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Persistent spin textures (PSTs) in solid-state materials arise from a unidirectional spin-orbit field in momentum space and offer a route to deliver long carrier spin lifetimes sought for future quantum microelectronic devices. Nonetheless, few three-dimensional materials are known to host PSTs owing to crystal symmetry and chemical requirements. There are even fewer examples demonstrated experimentally. Here we report that high-quality persistent spin textures can be obtained in the polar point groups containing an odd number of mirror operations. We use representation theory analysis and electronic structure calculations to formulate general discovery principles to identify PSTs hidden in known complex ternary layered and perovskite structures with large electric polarizations. We then show some of these materials exhibit PSTs without requiring any special crystalline symmetries. This finding removes the limitation imposed by mirror-symmetry protected PSTs that has limited compound discovery. Our general design approach enables the pursuit of persistent spin helices in materials exhibiting the $C_{3v}$ crystal class adopted by many quantum materials exhibiting large Rashba coefficients.

Materials property predictions have improved from advances in machine learning algorithms, delivering materials discoveries and novel insights through data-driven models of structure-property relationships. Nearly all available models rely on featurization of materials composition, however, whether the exclusive use of structural knowledge in such models has the capacity to make comparable predictions remains unknown. Here we employ a deep neural network model to decode structure-property relationships in crystalline materials without explicitly considering chemical compositions. The focus is on classification of crystal systems, mechanical elasticity, electronic band gap, and phase stability. Our model utilizes a three-dimensional (3D) momentum space representation of structure from elastic x-ray scattering theory that exhibits rotation and permutation invariance. We perform novel ablation studies to help interpret the model performance by perturbing the physically meaningful input features (i.e., the diffraction patterns) instead of tuning the architecture of the learning model as in conventional ablation methods. We find that the spatial symmetry of the 3D diffraction patterns, which reflects crystalline symmetry operations, is more important than the diffraction intensities contained within for the model to make a successful classification. Our work showcases the potential of using statistical learning models to help understand materials physics, rather than performing predictive and generative tasks as in most materials informatics research. We also argue that learning the crystal structure genome in a chemistry-agnostic manner demonstrates that some crystal structures inherently host high propensities for optimal materials properties, which enables the decoupling of structure and composition for future codesign of multifunctionality., Comment: 14 pages, 6 Figures

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.

Room-temperature electric-field control of magnetism is actively sought to realize electric-field assisted changes in perpendicular magnetic anisotropy (PMA), which is important to magnetic random access memories (MRAMs) and future spin-orbit based logic technologies. Traditional routes to achieve such control rely on heterostructures of ferromagnets and/or ferroelectrics, exploiting interfacial effects, including strain generated by the substrate, or electric-field induced changes in the interfacial electronic structures. Here we present design rules based on $d$-orbital splitting in an octahedral field and crystallographic symmetries for electric-field control of PMA utilizing hybrid improper ferroelectricity by scaffolding simple perovskite oxides into ultrashort period superlattices, ${(\mathrm{AB}{\mathrm{O}}_{3})}_{1}/{({\mathrm{A}}^{\ensuremath{'}}\mathrm{B}{\mathrm{O}}_{3})}_{1}$, and in multiferroic ${\mathrm{AA}}^{\ensuremath{'}}{\mathrm{BB}}^{\ensuremath{'}}{\mathrm{O}}_{6}$ double perovskites. We validate the strategy using first principles calculations and a single-ion anisotropic model. We find a change in the magnetic anisotropy from the in-plane to the out of plane direction in ${(\mathrm{BiFe}{\mathrm{O}}_{3})}_{1}/{(\mathrm{LaFe}{\mathrm{O}}_{3})}_{1}$ and a 50% decrease of the magnetization along the out of plane direction in $\mathrm{LaYNiMn}{\mathrm{O}}_{6}$, when a polar to nonpolar phase transition occurs with strain. The origin of the PMA control is due to the structural tunable competitions among the ${t}_{2g}$ and ${e}_{g}$ orbital interactions on the magnetic ions arising from relativistic spin-orbital interactions that are susceptible to changes in the oxygen octahedral tilts across the field-tunable transition. Our results allow us to search rapidly for other promising multiferroics materials with voltage-controlled magnetic anisotropy for applications in low-energy information storage and logic devices.

Materials exhibiting negative thermal expansion (NTE) are important for the fabrication and operation of microelectronic devices and optical systems. As an important group of Ruddlesden-Popper (RP) perovskites, calcium titanates ${\mathrm{Ca}}_{n+1}{\mathrm{Ti}}_{n}{\mathrm{O}}_{3n+1}$ [(CTO), $n=1,2,...,\ensuremath{\infty}$] have layered structures and may exhibit quasi-two-dimensional (quasi-2D) NTE within their three-dimensional structural architectures. In this paper, combining density-functional-theory calculations and the self-consistent quasiharmonic approximation method, we investigate the variation of the quasi-2D character of the phonon spectra and thermal expansion in the ${\mathrm{Ca}}_{n+1}{\mathrm{Ti}}_{n}{\mathrm{O}}_{3n+1}$ family ($n=1--3$, and $\ensuremath{\infty}$) with respect to $n$. We find that a quasi-2D NTE mechanism is active in the RP-CTOs at $n$ of 1--3, whereas a quasi-rigid-unit mode mechanism is active at $n=\ensuremath{\infty}$ (i.e., the perovskite phase). We find a NTE trend with layer number for the orthorhombic materials comprising the RP series, but the monoclinic polymorph is an outlier. For the orthorhombic members, we find the critical pressure for NTE increases with increasing $n$, but the NTE critical temperature decreases (when materials are compared at the same pressure). Additionally, the elastic moduli can be used as effective descriptors for this layer-dependent behavior of the NTE, i.e., the stiffer the RP-CTO then the lower its NTE. We also propose the integrated NTE capacity to capture the correlation between the quasi-2D NTE and $n$, and it monotonically decreases with increasing $n$.

The solid solution $\mathrm{Ba}{\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{S}}_{2}$ exhibits an insulator-to-metal transition close to $x=0.21$. Questions of whether this transition is coupled with structural changes remain open. Here we follow the structural evolution as a function of the Ni content $x$ using synchrotron powder x-ray diffraction and pair distribution function analyses to reveal significant basal sulfide anion displacements occurring preferentially along the ${\mathrm{CoS}}_{5}$ pyramidal edges comprising the edge-connected bond network in $\mathrm{Ba}{\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{S}}_{2}$. These displacements decrease in magnitude as $x$ increases and are nearly quenched in $x=1\phantom{\rule{4pt}{0ex}}{\mathrm{BaNiS}}_{2}$. Density-functional-theory-based electronic structure calculations on $x=0\phantom{\rule{4pt}{0ex}}{\mathrm{BaCoS}}_{2}$ suggest that these displacements arise as a dynamic first-order Jahn-Teller effect owing to partial occupancy of nominally degenerate ${\mathrm{Co}}^{2+}\phantom{\rule{4pt}{0ex}}{d}_{xz}$ and ${d}_{yz}$ orbitals, leading to local structural symmetry breaking in the $xy$-plane of the Co-rich phases. The Jahn-Teller instability is associated with the opening of a band gap that is further strengthened by electronic correlation. The Jahn-Teller effect is reduced upon increased electron filling as $x\ensuremath{\rightarrow}1$, indicating that the local structure and band filling cooperatively result in the observed insulator-to-metal transition.

The presence of ordered oxygen vacancies in perovskites governs magnetic phase stability owing to changes in crystal-field splitting with different anion geometries, polyhedral arrangements, and electronic configurations of the transition-metal cations. Here we use density functional theory calculations to assess the magnetic phase stability of ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ (with a ${d}^{5}$ electronic configuration) and ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ (${d}^{4}$ configuration), exhibiting the ${\mathrm{Ca}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$-type oxygen-deficient perovskite structure, with hydrostatic pressure. The ${\mathrm{Ca}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$-type structure is composed of square pyramidal units, the crystal-field splitting and polyhedral connectivities of which support different ground-state magnetic orders depending on $d$-orbital filling: E-type antiferromagnetic (AFM-E) for ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ (${d}^{4}$) and G-type antiferromagnetic (AFM-G) for ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ (${d}^{5}$). We show that hydrostatic pressure enhances the crystal-field splitting and affects the magnetic stability. We find that the AFM-E order exhibited by ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ is robust over the surveyed ranges of applied pressures, whereas ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ shows a magnetic transition from AFM-G to ferromagnetic spin order at $\ensuremath{\approx}24.5$ GPa. We also discuss the effect of correlation strength, treated using the Hubbard $U$ correction, which we find suppresses a spin crossover transition in ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ and shifts it to higher pressures.

We acknowledge support from the National Science Foundation (NSF) under Grant No. 1751736, “CAREER: Fundamentals of Complex Chalcogenide Electronic Materials,” from the MIT Skoltech Program, and from “la Caixa” Foundation MISTI Global Seed Funds. Financial support from the Spanish Ministry of Economy, Competitiveness and Universities, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (Grant No. SEV-2015-0496) and Projects No. MAT2015-73839-JIN (MINECO/FEDER, EU) and No. PID2019-107727RB-I00, and from Generalitat de Catalunya (Grant No. 2017 SGR 1377) is acknowledged. I.F. acknowledges Ramon y Cajal Contract No. RYC-2017- 22531. S.F. acknowledges support from the NSF Graduate Research Fellowship under Grant No. 1122374. The work at Caltech was supported by National Science Foundation Grant No. DMR-1606858. J.R., B.Z., and S.N. acknowledge support from Army Research Office under Award No. W911NF-19- 1-0137 and Air Force Office of Scientific Research under Award No. FA9550-16-1-0335. N.Z.K. and J.M.R. acknowledge support from the U.S. Department of Energy under Grant No. DE-SC0012375 and the DOD-HPCMP for computational resources. N.Z.K. thanks Dr. Michael Waters and Dr. Xuezeng Lu for helpful discussions. S.F. and R.J. acknowledge David Bono and Brian Neltner for helpful discussions and technical assistance.

The discovery of polar metals, bulk band conductors which combine broken inversion symmetry and metallic conductivity, has disrupted the long-standing assumption that polar order and metallicity are incompatible. Despite recent progress, however, the circumstances which allow for this property convergence remain ambiguous. Here, we perform a first-principles analysis of the evolution in the polar distortions in perovskite ${\mathrm{BaTiO}}_{3}$ under electrostatic doping to ascertain the dependencies of acentricity and electrical conductivity at the microscopic level. We focus on the role of local off-centering displacements, driven by the second-order Jahn-Teller effect, rather than discussing bulk homogeneous and cooperative lifting of inversion symmetry, which relies on long-range Coulombic interactions. We show that $n$-type doping disrupts the Ti-O covalent bond and advocate for using chemical bonding arguments as a framework for understanding the interplay between local distortions and free charge carriers rather than solely principles of dielectric screening in metals. In the process, we develop several tools and electronic descriptors that we expect could enable a more direct comparison between the behavior of doped ferroelectrics, such as ${\mathrm{BaTiO}}_{3}$, and polar metals, such as ${\mathrm{LiOsO}}_{3}$, helping us to better understand their similarities and differences under external perturbation.

The authors predict that a charge and spin density-wave transition occurs in the correlated polar metal Ca3Ru2O7 using first-principles calculations and angle resolved photoelectron spectroscopy. The paper presents a model that describes existing experimental data describing the low-temperature fermiology.

We perform systematic density functional theory (DFT) calculations to assess the performance of various exchange-correlation potentials ${V}_{xc}$ in describing the chalcogenide $\mathrm{Ga}{M}_{4}{Q}_{8}$ lacunar spinels ($M=\text{Mo}$, V, Nb, Ta; $Q=\text{S}$, Se). We examine the dependency of crystal structure (in cubic and rhombohedral symmetries), electronic structure, magnetism, optical conductivity, and lattice dynamics in lacunar spinels at four different levels of ${V}_{xc}$: The local density approximation (LDA), generalized gradient approximation (GGA), meta-GGA, and hybrid with fractional Fock exchange. We find that LDA underperforms the Perdew-Burke-Ernzerhof (PBE) and PBE revised for solids (PBEsol) GGA functionals in predicting lattice constants as well as reasonable electronic structures. The performance of LDA and GGAs can be improved both quantitatively and qualitatively by including an on-site Coulomb interaction (LDA/$\mathrm{GGA}+U$) with a Hubbard $U$ value ranging from 2 to 3 eV. We find that the PBE functional is able to produce a semiconducting state in the distorted polar $R3m$ phase without on-site Coulomb interactions. The meta-GGA functional SCAN (strongly constrained and appropriately normed) predicts reasonable lattice constants and electronic structures; it exhibits behavior similar to the $\mathrm{GGA}+U$ functionals for small $U$ values of 1 to 2 eV. The hybrid functional HSE06 is accurate in predicting the lattice constants but leads to a band gap greater than the experimental estimation of 0.2 eV in this family. All of the lacunar spinels in the cubic phase are metallic at these levels of band theory; however, the predicted valence bandwidths are extremely narrow ($\ensuremath{\approx}0.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$). The DFT ground states of cubic vanadium chalcogenides are found to be highly spin polarized, which contrast with previous experimental results. With spin-orbit coupling (SOC) interactions and a Hubbard $U$ value of 2 to 3 eV, we predict a semiconducting cubic phase in all compounds studied. SOC does not strongly impact the electronic structures of the symmetry-broken $R3m$ phase. We also find that these ${V}_{xc}$ potentials do not quantitatively agree with the available experimental optical conductivity on ${\mathrm{GaV}}_{4}{\mathrm{S}}_{8}$; nonetheless, the LDA and GGA functionals correctly reproduce its lattice dynamical modes. Our findings suggest that accurate qualitative and quantitative simulations of the lacunar spinel family with DFT requires careful attention to the nuances of the exchange-correlation functional and considered spin structures.

The study of insulator-to-metal transitions is of interest from the viewpoint of fundamental understanding of the underlying physics, and materials at the brink of such transitions possess useful functionality. Driving this transition through compositional tuning can help engineer useful material properties. Here we study the role of disorder in the form of cation off-centering on the compositionally-controlled insulator-to-metal transition in the solid solution oxide pyrochlore (${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}{)}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$. Prior work has established site disorder by the ${\mathrm{Bi}}^{3+}$ cations shifting incoherently away from their ideal crystallographic site in the Bi end-member pyrochlore as a consequence of stereochemical activity of the lone pair of electrons. However, less is known about the consequences of such off-centering in solid solutions and its role in determining the electronic ground state. Here we demonstrate through total scattering studies that even a small substitution of Bi on the pyrochlore A site leads to site disorder that enhances the average effective size of the A-site cation. This indirectly increases Ru-O-Ru covalency, which appears to play a crucial role in the crossover from insulating to metallic behavior in the solid solution. Further, density functional electronic structure calculations suggest the combination of primary and secondary (due to size) electronic effects of the lone pair-driven incoherent cation displacements drive the solid solution into a metallic state.

Author(s): Yang, W; Chandrasena, RU; Gu, M; Dos Reis, RMS; Moon, EJ; Arab, A; Husanu, MA; Nemsak, S; Gullikson, EM; Ciston, J; Strocov, VN; Rondinelli, JM; May, SJ; Gray, AX | Abstract: Control of structural coupling at complex-oxide interfaces is a powerful platform for creating ultrathin layers with electronic and magnetic properties unattainable in the bulk. However, with the capability to design and control the electronic structure of such buried layers and interfaces at a unit-cell level, a new challenge emerges to be able to probe these engineered emergent phenomena with depth-dependent atomic resolution as well as element- and orbital selectivity. Here, we utilize a combination of core-level and valence-band soft x-ray standing-wave photoemission spectroscopy, in conjunction with scanning transmission electron microscopy, to probe the depth-dependent and single-unit-cell resolved electronic structure of an isovalent manganite superlattice [Eu0.7Sr0.3MnO3/La0.7Sr0.3MnO3]×15 wherein the electronic-structural properties are intentionally modulated with depth via engineered oxygen octahedra rotations/tilts and A-site displacements. Our unit-cell resolved measurements reveal significant transformations in the local chemical and electronic valence-band states, which are consistent with the layer-resolved first-principles theoretical calculations, thus opening the door for future depth-resolved studies of a wide variety of heteroengineered material systems.

We investigate the effect of the degree of metal-oxygen octahedral facesharing on the mechanical and electronic properties of ${d}^{0} {\mathrm{BaTiO}}_{3}$ and ${d}^{3} {\mathrm{BaMnO}}_{3}$. We find that increased facesharing softens the elastic constants of both materials due to the increased volume per atom, with polar distortions also contributing to the reduction in the bulk modulus. Owing to orbital filling in the $d$ manifold, we find the electronic band gap of ${\mathrm{BaTiO}}_{3}$ is relatively unaffected by changes in percent facesharing whereas the band gap of ${\mathrm{BaMnO}}_{3}$ increases by more than 200% as the percent facesharing increases from 0% (cubic perovskite) to 100% (hexagonal ${\mathrm{BaNiO}}_{3}$ perovskite). We identify that the trigonal distortions present in the face-connected polymorphs represent useful atomistic structural knobs to tune band structure in hexagonal perovskites. Our results indicate that facesharing hexagonal polymorphs provide an expanded oxides arena with additional structural flexibility beyond the usual fully corner-connected perovskites for property control.

We report the high-pressure, high-temperature synthesis of $\mathrm{BiV}{\mathrm{O}}_{3}$, a cubic perovskite that thus far has remained inaccessible under ambient pressure conditions. We created this material at $\ensuremath{\sim}25$ GPa and 1500 K in a laser-heated diamond-anvil cell and recovered it to ambient pressure and temperature. Our synthetic approach circumvents the oxidative chemistry that, at ambient pressures, previously rendered the cubic $\mathrm{BiV}{\mathrm{O}}_{3}$ perovskite inaccessible. We find through density-functional theory calculations that this material is a unique metallic and antiferromagnetic transition-metal oxide.

The authors report on comprehensive magnetic phase diagrams of the novel magnetic polar metal Ca${}_{3}$(Ru${}_{0.95}$Fe${}_{0.05}$)${}_{2}$O${}_{7}$. The small amount of Fe acts like a ``magnetic softener'', resulting in incommensurate spin order, in contrast to the collinear magnetic order seen in pure Ca${}_{3}$Ru${}_{2}$O${}_{7}$. The authors show that the complex magnetic order is extremely sensitive to magnetic field and that Ca${}_{3}$(Ru${}_{0.95}$Fe${}_{0.05}$)${}_{2}$O${}_{7}$ can exhibit a net ferromagnetic moment.

We demonstrate inversion symmetry breaking induced by oxygen octahedral rotations in layered perovskite oxides $\mathrm{K}{A}_{R}{\mathrm{TiO}}_{4}$ (${A}_{R}$ = rare earth) using a combined experimental and theoretical approach including synchrotron x-ray diffraction, optical second harmonic generation, and first-principles lattice dynamics calculations. We experimentally find an interesting but counterintuitive phenomenon, i.e., the acentric-to-centric phase transition temperatures for K family are higher than those for previously reported Na family, in contrast to expectations based on the Goldschmidt tolerance factor, where the octahedral rotation instability toward the acentric phases would reduce with an increase in the radius of $A$-site alkali metal ions. Our detailed analysis of first-principles calculations for ${A}_{A}{A}_{R}{\mathrm{TiO}}_{4}$ (${A}_{A}=\mathrm{Na}$, K, Rb) reveals that the alkali metal and rare-earth ions play quite different roles in driving the octahedral rotations. Since rare-earth ions attract oxide ions more strongly than alkali metal ions due to the higher valence of the former in comparison with the latter (${A}_{R}^{3+}$ vs ${A}_{A}^{+}$), the optimization of coordination environment of rare-earth ions is the primary driving force of the octahedral rotations. Alkali metal ions serve to impose ``bond strains'' parallel to the layers, playing a secondary role in the octahedral rotations. Incorporation of large alkali metal ions generates a significant in-plane biaxial bond strain in ${A}_{R}\mathrm{O}$ and ${\mathrm{TiO}}_{2}$ layers through the expanded ${A}_{A}\mathrm{O}$ layers, and thereby facilitates the octahedral rotations because of the otherwise highly underbonding of rare-earth ions. Thus, the effect of $A$-site alkali metal size on the octahedral rotation instability can be explained in terms of the interlayer lattice mismatch. This understanding allows us to propose a geometric descriptor governing the structural instability in ${A}_{A}{A}_{R}{\mathrm{TiO}}_{4}$ layered perovskites. We believe that control over the interlayer lattice mismatch could be a useful strategy to tune the octahedral rotations in layered compounds.

The correlated polar semimetal ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ exhibits a rich phase diagram including two magnetic transitions (${T}_{N}=56$ K and ${T}_{C}=48$ K) with the appearance of an insulating-like pseudogap (at ${T}_{C}$). In addition, there is a crossover back to metallic behavior at ${T}^{*}=30$ K, the origin of which is still under debate. We utilized ultrafast optical-pump optical-probe spectroscopy to investigate quasiparticle dynamics as a function of temperature in this enigmatic quantum material. We identify two dynamical processes, both of which are influenced by the onset of the pseudogap. This includes electron-phonon relaxation and, below ${T}_{C}$, the onset of a phonon bottleneck hindering the relaxation of quasiparticles across the pseudogap. We introduce a gap-modified two-temperature model to describe the temperature dependence of electron-phonon thermalization, and use the Rothwarf-Taylor to model the phonon bottleneck. In conjunction with density functional theory, our experimental results synergistically reveal the origin of the $T$-dependent pseudogap. Further, our data and analysis indicate that ${T}^{*}$ emerges as a natural consequence of $T$-dependent gapping out of carriers, and does not correspond to a separate electronic transition. Our results highlight the value of low-fluence ultrafast optics as a sensitive probe of low-energy electronic structure, thermodynamic parameters, and transport properties of Ruddlesden-Popper ruthenates.

Polar metals counterintuitively bring two well-known phenomena into coexistence, namely, bulk polar displacements, and an electronic Fermi surface giving rise to metallic conduction. However, little is known about the polar domains or domain walls in such materials. Using atomic resolution electron microscopy imaging combined with first principles density functional theory, we show that uncharged head-to-tail walls, and ``charged'' head-to-head and tail-to-tail walls can exist in the bulk of such crystals of polar metals ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$, where both structural changes at the wall as well as electrostatic considerations define the wall nature. Significant built-in potentials of 30--170 meV are predicted at such walls.

We report on measurements of the anisotropic magnetoresistance (AMR) of doped $\mathrm{EuTi}{\mathrm{O}}_{3}$. It is shown that the primary contribution to the AMR is the crystalline component, which depends on the relative orientation between the magnetic moments and the crystal axes. With increasing magnetic field, a fourfold crystalline AMR undergoes a change in its alignment with respect to the crystal axes. The results are discussed in the context of the coupling between spin canting, electronic structure, and transport. We discuss the potential role of Weyl points in the band structure. At high fields, the AMR transitions to uniaxial symmetry, which is lower than that of the lattice, along with a crossover from positive to negative magnetoresistance.

We design a compound belonging to a class of materials designated as polar half-metallic ferromagnets, where a 100% spin polarization coexists with polar distortions that globally lift inversion symmetry. Using electronic structure calculations, we predict that the ultrashort period ${({\mathrm{LaNiO}}_{3})}_{1}/{({\mathrm{YCrO}}_{3})}_{1}$ superlattice belongs to this materials class, exhibiting an integer magnetic moment of $4\phantom{\rule{3.33333pt}{0ex}}{\ensuremath{\mu}}_{B}$. The minority channel electric polarization, as computed using Berry phase theory, is as high as $\ensuremath{\sim}13.0\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, and we discuss experimental strategies to access the polarization. We propose that polar ferromagnetic half-metals exhibit multiferroism and can be exploited to realize nonreciprocal effects and directional anisotropy owing to the absence of both space-inversion and time-reversal symmetries.

We report the synthesis of $\mathrm{SrMn}{\mathrm{O}}_{3\text{\ensuremath{-}}\ensuremath{\delta}}{\mathrm{F}}_{\ensuremath{\gamma}}$ perovskite oxyfluoride thin films using a vapor transport method to fluorinate as-grown $\mathrm{SrMn}{\mathrm{O}}_{2.5}$ epitaxial thin films. The influence of the fluoropolymer, which acts as a fluorine vapor source, was investigated by utilizing polyvinyl fluoride (PVF), polyvinylidene difluoride (PVDF), and polytetrafluoroethylene (PTFE) in the reaction. The same process was carried out with polyethylene to isolate the role of carbon in the vapor transport process. The F distribution was probed by x-ray photoemission spectroscopy, which confirmed the incorporation of F into the films and revealed higher F concentrations in films exposed to PVF and PVDF compared to PTFE. The $c$-axis parameter expands after fluorination, a result consistent with density functional theory calculations that attribute the volume expansion to elongated Mn-F bonds compared to shorter Mn-O bonds. Using x-ray absorption spectroscopy, we show that the fluorination process reduces the nominal Mn oxidation state suggesting that F substitutes on O sites in the lattice as opposed to filling anion vacancy sites, a finding further supported by calculated formation energies of different F site occupancies. These results provide insights into topochemical fluorination of perovskite oxides, which should enable future synthesis and design efforts focused on oxyfluoride heterostructures.

In this work, we use density functional theory calculations to demonstrate how spontaneous electric polarizations can be induced via a hybrid improper ferroelectric mechanism in iodide perovskites, a family well known to display solar-optimal band gaps, to create materials for photoferroic applications. We first assemble three chemically distinct $(A{A}^{\ensuremath{'}})(B{B}^{\ensuremath{'}}){\mathrm{I}}_{6}$ double perovskites using centrosymmetric $AB{\mathrm{I}}_{3}$ perovskite iodides (where $A=\mathrm{Cs}$, Rb, K and $B=\mathrm{Sn}$, Ge) as building units. In each superlattice, we investigate the effects of three types of $A$- and $B$-site cation ordering schemes and three different $B{\mathrm{I}}_{6}$ octahedral rotation patterns. Out of these 27 combinations, we find that 15 produce polar space groups and display spontaneous electric polarizations ranging from 0.26 to $23.33\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{cm}}^{2}$. Furthermore, we find that a layered $A$-site/rock salt $B$-site ordering, in the presence of an ${a}^{0}{a}^{0}{c}^{+}$ rotation pattern, produces a chiral vortex-like $A$-site displacement pattern. We then investigate the effect of epitaxial strain on one of these compounds, (CsRb)(SnGe)${\mathrm{I}}_{6}$, in layered and rock salt ordered configurations. In both phases, we find strong competition between the cation ordering schemes as well as an enhancement of the spontaneous polarization magnitude under tensile strain. Finally, using a hybrid density functional with fractional Fock exchange, we find the iodide superlattices display semiconducting band gaps ranging from 0.2 to 1.3 eV. These results demonstrate that cation ordering and epitaxial strain are powerful ways to induce and control functionalities in technologically useful families of materials.

Electric-field tunable indirect-to-direct band gap transitions occur in thin-film silicon and transition metal dichalcogenides; however, they remain challenging to access in three-dimensional transition metal oxides. Very recently, an unusual polar-to-nonpolar phase transition under epitaxial strain was discovered in ${\mathrm{A}}_{3}{\mathrm{B}}_{2}{\mathrm{O}}_{7}$ hybrid improper ferroelectrics (HIFs), which supports controllable dielectric anisotropy and magnetization. Here we examine HIF ${(\mathrm{AB}{\mathrm{O}}_{3})}_{1}/{({\mathrm{A}}^{\ensuremath{'}}\mathrm{B}{\mathrm{O}}_{3})}_{1}$ superlattices and ${\mathrm{AA}}^{\ensuremath{'}}{\mathrm{BB}}^{\ensuremath{'}}{\mathrm{O}}_{6}$ double perovskites and predict a competing nonpolar antiferroelectric phase, demonstrating it is hidden in hybrid improper ferroelectrics exhibiting corner-connected $\mathrm{B}{\mathrm{O}}_{6}$ octahedra. Furthermore, we show the transition between the polar and nonpolar phases enables an in-plane electric field to control the indirect-to-direct band gap transition at the phase boundary in the ${(\mathrm{AB}{\mathrm{O}}_{3})}_{1}/{({\mathrm{A}}^{\ensuremath{'}}\mathrm{B}{\mathrm{O}}_{3})}_{1}$ superlattices and ${\mathrm{AA}}^{\ensuremath{'}}{\mathrm{BB}}^{\ensuremath{'}}{\mathrm{O}}_{6}$ double perovskites, which may be tuned through static strain or chemical substitution. Our findings establish HIFs as a functional electronics class from which to realize direct gap materials and enables the integration of a broader palette of chemistries and compounds for linear and nonlinear optical applications.

We investigate the crystal structure and the electronic properties of the trilayer nickelate ${\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ by means of quantum-mechanical calculations in the framework of the density-functional theory. We find that, at low temperature, ${\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ undergoes a hitherto unreported structural phase transition and transforms to a new monoclinic $P{2}_{1}/a$ phase. This phase exhibits electronic properties in agreement with recent angle-resolved photoemission spectroscopy data reported in H. Li et al., [Nat. Commun. 8, 704 (2017)] and should be considered in models focused on explaining the observed $\ensuremath{\sim}140$ K metal-to-metal phase transition.

We use density functional theory calculations to show nanofilms of covalent ${\mathrm{MoSi}}_{2}$ exhibit tunable carrier densities and high metallicity at the Fermi level. We determine the nanofilm stabilities by assessing their formation energies, surface energies, and phonon spectra. The dependence of metallicity on the nanofilm geometry is explained using surface-termination-induced electronic structure changes and confinement effects. Understanding this metallicity not only is important to further improve the oxidation resistance of related alloys, but also will facilitate the identification of various thin films of ${\mathrm{MoSi}}_{2}$ alloys and should lead to their use in nanoelectronic, electrochemical catalysis and low-dimensional transport experiments.

We use density functional calculations to show that heterovalent cation-order sequences enable control over band-gap variations up to several eV and band-gap closure in the bulk band insulator ${\mathrm{LaSrAlO}}_{4}$. The band-gap control originates from the internal electric fields induced by the digital chemical order, which induces picoscale band bending; the electric-field magnitude is mainly governed by the inequivalent charged monoxide layers afforded by the layered crystal habit. Charge transfer and ionic relaxations across these layers play secondary roles. This understanding is used to construct and validate a descriptor that captures the layer-charge variation and to predict changes in the electronic gap in layered oxides exhibiting antisite defects and in other chemistries.

The linear and nonlinear phononic interactions between an optically excited infrared (IR) or hyper-Raman mode and a driven Raman mode are computed for the ${d}^{0} ({\mathrm{CaTiO}}_{3})$ and ${d}^{1} ({\mathrm{LaTiO}}_{3})$ titanates within a first-principles density functional framework. We calculate the potential energy surface expanded in terms of the ${A}_{g}$ or ${B}_{1g}$ mode amplitudes coupled to the ${A}_{u}$ or the ${B}_{3u}$ mode and determine the coupling coefficients for these multimode interactions. We find that the linear-quadratic coupling dominates the anharmonicities over the quadratic-quadratic interaction in the perovskite titanates. The IR and Raman modes both modify the electronic structure with the former being more significant but occurring on a different time scale; furthermore, the coupled-mode interactions lead to sizable perturbations to the valence bandwidth ($\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\text{meV}$) and band gap ($\ensuremath{\sim}50$ meV). By comparing the coupling coefficients of undoped ${\mathrm{CaTiO}}_{3}$ and ${\mathrm{LaTiO}}_{3}$ to those for electron-doped $({\mathrm{CaTiO}}_{3})$ and hole-doped $({\mathrm{LaTiO}}_{3})$ titanates, we isolate the role of orbital filling in the nonlinear coupling process. We find that with increasing occupancy of the $d$ manifold, the linear-quadratic interaction decreases by approximately 30% with minor changes induced by the cation chemistry (that mainly affect the phonon mode frequencies) or by electron correlation. We identify the importance of the Ti-O bond stiffness, which depends on the orbital filling, in governing the lattice anharmonicitiy. This microscopic understanding can be used to increase the nonlinear coupling coefficient to facilitate more facile access of nonequilibrium structures and properties through ionic Raman scattering processes.

Motivated by the resurgence of electronic and optical property design in ordered fluoride and oxyfluoride compounds, we present a density functional theory (DFT) study on 19 materials with structures, ranging from simple to complex, and variable oxygen-to-fluorine ratios. We focus on understanding the accuracy of the exchange-correlation potentials ($V_{xc}$) to DFT for the prediction of structural, electronic, and lattice dynamical properties at four different levels of theory, \emph{i.e.}, the local density approximation (LDA), generalized gradient approximation (GGA), metaGGA, and hybrid functional level which includes fractions of exact exchange. We investigate in detail the metaGGA functionals MS2 [Sun \emph{et al}., Phys.\ Rev.\ Lett., \textbf{111}, 106401 (2013)] and SCAN [Sun \emph{et al}., Phys.\ Rev.\ Lett., \textbf{115}, 036402 (2015)], and show that although the metaGGAs show improvements over the LDA and GGA functionals in describing the electronic structure and phonon frequencies, the static structural properties of fluorides and oxyfluorides are often more accurately predicted by the GGA-level functional PBEsol. Results from LDA calculations are unsatisfactory for any compound regardless of oxygen concentration. PBEsol or HSE06 gives good performance in all oxide or all fluoride compounds. For the oxyfluorides, PBEsol is consistently more accurate for structural properties across all oxygen concentrations, however, we stress the need for detailed property assessment with various functionals for oxyfluorides with variable composition. The "best" functional is anticipated to be more strongly dependent on the property of interest. Our study provides useful insights in selecting an $V_{xc}$ that achieves the best performance comprise, enabling more accurate predictive design of functional fluoride-based materials with density functional theory., Comment: 17 pages, 11 figures

We identify a quasi-two-dimensional (quasi-2D) phonon mode in the layered-perovskite Ca_{3}Ti_{2}O_{7}, which exhibits an acoustic branch with quadratic dispersion. Using first-principles methods, we show this mode exhibits atomic displacements perpendicular to the layered [CaTiO_{3}]_{2} blocks comprising the structure and a negative Grüneisen parameter. Owing to these quasi-2D structural and dynamical features, we find that the mode can be utilized to realize unusual membrane effects, including a tunable negative thermal expansion (NTE) and a rare pressure-independent thermal softening of the bulk modulus. Detailed microscopic analysis shows that the NTE relies on strong intralayer Ti-O covalent bonding and weaker interlayer interactions, which is in contrast to conventional NTE mechanisms for perovskites, such as rigid-unit modes, structural transitions, and electronic or magnetic ordering. The general application of the quasi-2D lattice dynamics opens exciting avenues for the control of lattice dynamical and thermodynamic responses of other complex layered compounds through rational chemical substitution, as we show in A_{3}Zr_{2}O_{7} (A=Ca, Sr), and by heterostructuring.

Jian, Liu et al., By using a combination of heteroepitaxial growth, structure refinement based on synchrotron x-ray diffraction, and first-principles calculations, we show that the symmetry-protected Dirac line nodes in the topological semimetallic perovskite SrIrO3 can be lifted simply by applying epitaxial constraints. In particular, the Dirac gap opens without breaking the Pbnm mirror symmetry. In virtue of a symmetry-breaking analysis, we demonstrate that the original symmetry protection is related to the n-glide operation, which can be selectively broken by different heteroepitaxial structures. This symmetry protection renders the nodal line a nonsymmorphic Dirac semimetallic state. The results highlight the vital role of crystal symmetry in spin-orbit-coupled correlated oxides and provide a foundation for experimental realization of topological insulators in iridate-based heterostructures., We thank C. Fang, P. J. Ryan, and J.-W. Kim for insightful discussions. We thank J. Rubio for experiment assistance. J.L. is sponsored by the Science Alliance Joint Directed Research and Development Program at the University of Tennessee.We are thankful for support from the director of the Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No.DE-AC02-05CH11231 through the Quantum Materials FWP.We acknowledge additional support of the material synthesis facility through the DOD ARO MURI, E3S, and DARPA. D.K. acknowledges the support from the Austrian Science Fund (FWF): J3523-N27. We acknowledge support from the Grant Agency of the Czech Republic Grant no. 14-37427. Financial support from the Spanish MINECO (MAT2012-33207) is acknowledged. We acknowledge ESRF for the provision of beam time. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. J.M.R. and D.P. acknowledge support from the Army Research Office under Grant No. W911NF-15-1-0017 and the High Performance Computing Modernization Program (HPCMP) of the DOD for providing computational resources that have contributed to the research results reported herein.

Developing an accurate simulation method for the electrochemical stability of solids, as well as understanding the physics related with its accuracy, is critically important for improving the performance of compounds and predicting the stability of new materials in aqueous environments. Herein we propose a workflow for the accurate calculation of first-principles electrochemical phase (Pourbaix) diagrams. With this scheme, we study the electrochemical stabilities of Ti and Ti oxides using density-functional theory. First, we find the accuracy of an exchange-correlation functional in predicting formation energies and electrochemical stabilities is closely related with the electronic exchange interaction therein. Second, the metaGGA and hybrid functionals with a more precise description of the electronic exchange interaction lead to a systematic improvement in the accuracy of the Pourbaix diagrams. Furthermore, we show that accurate Ti Pourbaix diagrams also require that thermal effects are included through vibrational contributions to the free energy. We then use these diagrams to explain various experimental electrochemical phenomena for the Ti--O system, and show that if experimental formation energies for Ti oxides, which contain contributions from defects owing to their generation at high (combustion) temperatures, are directly used to predict room temperature Pourbaix diagrams then significant inaccuracies result. In contrast, the formation energies from accurate first-principles calculations, e.g., using metaGGA and hybrid functionals, are found to be more reliable. Finally, to facilitate the future application of our accurate electrochemical phase equilibria diagrams, the variation of the Ti Pourbaix diagrams with aqueous ion concentration is also provided.

The equilibrium structure and functional properties exhibited by brownmillerite oxides, a family of perovskite-derived structures with alternating layers of $B$O$_6$ octahedra and $B$O$_4$ tetrahedra, viz., ordered arrangements of oxygen vacancies, is dependent on a variety of competing crystal-chemistry factors. We use electronic structure calculations to disentangle the complex interactions in two ferrates, Sr$_2$Fe$_2$O$_5$ and Ca$_2$Fe$_2$O$_5$, relating the stability of the equilibrium (strain-free) and thin film structures to both previously identified and newly herein proposed descriptors. We show that cation size and intralayer separation of the tetrahedral chains provide key contributions to the preferred ground state. We show the bulk ground state structure is retained in the ferrates over a range of strain values; however, a change in the orientation of the tetrahedral chains, i.e., a perpendicular orientation of the vacancies relative to the substrate, is stabilized in the compressive region. The structure stability under strain is largely governed by maximizing the intraplane separation of the `dipoles' generated from rotations of the FeO$_4$ tetrahedra. Lastly, we find that the electronic band gap is strongly influenced by strain, manifesting as an unanticipated asymmetric-vacancy alignment dependent response. This atomistic understanding establishes a practical route for the design of novel functional electronic materials in thin film geometries., 10 pages, 9 figures

Ferroelectricity in strain-free and strained double perovskite fluorides, ${\mathrm{Na}}_{3}{\mathrm{ScF}}_{6}$ and ${\mathrm{K}}_{2}{\mathrm{NaScF}}_{6}$, is investigated using first-principles density functional theory. Although the experimental room temperature crystal structures of these fluoroscandates are centrosymmetric, i.e., ${\mathrm{Na}}_{3}{\mathrm{ScF}}_{6}$ ($P{2}_{1}/n$) and ${\mathrm{K}}_{2}{\mathrm{NaScF}}_{6}$ ($Fm\overline{3}m$), lattice dynamical calculations reveal that soft polar instabilities exist in each prototypical cubic phase and that the modes harden as the tolerance factor approaches unity. Thus the double fluoroperovskites bear some similarities to $AB{\mathrm{O}}_{3}$ perovskite oxides; however, in contrast, these fluorides exhibit large acentric displacements of alkali metal cations (Na, K) rather than polar displacements of the transition metal cations. Biaxial strain investigations of the centrosymmetric and polar ${\text{Na}}_{3}{\text{ScF}}_{6}$ and ${\text{K}}_{2}{\text{NaScF}}_{6}$ phases reveal that the paraelectric structures are favored under compressive strain, whereas polar structures with in-plane electric polarizations ($\ensuremath{\sim}5--18\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{C}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$) are realized at sufficiently large tensile strains. The electric polarization and stability of the polar structures for both chemistries are found to be further enhanced and stabilized by a coexisting single octahedral tilt system. Our results suggest that polar double perovskite fluorides may be realized by suppression of octahedral rotations about more than one Cartesian axis; structures exhibiting in- or out-of-phase octahedral rotations about the $c$ axis are more susceptible to polar symmetries.

Density functional theory and group-theoretical methods are used to explore the origin for ferroelectricity in cation ordered ${\mathrm{LaSrMnO}}_{4}$ with the Ruddlesden-Popper structure. The equilibrium phase exhibits the polar $Pca{2}_{1}$ space group where small polar displacements of ${d}^{4}\phantom{\rule{0.28em}{0ex}}{\mathrm{Mn}}^{3+}$ coexist with antiferrodistortive octahedral rotations and Jahn-Teller distortions. We find that the octahedral rotations and Jahn-Teller distortion stabilize the polar structure and induce polar displacements through high-order anharmonic interactions among the three modes, making ${\mathrm{LaSrMnO}}_{4}$ a hybrid-improper ferroelectric material. The rotations result from the ionic size mismatch between $A$ cations and Mn whereas the Jahn-Teller distortions are energetically favored owing to the coupling between the local ${e}_{g}$ orbital polarization of the two nearest-neighboring Mn cations in the two-dimensional ${\mathrm{MnO}}_{2}$ sheets. Our results indicate that anharmonic interactions among multiple centric modes can be activated by cation ordering to induce polar displacements in layered oxides, making it a reliable approach for realizing acentric properties in artificially constructed materials.

The effect of octahedral tilting on the acentric structural transitions in AGaO$_{3}$/A'GaO$_{3}$ [001], [110], and [111] superlattices (A, A' = La, Pr, Nd) is studied using density functional theory. We find the displacive transitions are driven by two octahedral rotations modes (a$^{-}$a$^{-}$c$^{0}$ and a$^{0}$a$^{0}$c$^{+}$ tilting), with amplitudes that depend on the A and A' chemistry and cation ordering direction. We find the ground states structures of the [001] and [111] ordered superlattices are polar. The coupling of octahedral tilting modes through a hybrid improper ferroelectric mechanism induces the polar displacements and produces the macroscopic electric polarizations.

Using first-principles density functional theory calculations, we investigate the hydrostatic pressure-induced reorientation of the Mn--F Jahn-Teller bond axis in the fluoride cryolite Na$_3$MnF$_6$. We find a first-order isosymmetric transition occurs between crystallographically equivalent monoclinic structures at approximately 2.15 GPa, consistent with earlier experimental studies. Analogous calculations for isostructural $3d^0$ Na$_3$ScF$_6$ show no evidence of a transition up to 6.82 GPa. Mode crystallography analyses of the pressure-dependent structures in the vicinity of the transition reveals a clear evolution of the Jahn-Teller bond distortions in cooperation with an asymmetrical stretching of the equatorial fluorine atoms in the MnF$_6$ octahedral units. We identify a change in orbital occupancy of the $e_g$ manifold in the $3d^4$ Jahn-Teller active Mn(III) to be responsible for the transition, which stabilizes one monoclinic $P2_1/n$ variant over the other., 10 pages, 9 figures

High-temperature electronic materials are in constant demand as the required operational range for various industries increases. Here we design ($A$,${A}^{\ensuremath{'}}$)${B}_{2}{\mathrm{O}}_{6}$ perovskite oxides with [111] ``rock salt'' $A$-site cation order and predict them to be potential high-temperature piezoelectric materials. By selecting bulk perovskites which have a tendency towards only out-of-phase $B{\mathrm{O}}_{6}$ rotations, we avoid possible staggered ferroelectric to paraelectric phase transitions while also retaining noncentrosymmetric crystal structures necessary for ferro- and piezoelectricity. Using density functional theory calculations, we show that (La,Pr)${\mathrm{Al}}_{2}{\mathrm{O}}_{6}$ and (Ce,Pr)${\mathrm{Al}}_{2}{\mathrm{O}}_{6}$ display spontaneous polarizations in their polar ground state structures; we also compute the dielectric and piezoelectric constants for each phase. Additionally, we predict the critical phase transition temperatures for each material from first-principles to demonstrate that the piezoelectric responses, which are comparable to traditional lead-free piezoelectrics, should persist to high temperature. These features make the rock salt $A$-site-ordered aluminates candidates for high-temperature sensors, actuators, or other electronic devices.

A novel class of materials with broken inversion symmetry is predicted using group theoretical analysis and confirmed experimentally by synchrotron x-ray diffraction.

We investigate the structure--property relationships in $AB$O$_3$ perovskites exhibiting octahedral rotations and cooperative octahedral breathing distortions (CBD) using group theoretical methods. Rotations of octahedra are ubiquitous in the perovskite family, while the appearance of breathing distortions -- oxygen displacement patterns that lead to approximately uniform dilation and contraction of the $B$O$_6$ octahedra -- are rarer in compositions with a single, chemically unique $B$-site. The presence of a CBD relies on electronic instabilities of the $B$-site cations, either orbital degeneracies or valence-state fluctuations, and often appear concomitant with charge order metal--insulator transitions or $B$-site cation ordering. We enumerate the structural variants obtained from rotational and breathing lattice modes and formulate a general Landau functional describing their interaction. We use this information and combine it with statistical correlation techniques to evaluate the role of atomic scale distortions on the critical temperatures in representative charge ordering nickelate and bismuthate perovskites. Our results provide new microscopic insights into the underlying structure--property interactions across electronic and magnetic phase boundaries, suggesting plausible routes to tailor the behavior of functional oxides by design., Comment: 14 pages, 8 figures

First-principles density functional calculations on the metal-insulator transition (MIT) in perovskite CaFeO${}_{3}$ point to local ferromagnetic coupling as the microscopic origin for the electronic ``charge order'' transition. Our atomic, electronic, and magnetic structure analyses reveal that the MIT results from a spin-assisted covalent bonding mechanism between the O 2$p$ and Fe 3$d$ states with anisotropic Fe-O bonds and negligible intersite Fe-Fe charge transfer. We suggest that control of the lattice distortions, which mediate the covalent bond formation, in oxides containing late transition-metal row cations in high valence states provides a platform to tailor electronic transitions.