-orbital entangled iridates and Kitaev materials.

We report a comprehensive investigation of $L{n}_{2}{\mathrm{NiIrO}}_{6}$ ($Ln=\text{La}$, Pr, Nd) using thermodynamic and transport properties, neutron powder diffraction, resonant inelastic x-ray scattering, and density-functional theory (DFT) calculations to investigate the role of A-site cations on the magnetic interactions in this family of hybrid $3d\text{\ensuremath{-}}5d\text{\ensuremath{-}}4f$ compositions. Magnetic structure determination using neutron diffraction reveals antiferromagnetism for ${\mathrm{La}}_{2}{\mathrm{NiIrO}}_{6}$, a collinear ferrimagnetic Ni and Ir state that is driven to long-range antiferromagnetism upon the onset of Nd ordering in ${\mathrm{Nd}}_{2}{\mathrm{NiIrO}}_{6}$, and a noncollinear ferrimagnetic Ni and Ir sublattice interpenetrated by a ferromagnetic Pr lattice for ${\mathrm{Pr}}_{2}{\mathrm{NiIrO}}_{6}$. For ${\mathrm{Pr}}_{2}{\mathrm{NiIrO}}_{6}$, heat-capacity results reveal the presence of two independent magnetic sublattices, and transport resistivity indicates insulating behavior and a conduction pathway that is thermally mediated. A first principles DFT calculation elucidates the existence of the two independent magnetic sublattices within ${\mathrm{Pr}}_{2}{\mathrm{NiIrO}}_{6}$ and offers insight into the behavior in ${\mathrm{La}}_{2}{\mathrm{NiIrO}}_{6}$ and ${\mathrm{Nd}}_{2}{\mathrm{NiIrO}}_{6}$. Resonant inelastic x-ray scattering is consistent with spin-orbit coupling splitting the ${t}_{2g}$ manifold of octahedral ${\mathrm{Ir}}^{4+}$ into a ${J}_{\mathrm{eff}}=\frac{1}{2}$ and ${J}_{\mathrm{eff}}=\frac{3}{2}$ state for all members of the series considered.

We report magnetic properties in the random spinel magnet CoGa2O4. Rietveld analysis of the x-ray diffraction profile for CoGa2O4 reveals that the Co and Ga ions are distributed randomly in the tetrahedral A-sites and octahedral B-sites in the cubic spinel structure. CoGa2O4 exhibits a spin-glass transition at TSG = 8.2 K that is confirmed by measurements of the dc- and ac-susceptibilities and thermoremanent magnetization (TRM) that develops below TSG. From the frequency dependence of the freezing temperature Tf for CoGa2O4, it is indicated that the relaxation time follows a Vogel-Fulcher law. Magnetic entropy is considerably reduced, probably because magnetic cluster formation developed even at T > TSG. The relaxation rate of TRM is considerably enhanced at TSG and decays rapidly above and below TSG. The time course of TRM is reproduced by non-exponential relaxation forms, such as a stretched exponential (Kohlrausch) as well as Ogielski and Weron relaxation forms. This behavior is displayed universally in glass systems, and the characteristic parameters associated with these functions were reasonable., 34 pages, 14 figures, 3 table

The magnetism, magnetotransport, and Seebeck coefficients $(S)$ for three ruthenates ${\mathrm{Ba}}_{1\ensuremath{-}\ensuremath{\delta}}{M}_{2+x}{\mathrm{Ru}}_{4\ensuremath{-}x}{\mathrm{O}}_{11}$ $(\ensuremath{\delta}=0.06;$ $M=\mathrm{Mn}, \mathrm{Co}; x=0.4)$ and ${\mathrm{Sr}}_{1\ensuremath{-}\ensuremath{\delta}}{M}_{2+x}{\mathrm{Ru}}_{4\ensuremath{-}x}{\mathrm{O}}_{11}$ $(\ensuremath{\delta}=0.02; M=\mathrm{Fe}; x=0.7)$ compositions have been studied. Their crystallographic structures contain three metal sites, edge-sharing octahedra forming kagome lattices, face-shared octahedra with the shortest $\mathrm{Ru}(M)\text{\ensuremath{-}}\mathrm{Ru}(M)$ distance, and ${M\mathrm{O}}_{5}$ trigonal bipyramids. These three compositions have been selected for their transport behavior exhibiting small resistivity values (\ensuremath{\sim}m\ensuremath{\Omega} cm) together with a complex ferrimagnetic behavior, with localization increasing from $M=\mathrm{Co}$ to $M=\mathrm{Fe}$. This enabled the thermopower to be measured in hexagonal ruthenates in which the conducting kagome layers are more or less diluted by three different magnetic cations substituted for Ru. The positive Seebeck coefficient of the three compounds is found to increase up to 750 K to values in the range of 22 to $35\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\text{--}1}$. Such values, similar to those of perovskite ruthenates, reveal a Seebeck coefficient dominated by the Ru network at high temperature whatever the foreign magnetic cation is. In addition, below about 50 K, the values of $S$ are very small for $M=\mathrm{Mn}$ and Co, and the $S(T)$ curves of the ${\mathrm{Ba}}_{1\ensuremath{-}\ensuremath{\delta}}{M}_{2.4}{\mathrm{Ru}}_{3.6}{\mathrm{O}}_{11}$ compounds exhibit similarities with that of ruthenium metal. This is interpreted by shorter Ru-Ru distances as compared with perovskite ruthenates allowing a metallic direct exchange. The ferrimagnetism associated with the $M$ cation does not seem to play a major role in transport, as there is almost no impact of the magnetic ordering on thermopower and electrical resistivity and the values of magnetoresistance remain very small, reaching at most \ensuremath{-}1% in 9 T at 5 K for $M=\mathrm{Mn}$, and \ensuremath{-}0.4% at ${T}_{\mathrm{C}}$ for $M=\mathrm{Co}$. The present results obtained in these phases containing hexagonal Ru networks show that Hund's metal model developed to describe the thermopower of perovskite ruthenates with a Ru square lattice can have a broader range of validity.

We report the synthesis of a quasi-one-dimensional (1D) iron selenide ${\mathrm{Ba}}_{9}{\mathrm{Fe}}_{3}{\mathrm{Se}}_{15}$. It was synthesized at high temperature and high pressure of 5.5 GPa and systematically studied via structural, magnetic, and transport measurements at both ambient and high pressure. ${\mathrm{Ba}}_{9}{\mathrm{Fe}}_{3}{\mathrm{Se}}_{15}$ crystallizes in a monoclinic structure and consists of face-sharing $\mathrm{Fe}{\mathrm{Se}}_{6}$ octahedral chains running in $c$-direction. It shows a sizable unquenched orbital moment and undergoes a ferromagnetic-like phase transition at 14 K. At ambient pressure, ${\mathrm{Ba}}_{9}{\mathrm{Fe}}_{3}{\mathrm{Se}}_{15}$ exhibits an insulating behavior with a band gap of \ensuremath{\sim}460 meV. Under high pressure, a complete metallization occurs at \ensuremath{\sim}29 GPa, which is accompanied by a spin state crossover from high-spin to low-spin state.

Structural parameters in quasi-one-dimensional organic conductors ${(\mathrm{TMTTF})}_{2}X$ ($X=\mathrm{Nb}{\mathrm{F}}_{6}, \mathrm{As}{\mathrm{F}}_{6}, \mathrm{P}{\mathrm{F}}_{6}$, and Br) are investigated by synchrotron x-ray diffraction. The temperature dependences of the crystal structures differ significantly between octahedral and monatomic anion systems, corresponding to the presence or absence of a charge-ordering transition. Changes in temperature and chemical pressure control the dimensionality of the lattice and the degree of dimerization. Furthermore, the antiferromagnetic and spin-Peierls ground states in this system depend on the spin frustration due to transfer integral paths between the one-dimensional chain molecules.

Systematic change of structural transition and high-spin (HS) to low-spin (LS) transition of ${\mathrm{Fe}}^{2+}$ in synthetic $({\mathrm{Mg}}_{0.6}{\mathrm{Fe}}_{0.4})\mathrm{O}$-ferropericlase for pressures up to that of the lower mantle region were investigated using synchrotron x-ray diffraction (XRD) and synchrotron M\"ossbauer spectroscopic methods. The XRD patterns and the M\"ossbauer spectra were measured up to 160 GPa at room temperature. The results of the synchrotron XRD analysis indicate that the cubic structure of $({\mathrm{Mg}}_{0.6}{\mathrm{Fe}}_{0.4})\mathrm{O}$-ferropericlase is maintained up to 160 GPa. The M\"ossbauer spectra at 19.8 and 24.0 GPa consist of three doublets assigned to HS ${\mathrm{Fe}}^{2+}$ at the octahedral site. At pressures from 61 to 136 GPa, a singlet assigned to LS ${\mathrm{Fe}}^{2+}$ is added to the three HS ${\mathrm{Fe}}^{2+}$ doublets, and its area ratio with respect to the HS ${\mathrm{Fe}}^{2+}$ doublets gradually increase with increasing pressure. At pressures above 136 GPa, the M\"ossbauer spectra consist of only an LS ${\mathrm{Fe}}^{2+}$ singlet, implying that all Fe at these pressures is in a LS state. The resulting spin crossover pressure interval from 61 to 136 GPa indicates the coexistence of both HS and LS ${\mathrm{Fe}}^{2+}$ at pressure conditions from the upper part to the bottom of the lower mantle.

Lead halide perovskites undergo pressure-induced phase transition, facilitated by bond compressions and different tilt patterns of the ${\mathrm{PbI}}_{6}$ octahedra. Utilizing first-principles calculations we investigated 14 possible tilt systems of ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$ under compression, derived from the high-symmetry $Pm\overline{3}m$ structure. Fr substitution is adopted to mimic the rotational disorder effect of the ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}$ cation with the similar size (at room temperature or higher). Analyses of these phases reveal an interplay between tilting and distortion of the ${\mathrm{PbI}}_{6}$ octahedra. Dynamical fluctuations at finite temperature provide additional insight into the phases' stability. Drawing from the trends observed in $\mathrm{Fr}\mathrm{Pb}{\mathrm{I}}_{3}$ and additional calculations for perovskites involving differently ordered organic cations, we propose that phase transition in ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$ occurs from tetragonal at ambient pressure to orthorhombic under high pressure via the pathway I4/mcm$\ensuremath{\rightarrow}$P4/mbm$\ensuremath{\rightarrow}$Imm2. The distinct discontinuity in the calculated volume-pressure curve of I4/mcm and band-gap evolution of the proposed phases are consistent with photoluminescence shifts observed in ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$ under pressure.

Very recently, carbon-boron clathrate $\mathrm{Sr}{\mathrm{B}}_{3}{\mathrm{C}}_{3}$ has been successfully synthesized, in which carbon and boron atoms form $s{p}^{3}$-bonded truncated octahedral cages. Interestingly, the $s{p}^{3}$-hybridized $\ensuremath{\sigma}$-bonding bands are partially occupied. This may drive $\mathrm{Sr}{\mathrm{B}}_{3}{\mathrm{C}}_{3}$ into a superconducting state, like boron-doped diamond. By means of density functional first-principles calculations and Wannier interpolation technique, we have investigated the electron-phonon coupling and phonon-mediated superconductivity in $\mathrm{Sr}{\mathrm{B}}_{3}{\mathrm{C}}_{3}$. Our calculations reveal that there exists strong coupling between $s{p}^{3}$-hybridized $\ensuremath{\sigma}$-bonding bands and boron-associated ${E}_{g}$ phonon modes. Based on the Migdal-Eliashberg theory, we self-consistently solve the anisotropic Eliashberg equations. It is found that $\mathrm{Sr}{\mathrm{B}}_{3}{\mathrm{C}}_{3}$ is a single-gap superconductor, with superconducting transition temperature being 40 K. The anisotropic ratio of superconducting energy gap is computed to be 32.8%. Further replacing Sr with Ba, the transition temperature can be boosted to 43 K in $\mathrm{Ba}{\mathrm{B}}_{3}{\mathrm{C}}_{3}$ due to phonon softening. These findings suggest that $\mathrm{Sr}{\mathrm{B}}_{3}{\mathrm{C}}_{3}$ and $\mathrm{Ba}{\mathrm{B}}_{3}{\mathrm{C}}_{3}$ are phonon-mediated high-temperature anisotropic $s$-wave superconductors.

Structural models of the prototypal phase change material Ge-Sb-Te are generated from first-principles molecular dynamics simulations with a particular attention to the ${\mathrm{Ge}}_{x}{\mathrm{Sb}}_{x}{\mathrm{Te}}_{100\ensuremath{-}2x}$ join. Constraint counting algorithms permit us to analyze the obtained amorphous networks within the framework of topological constraint (rigidity) theory. A flexible-to-rigid transition is found at $x\ensuremath{\simeq}8.5$% which satisfies the Maxwell isostatic criterion and is characterized by an important topological disorder with large amounts of mixed geometries and miscoordinations. The angular constraint count furthermore leads to the identification of tetrahedral Ge sites whose population decreases with growing Ge/Sb content and is about 55% for ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$. Sb sites change from a dominant pyramidal geometry typical of Group V chalcogenides to a defect octahedral one which is reminiscent of the crystalline polymorph of ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$.

We report the synthesis and initial characterization of ${\mathrm{Ba}}_{6}{\mathrm{Y}}_{2}{\mathrm{Rh}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{17\ensuremath{-}\ensuremath{\delta}}$, a previously unreported material, to the best of our knowledge, with a hexagonal symmetry structure. Face-sharing $\mathrm{Rh}{\mathrm{O}}_{6}$ octahedra form triangular planes of ${\mathrm{Rh}}_{2}{\mathrm{O}}_{9}$ dimers that are widely separated in the perpendicular direction. The material displays a small effective magnetic moment, due to the Rh ions present, and a negative Curie-Weiss temperature. The charge transport and optical band gaps are very similar, near 0.16 eV. A large upturn in the heat capacity at temperatures below 1 K, suppressed by applied magnetic fields larger than ${\ensuremath{\mu}}_{0}\mathrm{H}=2\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, is observed. A large T-linear term in the specific heat ($\ensuremath{\gamma}=166\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{mol}\phantom{\rule{0.16em}{0ex}}\mathrm{f}.\mathrm{u}\phantom{\rule{0.16em}{0ex}}\mathrm{K}}^{2}$) is seen, although the material is insulating at low temperatures. These results suggest the possibility of a spin liquid ground state in this material.

Environmentally benign $(1\text{\ensuremath{-}}x)\mathrm{Ba}({\mathrm{Ti}}_{0.8}{\mathrm{Zr}}_{0.2}){\mathrm{O}}_{3}\text{\ensuremath{-}}x({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}){\mathrm{TiO}}_{3}$ (BZT-BCT) ceramics are promising materials due to their remarkable high piezoresponse [Liu and Ren, Phys. Rev. Lett. 103, 257602 (2009)]. In this Letter, by focusing on local and average structure in combination with macroscopic electromechanical and dielectric measurements we demonstrate the structure property relationship in the tetragonal BZT-BCT ceramic. During high-temperature cubic to tetragonal phase transformation, polar nanoregions are manifested through the spontaneous volume ferroelectrostriction at temperatures below $\ensuremath{\sim}477$ K. Temperature-dependent local structural investigations across the Zr $K$ edge extended x-ray absorption fine-structure spectroscopy reveal an anomalous collaboration between the ${\mathrm{ZrO}}_{6}$ and ${\mathrm{TiO}}_{6}$ octahedra. These octahedra compromise their individuality during polarization development. The presence of domains of submicron size embedded inside the macroscopic ferroelectric regions below ${T}_{m}$, as well as their hierarchical arrangement, is observed by piezoresponse force microscopy. Effects of the existence of the structural/polar heterogeneities below ${T}_{m}$ are observed also when polarizabilities of the poled and unpoled samples are compared; the poled sample is found to be more susceptible to the electric field. In addition, by using electric field dependent x-ray diffraction studies we also show that this ceramic under field exhibits a reduction of tetragonal distortion, which is consistent with earlier reports.

Crystal structure, electronic state of ${\mathrm{Ir}}^{4+}$, and magnetic properties of the antifluorite compound ${\mathrm{K}}_{2}{\mathrm{IrBr}}_{6}$ are studied using high-resolution synchrotron x-ray diffraction, resonant inelastic x-ray scattering (RIXS), thermodynamic and transport measurements, and ab initio calculations. The crystal symmetry is reduced from cubic at room temperature to tetragonal below 170 K and eventually to monoclinic below 122 K. These changes are tracked by the evolution of the noncubic crystal-field splitting $\mathrm{\ensuremath{\Delta}}$ measured by RIXS. Nonmonotonic changes in $\mathrm{\ensuremath{\Delta}}$ are ascribed to the competing effects of the tilt, rotation, and deformation of the ${\mathrm{IrBr}}_{6}$ octahedra as well as tetragonal strain on the electronic levels of ${\mathrm{Ir}}^{4+}$. The N\'eel temperature of ${T}_{N}=11.9$ K exceeds that of the isostructural ${\mathrm{K}}_{2}{\mathrm{IrCl}}_{6}$, and the magnitude of frustration on the fcc spin lattice decreases. We argue that the replacement of Cl by Br weakens electronic correlations and enhances magnetic couplings.

First-principles calculations are performed to investigate and analyze properties of (001) thin films made of the most complex perovskite system, namely, $\mathrm{Na}\mathrm{Nb}{\mathrm{O}}_{3}$ (NNO), and are subject to epitaxial strain. In particular, an energy-versus-misfit strain phase diagram is constructed and reveals the existence of three different ground states for different strain regimes. For large compressive strain and up to moderate tensile strain, a monoclinic $Cc$ phase occurred, with its polarization and axis of antiphase tilting both rotating within a ($\overline{1}10$) plane with the magnitude of the strain. For large tensile strain, a ferroelectric orthorhombic state of $Pmc{2}_{1}$ symmetry emerges with polarization lying along the [110] in-plane direction together with an octahedral tilting adopting the ${a}^{\ensuremath{-}}{a}^{\ensuremath{-}}{c}^{+}$ pattern and an antiferroelectric vector associated with the reciprocal zone-border $X$ point. Finally, in between and for a narrow region of strain, a complex ground state is found. It has orthorhombic $Pca{2}_{1}$ symmetry, for which a complex tilting pattern coexists with polarization pointing along the [001] out-of-plane direction and antiferroelectric displacement associated with the $\mathrm{\ensuremath{\Delta}} k$ point located halfway between the zone-center $\mathrm{\ensuremath{\Gamma}}$ and zone-border $X$ point. Ferroelectric, antiferroelectric, and antiferrodistortive properties are also reported and discussed as a function of misfit strain.

Using first-principles calculations, we predict a unique $\mathrm{BiFe}{\mathrm{O}}_{3}$ (BFO) phase, which is coined $I$-BFO, with layered characteristics based on the ilmenite structure. The interactions between interlayer bismuth atoms make it easy to exfoliate monolayer BFO with a configuration in which Bi ions are above and below two-dimensional (2D) iron-oxygen octahedra sharing common edges. The calculated phonon spectra reveal the dynamic stability of both bulk and monolayer forms of $I$-BFO. Combining total energy computation and crystal structure search, we corroborate that the $I$-BFO monolayer maintains the lowest-energy ground state, as compared to other 2D BFO exfoliations. We also discover that monolayer $I$-BFO is a robust antiferromagnetic material with a N\'eel temperature (${T}_{N}$) of 4 K in the fully relaxed case. Such ${T}_{N}$ can be prominently improved by strain, for example, ${T}_{N}=25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at 6% tension. Furthermore, our electronic structure calculations with PBE and hybrid HSE functionals show the $I$-BFO monolayer to be an indirect-gap semiconductor. Finally, we indicate that other ternary $AB{\mathrm{O}}_{3}$ compounds also have the potential to produce layered materials from the ilmenite structure. This work thus provides guidance to explore unconventional layered materials, and further broadens the application of BFO and other promising ilmenite oxides in two dimensions.

We determined the crystal structure of ${\mathrm{Cs}}_{2}{\mathrm{LiRuCl}}_{6}$, which was synthesized in this work, and investigated its magnetic properties. ${\mathrm{Cs}}_{2}{\mathrm{LiRuCl}}_{6}$ has a hexagonal structure composed of linear chains of face-sharing ${\mathrm{RuCl}}_{6}$ and ${\mathrm{LiCl}}_{6}$ octahedra. In two thirds of the structural chains, ${\mathrm{Ru}}^{3+}$ and ${\mathrm{Li}}^{+}$ sites are almost ordered, while in the other chains their sites are disordered. This situation is analogous to the ground state of the antiferromagnetic Ising model on a triangular lattice. Using electron paramagnetic resonance, we evaluated the $g$ factors of ${\mathrm{Ru}}^{3+}$ with effective spin-$\frac{1}{2}$ as ${g}_{c}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}2.72$ and ${g}_{ab}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}1.50$ for magnetic fields $H$ parallel and perpendicular to the $c$ axis, respectively. Magnetization curves for $H\ensuremath{\parallel}c$ and $H\phantom{\rule{0.16em}{0ex}}\ensuremath{\perp}\phantom{\rule{0.16em}{0ex}}c$ are highly anisotropic. However, these magnetization curves approximately coincide when normalized by the $g$ factors. It was found from the magnetization and specific heat results that ${\mathrm{Cs}}_{2}{\mathrm{LiRuCl}}_{6}$ can be described as a coupled one-dimensional $S\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}1/2$ Heisenberg-like antiferromagnet with $J/{k}_{\mathrm{B}}\phantom{\rule{0.16em}{0ex}}\ensuremath{\simeq}\phantom{\rule{0.16em}{0ex}}3.7$ K. Three-dimensional ordering occurs at ${T}_{\mathrm{N}}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}0.48$ K. A magnetic phase diagram for $H\ensuremath{\parallel}c$ is also presented.

In Cu-doped ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ nanoparticles $({\mathrm{Cu}}_{x}{\mathrm{Co}}_{3\text{\ensuremath{-}}x}{\mathrm{O}}_{4};\phantom{\rule{0.16em}{0ex}}0\ensuremath{\le}x\ensuremath{\le}0.5)$ Cu occupies both octahedral and tetrahedral sites with a $2+$ oxidation state. As the Cu doping increases, we observe changes in the crystal structure corresponding to a Jahn-Teller distortion of the ${\mathrm{Cu}}^{2+}$ sites. To mediate charge balance with ${\mathrm{Cu}}^{2+}$ entering the octahedral sites, a hole forms in the O $2p$ orbitals bonded to the ${\mathrm{Cu}}^{2+}({O}_{h})$. ${\mathrm{Cu}}^{2+}({T}_{d})$ is noninteracting and disrupts the existing antiferromagnetic interactions between the ${\mathrm{Co}}^{2+}({T}_{d})$ ions, while ${\mathrm{Cu}}^{2+}({O}_{h})$ exhibits a ferromagnetic response as a result of a hybrid form of exchange occurring between ${\mathrm{Cu}}^{2+}({O}_{h})$ and ${\mathrm{Co}}^{2+}({T}_{d})$. Emergence of the $3{d}^{9}$ ligand hole is directly responsible for the origin of ferromagnetic Cu in the octahedral sites and this results in the unusual magnetism in ${\mathrm{Cu}}_{x}{\mathrm{Co}}_{3\text{\ensuremath{-}}x}{\mathrm{O}}_{4}$.

Concerning the magnetic anisotropy and magnetic moments of the ${M}^{3+}$ $(M=\mathrm{V}, \mathrm{Cr})$ ions in ferromagnetic $(\mathrm{FM}){M\mathrm{I}}_{3}$ monolayers, which have a honeycomb pattern of edge-sharing ${M\mathrm{I}}_{6}$ octahedra, conflicting observations have been reported in experimental and theoretical studies. We resolve these conflicts by determining the magnetic anisotropy energies for the ${M}^{3+}$ ions of ${M\mathrm{I}}_{3}$ monolayers, by analyzing their preferred spin orientations in terms of the selection rules based on the highest occupied molecular orbital--lowest unoccupied molecular orbital interactions of the ${M\mathrm{I}}_{6}$ octahedra, and by discussing whether or not the ${M}^{3+}$ ions are uniaxial. Here we show that the FM monolayer ${\mathrm{VI}}_{3}$ is uniaxial, but that of ${\mathrm{CrI}}_{3}$ is not. The magnetic anisotropy energy for the ${\mathrm{V}}^{3+}({d}^{2},S=1)$ ion of ${\mathrm{VI}}_{3}$ is greater than that for the ${\mathrm{Cr}}^{3+}$ $({d}^{3},$ $S=3/2)$ ion of ${\mathrm{CrI}}_{3}$ by more than an order of magnitude (i.e., \ensuremath{\sim}8 vs \ensuremath{\sim}0.6 meV). The ${\mathrm{V}}^{3+}$ ion exhibits uniaxial magnetism because its orbital quantum number L is not zero $(L=\phantom{\rule{0.16em}{0ex}}1)$, in contrast to the ${\mathrm{Cr}}^{3+}$ ion $(L=0)$.

Transition metal oxides in the ${d}^{0}$ perovskite class, defined broadly to include $\mathrm{W}{\mathrm{O}}_{3}$, have shown promise as light absorbers for photocatalytic applications. These compounds are highly tunable in their composition and atomic structure, and consequently, in their electronic structure and properties. Through the lens of atomic orbital interactions and density functional theory calculations, the present work explores how and why certain structural distortions tune the band gaps and band edges of $\mathrm{W}{\mathrm{O}}_{3}$ and ${d}^{0}$ perovskite oxides. It is determined that, because an individual distortive mode (e.g., polar distortion along or octahedral rotation around a cubic perovskite axis) changes the character of only some band-edge orbitals, combinations of distortive modes may tune band gaps much more significantly than the sum of individual modes. The implications of this tunability are discussed for $\mathrm{W}{\mathrm{O}}_{3}$ as a photocatalyst, and for ${d}^{0}$ perovskite oxides more generally.

Palladium hydride alloys are superconductors and hydrogen storage materials. One synthesis route is compression of Pd to high pressure in a hydrogen-rich environment. Here we report the evolution of the unit cell volume of ${\mathrm{PdH}}_{x}$ synthesized by compressing Pd in a pure ${\mathrm{H}}_{2}$ medium to pressures from 0.2 to 8 GPa in a diamond anvil cell at room temperature. The volume of the face-centered cubic unit cell changes nonmonotonically with pressure, increasing upon compression from 0.2 to 1 GPa and decreasing upon compression from 1 to 8 GPa. Volume is reversible upon decompression and is independent of whether the sample was heated to 600 K at low pressure ($Pl2$ GPa). The x-ray diffraction data show no evidence for a phase transition between 0.2 and 8 GPa. The volume maximum at 1 GPa must be caused by progressive hydrogenation from 0 to 1 GPa. Assuming a pressure-volume-composition equation of state derived from previously published data, the [H]:[Pd] ratio in this study increases to a maximum value of $x=1\ifmmode\pm\else\textpm\fi{}0.02$ at $2\ifmmode\pm\else\textpm\fi{}0.5$ GPa and remains stable upon further compression to and from 8 GPa. These results add to a mounting body of evidence that ${\mathrm{PdH}}_{1\ifmmode\pm\else\textpm\fi{}\ensuremath{\epsilon}}$ is in thermodynamic equilibrium with pure ${\mathrm{H}}_{2}$ at room temperature from 2 GPa to at least 8 GPa. The simplest interpretation is that H atoms occupy all octahedral sites and no tetrahedral sites in face-centered cubic ${\mathrm{PdH}}_{1.0}$.

The recently discovered cuprate superconductor ${\mathrm{Ba}}_{2}\mathrm{Cu}{\mathrm{O}}_{3+\ensuremath{\delta}}$ exhibits a high ${T}_{c}\ensuremath{\simeq}73\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ at $\ensuremath{\delta}\ensuremath{\simeq}0.2$. The polycrystal grown under high pressure has a structure similar to ${\mathrm{La}}_{2}\mathrm{Cu}{\mathrm{O}}_{4}$ but with dramatically different lattice parameters due to the $\mathrm{Cu}{\mathrm{O}}_{6}$ octahedron compression. The crystal field in the compressed ${\mathrm{Ba}}_{2}\mathrm{Cu}{\mathrm{O}}_{4}$ leads to an inverted Cu $3d\phantom{\rule{4pt}{0ex}}{e}_{g}$ complex with the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital sitting below the ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ and an electronic structure highly unusual compared to the conventional cuprates. We construct a two-orbital Hubbard model for the Cu ${d}^{9}$ state at hole doping $x=2\ensuremath{\delta}$ and study the orbital-dependent strong correlation and superconductivity. For the undoped case at $x=0$, we found that strong correlation drives an orbital-polarized Mott-insulating state with the spin-$1/2$ moment of the localized ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbital. In contrast to the single-band cuprates where superconductivity is suppressed in the overdoped regime, hole doping the two-orbital Mott insulator leads to orbital-dependent correlations and the robust spin and orbital exchange interactions produce a high-${T}_{c}$ antiphase $d$-wave superconductor even in the heavily doped regime at $x=0.4$. We conjecture that ${\mathrm{Ba}}_{2}\mathrm{Cu}{\mathrm{O}}_{3+\ensuremath{\delta}}$ realizes mixtures of such heavily hole-doped superconducting ${\mathrm{Ba}}_{2}\mathrm{Cu}{\mathrm{O}}_{4}$ and disordered ${\mathrm{Ba}}_{2}\mathrm{Cu}{\mathrm{O}}_{3}$ chains in a single-layer or predominately separated bilayer structure. Our findings suggest that unconventional cuprates with liberated orbitals as doped two-band Mott insulators can be a direction for realizing high-${T}_{c}$ superconductivity with enhanced transition temperature ${T}_{c}$.

Quasiparticle self-consistent $GW$ calculations are presented for the band structures of $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_{2}$ and $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_{2}$ in the orthorhombic $Pna{2}_{1}$ tetrahedrally coordinated crystal structures, which are closely related to the wurtzite structure of ZnO. Symmetry labeling of the bands near the gap is carried out, and effective-mass tensors are extracted for the conduction-band minimum and crystal-field split valence-band maxima (VBM) at $\mathrm{\ensuremath{\Gamma}}$. The gap is found to be direct at $\mathrm{\ensuremath{\Gamma}}$ and is 5.81 eV in $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_{2}$ and 5.46 eV in $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_{2}$. Electron-phonon coupling zero-point normalization is estimated to lower these gaps by about $0.2\ifmmode\pm\else\textpm\fi{}0.1$ eV. Optical response functions are calculated within the independent-particle long-wavelength limit, and they show the expected anisotropy of the absorption onsets due to the crystal-field splitting of the VBM. The results show that both materials are promising candidates as ultrawide-gap semiconductors with wurtzite-based tetrahedrally bonded crystal structures. Direct transitions from the lowest conduction band to higher bands, relevant to $n$-type doped material and transparent conduction applications, are found to start only above 3.9 eV and are allowed for only one polarization, and several higher band transitions are forbidden by symmetry. Alternative crystal structures, such as $R\overline{3}m$ and a rocksalt-type phase with a tetragonally distorted $P4/mmm$ spacegroup, both with octahedral coordination of the cations, are also investigated. They are found to have higher energy but about 20% smaller volume per formula unit. The transition pressures to these phases are determined, and for $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_{2}$ they are found to be in good agreement with experimental studies. The $R\overline{3}m$ phase also has a comparably high but slightly indirect band gap, while the rocksalt-type phase is found to have a considerably smaller gap of about 3.1 eV in $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_{2}$ and 1.0 eV in $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_{2}$.

${\mathrm{Co}}^{2+}$ ions in an octahedral crystal field stabilize a ${j}_{\mathrm{eff}}=1/2$ ground state with an orbital degree of freedom and have been recently put forward for realizing Kitaev interactions, a prediction we have tested by investigating spin dynamics in two cobalt honeycomb lattice compounds, ${\mathrm{Na}}_{2}{\mathrm{Co}}_{2}{\mathrm{TeO}}_{6}$ and ${\mathrm{Na}}_{3}{\mathrm{Co}}_{2}{\mathrm{SbO}}_{6}$, using inelastic neutron scattering. We used linear spin wave theory to show that the magnetic spectra can be reproduced with a spin Hamiltonian including a dominant Kitaev nearest-neighbor interaction, weaker Heisenberg interactions up to the third neighbor, and bond-dependent off-diagonal exchange interactions. Beyond the Kitaev interaction that alone would induce a quantum spin liquid state, the presence of these additional couplings is responsible for the zigzag-type long-range magnetic ordering observed at low temperature in both compounds. These results provide evidence for the realization of Kitaev-type coupling in cobalt-based materials, despite hosting a weaker spin-orbit coupling than their $4d$ and $5d$ counterparts.

Magnetic susceptibility, magnetization, and specific-heat measurements under both in-plane and out-of-plane field on ${\mathrm{K}}_{2}{\mathrm{Co}}_{2}{(\mathrm{Se}{\mathrm{O}}_{3})}_{3}$ single crystals are reported. The Co dimers in this compound, which are made from face-sharing effective spin-1/2 $\mathrm{Co}{\mathrm{O}}_{6}$ octahedra, sit on the corners of a geometrically frustrating triangular planar lattice. No long-range magnetic ordering is found down to 0.35 K, which, together with the large negative Curie-Weiss temperature obtained from fitting the temperature-dependent magnetic susceptibility, indicates that the system is magnetically frustrated. Analysis of the field and orientation dependence of the magnetic and thermodynamic properties suggests the presence of an easy-axis anisotropy. The magnetic fluctuations present due to geometric frustration are more resistant to an in-plane field than a perpendicular-to-plane field.

Using parameter-free density functional approximation and maximally localized Wannier functions analysis, we demonstrate how the ${\mathrm{Ni}}^{3+}$-based heterointerface can present a complete orbital polarization of the Ni-${e}_{g}$ orbitals without introducing additional electronic correlation, which is very inspiring for superconductivity research in two-dimensional confined nickelate layers. The polarized internal electric field from the insulating $\mathrm{Dy}\mathrm{Sc}{\mathrm{O}}_{3}$ layer, spontaneous oxygen octahedral distortion, and in-plane tensile strain can fine-tune means for the generation of single occupied Ni-${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ band structures and cuperate-like Fermi surfaces. Furthermore, by including electronic correlation through on-site Hubbard $U$, we also find a stronger antiferromagnetic correlation than that in the recent synthetic infinite-layer $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_{2}$ with hole-doped ${\mathrm{Ni}}^{1+}$ superconductivity. The low-energy electronic and spin excitations in the nickelate layer resemble those of high-temperature cuprate superconductors. Our study sheds light on promising directions for the preparation of nickelate superconductors.

Heavy transition metal magnets with ${J}_{eff}=\frac{1}{2}$ electronic ground states have attracted recent interest due to their penchant for hosting new classes of quantum spin liquids and superconductors. Unfortunately, model systems with ideal ${J}_{eff}=\frac{1}{2}$ states are scarce due to the importance of noncubic local distortions in most candidate materials. In this work, we identify a family of iridium halide systems [i.e., ${\mathrm{K}}_{2}\mathrm{Ir}{\mathrm{Cl}}_{6}$, ${\mathrm{K}}_{2}\mathrm{Ir}{\mathrm{Br}}_{6}$, ${({\mathrm{NH}}_{4})}_{2}\mathrm{Ir}{\mathrm{Cl}}_{6}$, and ${\mathrm{Na}}_{2}\mathrm{Ir}{\mathrm{Cl}}_{6}\ifmmode\cdot\else\textperiodcentered\fi{}6({\mathrm{H}}_{2}\mathrm{O})]$ with ${\mathrm{Ir}}^{4+}$ electronic ground states exhibiting extremely small deviations from the ideal ${J}_{eff}=\frac{1}{2}$ limit. We also find ordered magnetic ground states for the three anhydrous systems, with single-crystal neutron diffraction on ${\mathrm{K}}_{2}\mathrm{Ir}{\mathrm{Br}}_{6}$ revealing type-I antiferromagnetism. This spin configuration is consistent with expectations for significant Kitaev exchange in a face-centered-cubic magnet. This work establishes that incorporating isolated $\mathrm{Ir}{X}_{6}$ octahedra in materials, where $X$ is a halogen ion with a low electronegativity, is an effective design principle for realizing unprecedented proximity to the pure ${J}_{eff}=\frac{1}{2}$ state. At the same time, we highlight undeniable deviations from this ideal state, even in clean materials with ideal $\mathrm{Ir}{X}_{6}$ octahedra as inferred from the global cubic crystal structures.

We report preparation, crystal structure, and physical properties of a quasi-one-dimensional Cr-based arsenide hydride ${\mathrm{K}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}{\mathrm{H}}_{x}$. Through an electrolysis using essentially nonsuperconducting samples as the cathode, additional hydrogen atoms can be successfully intercalated up to $x=0.45$ and, consequently, the in-plane and interplane $\mathrm{Cr}\text{--}\mathrm{Cr}$ bond distances in the chains of face-sharing Cr octahedra increase by 3.7% and 1.5%, respectively. The electrochemically hydrogenized samples show a broad superconducting transition at ${T}_{\mathrm{c}}=5.8$ K, a record in the K-Cr-As-H system, with nearly full magnetic shielding at 1.8 K. The electronic specific-heat coefficient extracted from the specific-heat measurement is as high as ${\ensuremath{\gamma}}_{\mathrm{n}}=47 \mathrm{mJ}$ ${\mathrm{K}}^{\ensuremath{-}2} \mathrm{mol}$ ${\mathrm{Cr}}^{\ensuremath{-}1}$, suggesting a stronger electron correlation that is likely to be associated with the expansions of $\mathrm{Cr}\text{--}\mathrm{Cr}$ bonds. Meanwhile, the dimensionless specific-heat jump $\mathrm{\ensuremath{\Delta}}C/({\ensuremath{\gamma}}_{\mathrm{n}}{T}_{\mathrm{c}})$ is only 0.30, about 20% of the expected value in the BCS weak-coupling scenario. Furthermore, the normal-state magnetism is characterized by Curie-Weiss paramagnetism with an enhanced effective localized moment of 1.33 ${\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Cr}$, suggesting that a nonsuperconducting phase with localized spins dominates. The $^{1}\mathrm{H}$ nuclear magnetic resonance measurement reveals two different spin-lattice relaxations, corresponding to superconducting and localized-spin phases, respectively. All the results point to phase separation with minority superconducting phase and majority nonsuperconducting phase in the quasi-one-dimensional ${\mathrm{K}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}{\mathrm{H}}_{x}$ system.

Bulk chalcogenide perovskite $\mathrm{BaZr}{\mathrm{S}}_{3}$ (BZS), with a direct band gap in the visible region, is an important photovoltaic material, albeit with limited applicability owing to its antiferroelectric (AF) nature. Presently, ferroelectric (FE) perovskite-based photovoltaics are attracting enormous attention for environmental stability and better energy conversion efficiency through enhanced charge separation. We report on AF-FE phases of BZS thin film using temperature-dependent Raman investigations and first-principles calculations. Origin of localized FE phase is established from an anomalous behavior of ${A}_{g}^{7}\ensuremath{\sim}300\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ and ${B}_{1g}^{5}\ensuremath{\sim}420\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ modes, which involve the vibration of atoms at the apical site of $\mathrm{Zr}{\mathrm{S}}_{6}$ octahedra. Additionally, ${B}_{1g}^{1}$ and ${B}_{2g}^{2}$ ($\ensuremath{\sim}85\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$) modes appear while antipolar ${B}_{2g}^{1}$ ($\ensuremath{\sim}60\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$) disappears below 60 K. Our first-principles calculations confirm that FE appears as a result of the loss of center of inversion symmetry in $\mathrm{Zr}{\mathrm{S}}_{6}$ due to the existence of oxygen impurities placed locally at apical sites of sulfur.

We report the detailed analysis of specific heat $[{C}_{P}(T)]$ and ac susceptibility for magnetically frustrated ${\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{CoNbO}}_{6}$ $(x=0--1)$ double perovskites to understand low-temperature complex magnetic interactions and their evolution with $x$. Interestingly, the observed Schottky anomaly in the $x\ensuremath{\leqslant}0.4$ samples shifts gradually toward higher temperatures with magnetic field as well as $x$, and the analysis reveals the persistence of the discrete energy states in these samples resulting from the spin-orbit coupling and octahedral distortion. Moreover, the extracted values of the Land\'e $g$ factor indicate the existence of high-spin state ${\mathrm{Co}}^{3+}$ ions energetically close to a nonmagnetic low-spin state. The specific-heat data show the $\ensuremath{\lambda}$-type anomaly for the $x\ensuremath{\geqslant}0.6$ samples due to evolution of the long-range antiferromagnetic ordering. Our analysis of low-temperature ${C}_{P}(T)$ data for the $x\ensuremath{\geqslant}0.6$ samples demonstrates the 3D isotropic Heisenberg antiferromagnetic (AFM) interactions and the temperature-induced second-order AFM-paramagnetic phase transition. More interestingly, we demonstrate the presence of the free ${\mathrm{Co}}^{2+}$-like Kramers doublet ground state in the $x=1$ sample. Further, the ac susceptibility and time evolution of the magnetization data reveal the low-temperature cluster-glass-like behavior in the $x=0--0.4$ samples, where spin-spin correlation strength decreases with $x$.

Apart from a handful of exceptions, all known complex oxide two-dimensional electron gases (2DEGs) are formed in ${\mathrm{SrTiO}}_{3}$-based heterostructures, and microscopic information about non-${\mathrm{SrTiO}}_{3}$ 2DEGs systems is scarce. Here, we report on the realization of metallic conductance in a ${\mathrm{CaTiO}}_{3}$-based system, ${\mathrm{CaTiO}}_{3}$/${\mathrm{LaTiO}}_{3}$ superlattices, epitaxially grown in a layer-by-layer way on a ${\mathrm{NdGaO}}_{3}$(110) substrate by pulsed laser deposition. The high quality of the crystal and electronic structures is characterized by in situ reflection high-energy electron diffraction, x-ray diffraction, scanning transmission electron microscopy, and x-ray photoemission spectroscopy. Electrical transport confirms the formation of metallic interfaces in the ${\mathrm{CaTiO}}_{3}$/${\mathrm{LaTiO}}_{3}$ superlattice. In addition, Hall measurements reveal that in the ${\mathrm{CaTiO}}_{3}$/${\mathrm{LaTiO}}_{3}$ superlattice the room-temperature carrier mobility is nearly three times higher than that of the ${\mathrm{CaTiO}}_{3}$/${\mathrm{YTiO}}_{3}$ superlattice, implying the importance of ${\mathrm{TiO}}_{6}$ octahedral tilts and rotations on the carrier mobility of a 2DEG. Since doped ${\mathrm{CaTiO}}_{3}$ is an A-site polar metal, our results provide a materials system for designing synthetic two-dimensional polar metals.