Search

E.J.C., O.M., and C.P. acknowledge financial support from the Leverhulme Trust Research Project Grant No. RPG-2017-109. O.M. is grateful for funding via the Leverhulme Trust Early Career Fellowship ECF-2021-170. A.S.G. acknowledges funding through an EPSRC Early Career Fellowship EP/ T011130/1. A.S.G. and H.T. acknowledge funding through the Humboldt Foundation and the Max Planck Institute for Solid State Research. The authors thank the Science and Technology Facilities Council for beamtime allocated at ISIS through proposal RB1990046 (DOI: 10. 5286/ISIS.E.RB1990046) and the Swiss Muon Source at the Paul Scherrer Institute through proposal numbers 20150959 and 20211440. The authors are grateful for access to the MPMS3 instrument at The Royce Discovery Centre at the University of Sheffield (EPSRC grant no. EP/R00661X/1) and the PPMS instrument at the University of St. Andrews (EPSRC grant no. EP/T031441/1). Ba2CuTeO6 has attracted significant attention as it contains a two-leg spin ladder of Cu2+ cations that lies in close proximity to a quantum critical point. Recently, Ba2CuTeO6 has been shown to accommodate chemical substitutions, which can significantly tune its magnetic behavior. Here, we investigate the effects of substitution for non-magnetic Zn2+ impurities at the Cu2+ site, partitioning the spin ladders. Results from bulk thermodynamic and local muon magnetic characterization on the Ba2Cu1 – xZnxTeO6 solid solution (0 ≤ x ≤ 0.6) indicate that Zn2+ partitions the Cu2+ spin ladders into clusters and can be considered using the percolation theory. As the average cluster size decreases with increasing Zn2+ substitution, there is an evolving transition from long-range order to spin-freezing as the critical cluster size is reached between x = 0.1 to x = 0.2, beyond which the behavior became paramagnetic. This demonstrates well-controlled tuning of the magnetic disorder, which is highly topical across a range of low-dimensional Cu2+-based materials. However, in many of these cases, the chemical disorder is also relatively strong in contrast to Ba2CuTeO6 and its derivatives. Therefore, Ba2Cu1 – xZnxTeO6 provides an ideal model system for isolating the effect of defects and segmentation in low-dimensional quantum magnets. Publisher PDF

Funding: This research was carried out in part due to funding from the Max Planck-UBC-UTokyo Centre for Quantum Materials. Part of the research described in this paper was performed at the Canadian Light Source, a national research facility of the University of Saskatchewan, which is supported by the Canada Foundation for Innovation, the Natural Sciences and Engineering Research Council, the National Research Council, the Canadian Institutes of Health Research, the Government of Saskatchewan and the University of Saskatchewan. We thank the Science and Technology Facilities Council ISIS facility for the provision of beamtime. (Ba,K)BiO3 constitute an interesting class of superconductors, where the remarkably high superconducting transition temperature Tc of 30 K arises in proximity to charge density wave order. However, the precise mechanism behind these phases remains unclear. Here, enabled by high-pressure synthesis, we report superconductivity in (Ba,K)SbO3 with a positive oxygen–metal charge transfer energy in contrast to (Ba,K)BiO3. The parent compound BaSbO3−δ shows a larger charge density wave gap compared to BaBiO3. As the charge density wave order is suppressed via potassium substitution up to 65%, superconductivity emerges, rising up to Tc = 15 K. This value is lower than the maximum Tc of (Ba,K)BiO3, but higher by more than a factor of two at comparable potassium concentrations. The discovery of an enhanced charge density wave gap and superconductivity in (Ba,K)SbO3 indicates that strong oxygen–metal covalency may be more essential than the sign of the charge transfer energy in the main-group perovskite superconductors. Publisher PDF

Funding: Experiments at the Diamond Light Source were performed as part of the Block Allocation Group award “Oxford/Warwick Solid State Chemistry BAG to probe composition-structure– property relationships in solids” (CY25166). Experiments at the ISIS pulsed neutron facility were supported by a beam time allocation from the STFC (RB 2000148). SM thanks Somerville College for an Oxford Ryniker Lloyd scholarship. ‘PSH and WZ thank the National Science Foundation (DMR-2002319) and Welch Foundation (Grant E-1457) for support. KBiNb2O7 was prepared from RbBiNb2O7 by a sequence of cation exchange reactions which first convert RbBiNb2O7 to LiBiNb2O7, before KBiNb2O7 is formed by a further K-for-Li cation exchange. A combination of neutron, synchrotron X-ray and electron diffraction data reveal that KBiNb2O7 adopts a polar, layered, perovskite structure (space group A11m) in which the BiNb2O7 layers are stacked in a (0, ½, z) arrangement, with the K+ cations located in half of the available 10-coordinate interlayer cation sites. The inversion symmetry of the phase is broken by a large displacement of the Bi3+ cations parallel to the y-axis. HAADF-STEM images reveal that KBiNb2O7 exhibits frequent stacking faults which convert the (0, ½, z) layer stacking to (½, 0, z) stacking and vice versa, essentially switching the x- and y-axes of the material. By fitting the complex diffraction peak shape of the SXRD data collected from KBiNb2O7 it is estimated that each layer has approximately a 9% chance of being defective-a high level which is attributed to the lack of cooperative NbO6 tilting in the material, which limits the lattice strain associated with each fault. Publisher PDF

We present a comprehensive structural and magnetic characterization of the barlowite family of S = 1/2 kagomé magnets, Cu4(OH)6FX, where X = Cl, Br, or I. Through high-resolution synchrotron X-ray and neutron powder diffraction measurements, we reveal two sources of structural complexity within this family of materials, namely, compositional disorder of the halide species that occupy sites in between the kagomé layers and the positional disorder of the interlayer Cu2+ ions that persists well into the Pnma structural ground state. We demonstrate that understanding these inherent structural disorders is key as they correlate with the degree of partial order in the magnetic ground states of these quantum frustrated magnets.

Funding: A. W. R. and A. S. G. were supported by the Engineering and Physical Sciences Research Council (grant numbers EP/P024564/1 and EP/T011130/1 respectively). This work has been supported in part by the Alexander von Humboldt Foundation. The band topology of magnetic semimetals is of interest both from the fundamental science point of view and with respect to potential spintronics and memory applications. Unfortunately, only a handful of suitable topological semimetals with magnetic order have been discovered so far. One such family that hosts these characteristics is the antiperovskites, A3BO, a family of 3D Dirac semimetals. The A=Eu2+ compounds magnetically order with multiple phases as a function of applied magnetic field. Here, by combining band structure calculations with neutron diffraction and magnetic measurements, we establish the antiperovskite Eu3PbO as a new topological magnetic semimetal. This topological material exhibits a multitude of different topological phases with ordered Eu moments which can be easily controlled by an external magnetic field. The topological phase diagram of Eu3PbO includes an antiferromagnetic Dirac phase, as well as ferro- and ferrimagnetic phases with both Weyl points and nodal lines. For each of these phases, we determine the bulk band dispersions, the surface states, and the topological invariants by means of ab initio and tight-binding calculations. Our discovery of these topological phases introduces Eu3PbO as a new platform to study and manipulate the interplay of band topology, magnetism, and transport. Publisher PDF

Inorganic chemistry 61(9), 4033 - 4045 (2022). doi:10.1021/acs.inorgchem.1c03655, Isovalent nonmagnetic d$^{10}$ and d$^0$ B″ cations have proven to be a powerful tool for tuning the magnetic interactions between magnetic B′ cations in A$_2$B′B″O$_6$ double perovskites. Tuning is facilitated by the changes in orbital hybridization that favor different superexchange pathways. This can produce alternative magnetic structures when B″ is d$^{10}$ or d$^0$. Furthermore, the competition generated by introducing mixtures of d$^{10}$ and d$^0$ cations can drive the material into the realms of exotic quantum magnetism. Here, Te$^{6+}$ d$^{10}$ was substituted by Wu$^{6+}$ d$^0$ in the hexagonal perovskite Ba$_2$CuTeO$_6$, which possesses a spin ladder geometry of Cu$^{2+}$ cations, creating a Ba$_2$CuTe$_{1–x}$W$_x$O$_6$ solid solution (x = 0–0.3). We find W$^{6+}$ is almost exclusively substituted for Te$^{6+}$ on the corner-sharing site within the spin ladder, in preference to the face-sharing site between ladders. The site-selective doping directly tunes the intraladder, J$_{rung}$ and J$_{leg}$, interactions. Modeling the magnetic susceptibility data shows the d$^0$ orbitals modify the relative intraladder interaction strength (J$_{rung}$/J$_{leg}$) so the system changes from a spin ladder to isolated spin chains as W$^{6+}$ increases. This further demonstrates the utility of d$^{10}$ and d$^0$ dopants as a tool for tuning magnetic interactions in a wide range of perovskites and perovskite-derived structures., Published by American Chemical Society, Washington, DC

HRPD-X is a proposal to completely replace the current high-resolution powder diffractometer (HRPD) at the ISIS Neutron and Muon Source. The new instrument is expected to deliver a factor of four increase in solid-angle coverage. Taking advantage of new detector technology and coupled with a non-magnetic sample tank and improved incident- and diffracted-beam collimation, the new instrument will substantially improve HRPD’s scientific capabilities to study magnetic structures and behaviour, high-pressure phenomena and supramolecular structures whilst strengthening its performance in already-established areas.

Na-for-Rb cation exchange followed by K-for-Na cation exchange of RbNdM2O7 (M = Nb, Ta) yields the corresponding, metastable KNdM2O7 phases. Synchrotron X-ray and neutron powder diffraction data, combined with powder SHG data, reveal that the KNdM2O7 phases adopt a polar structure (space group Im2m) consisting of NdM2O7 perovskite double sheets stacked in a (0, 1/2, z) manner with K+ cations ordered within the 6-coordinate prismatic interlayer sites. The perovskite double sheets adopt an (a0b+c0/a0-b+c0) tilting distortion; however, unlike other A′AB2O7 phases, this distortion is not the origin of the noncentrosymmetric structure, which is attributed to a second-order Jahn–Teller distortion of the MO6 units. First-principles density functional theory (DFT) calculations confirm that the polar Im2m phase is more stable than the corresponding centrosymmetric alternative. The role of the A′- and A- cations in directing the stacking patterns and tilting distortions of A′AB2O7 phases is discussed with reference to hybrid improper ferroelectric behavior.

The impact of Li doping on the temperature-induced phase transitions in silver niobates Ag1-xLixNbO3 has been investigated using a combination of high-resolution powder neutron diffraction and synchrotron X-ray diffraction. Considering both the cell metric and distortions of the NbO6 octahedra, estimated by Rietveld refinements, it is shown that the sequence of temperature-induced phases in AgNbO3 is P21am → Pcam → Cmcm → P4/mbm → Pm3m. This sequence is simpler than that proposed in earlier studies. Evidence is presented for a second-order Jahn-Teller distortion in the Pcam phase. At x > 0.05, Li doping favors the formation of a rhombohedral phase in space group R3c, and such samples display the temperature-induced sequence R3c → Pbnm → Cmcm → P4/mbm → Pm3m. Unusual volume changes associated with the phase transitions point to the potential importance of lattice matching in optimizing the properties of thin films of doped AgNbO3.

Recently, there has been much interest in hybrid improper ferroelectrics materials that adopt polar, ferroelectric structures due to a complex tilting and twisting of the MO6 octahedra which constitute perovskite and related structures. Using a combination of synchrotron X-ray powder diffraction (XRD) and high-resolution neutron powder diffraction data, the temperature-dependent phase transitions of a series of n = 2 Dion–Jacobson oxides have been investigated. RbNdM2O7 undergoes a transition from a polar, a–a–c+/–(a–a–c+) distorted I2cm phase to an antipolar, a–b0c–/–(a–b0)c– distorted Cmca phase at T = 790 and 500 K for M = Nb and Ta, respectively. There is a subsequent transition to an a0a0c–/a0a0–c– distorted I4/mcm structure at 865 and 950 K for M = Nb and Ta, respectively, before a transition to the undistorted P4/mmm aristotype structure. In contrast, CsNdM2O7 undergoes a transition from a polar, a–a–c+ distorted P21am structure to an antipolar, a–b0c– distorted C2/m phase at T = 625 and 330 K for M = Nb and Ta, respectively, with a subsequent phase transition to the undistorted P4/mmm aristotype structure at 800 and 820 K for M = Nb and Ta, respectively. A plot of Tc against the relative stability of the 4 polar Dion–Jacobson phases compared to the corresponding aristotype P4/mmm structures (calculated from first-principles density functional theory (DFT)) yields a strong linear relation, suggesting that Tc is not proportional to the enthalpy change at the ferroelectric phase transition.

Isovalent nonmagnetic d

KBiNb

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.

Solid state compounds which exhibit non-centrosymmetric crystal structures are of great interest due to the physical properties they can exhibit. The ‘hybrid improper’ mechanism – in which two non-polar distortion modes couple to, and stabilize, a further polar distortion mode, yielding an acentric crystal structure – offers opportunities to prepare a range of novel non-centrosymmetric solids, but examples of compounds exhibiting acentric crystal structures stabilized by this mechanism are still relatively rare. Here we describe a series of bismuth-containing layered perovskite oxide phases, RbBiNb2O7, LiBiNb2O7and NaBiNb2O7, which have structural frameworks compatible with hybrid-improper ferroelectricity, but also contain Bi3+cations which are often observed to stabilize acentric crystal structures due to their 6s2electronic configurations. Neutron powder diffraction analysis reveals that RbBiNb2O7and LiBiNb2O7adopt polar crystal structures (space groupsI2cmandB2cmrespectively), compatible with stabilization by a trilinear coupling of non-polar and polar modes. The Bi3+cations present are observed to enhance the magnitude of the polar distortions of these phases, but are not the primary driver for the acentric structure, as evidenced by the observation that replacing the Bi3+cations with Nd3+cations does not change the structural symmetry of the compounds. In contrast the non-centrosymmetric, but non-polar structure of NaBiNb2O7(space groupP212121) differs significantly from the centrosymmetric structure of NaNdNb2O7, which is attributed to a second-order Jahn-Teller distortion associated with the presence of the Bi3+cations.

Preparing materials which simultaneously exhibit spontaneous magnetic and electrical polarisations is challenging as the electronic features which are typically used to stabilise each of these two polarisations in materials are contradictory. Here we show that by performing low-temperature cation-exchange reactions on a hybrid improper ferroelectric material, Li2SrTa2O7, which adopts a polar structure due to a cooperative tilting of its constituent TaO6 octahedra rather than an electronically driven atom displacement, a paramagnetic polar phase, MnSrTa2O7, can be prepared. On cooling below 43 K the Mn2+ centres in MnSrTa2O7 adopt a canted antiferromagnetic state, with a small spontaneous magnetic moment. On further cooling to 38 K there is a further transition in which the size of the ferromagnetic moment increases coincident with a decrease in magnitude of the polar distortion, consistent with a coupling between the two polarisations., Fabricating materials with simultaneously spontaneous magnetic and electrical polarisations is challenging due to contradictory electronic features. Here, the authors report a synthesis path toward a perovskite MnSrTa2O7 by performing low-temperature cation-exchange reactions on Li2SrTa2O7.

Solid state compounds which exhibit non-centrosymmetric crystal structures are of great interest due to the physical properties they can exhibit. The 'hybrid improper' mechanism - in which two non-polar distortion modes couple to, and stabilize, a further polar distortion mode, yielding an acentric crystal structure - offers opportunities to prepare a range of novel non-centrosymmetric solids, but examples of compounds exhibiting acentric crystal structures stabilized by this mechanism are still relatively rare. Here we describe a series of bismuth-containing layered perovskite oxide phases, RbBiNb

The magnetically frustrated manganese nitride antiperovskite family displays significant changes of entropy under hydrostatic pressure that can be useful for the emerging field of barocaloric cooling. Here we show that barocaloric properties of metallic antiperovskite Mn nitrides can be tailored for room-temperature application through quaternary alloying. We find an enhanced entropy change of $|\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{t}}|=37\phantom{\rule{4pt}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}$ at the ${T}_{t}=300\phantom{\rule{4pt}{0ex}}\mathrm{K}$ antiferromagnetic transition of quaternary ${\mathrm{Mn}}_{3}{\mathrm{Zn}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{N}$ relative to the ternary end members. The pressure-driven barocaloric entropy change of ${\mathrm{Mn}}_{3}{\mathrm{Zn}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{N}$ reaches $|\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{BCE}}|=20\phantom{\rule{4pt}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}$ in 2.9 kbar. Our results open up a large phase space where compounds with improved barocaloric properties may be found.

ASG acknowledges funding through an EPSRC Early Career Fellowship EP/T011130/1. Spin-orbit coupling is a quantum effect that can give rise to exotic electronic and magnetic states in the compounds of the 4d and 5d transition metals. Exploratory synthesis, chemical tuning and structure-property characterisation of such compounds is an increasingly active area of research with both fundamental and application-related outlooks, but requires great care with regards to the chemistry of these materials that has not always been considered. This Perspective will give an accessible introduction to topical materials with strong spin-orbit coupling, their crystal chemistry, and their structure-property relationships, which overlaps with the contemporary investigation of some of the same materials within different communities. It will also outline some of the challenges faced in their synthesis and characterisation, and the contributions that materials chemists can make to overcoming these. Publisher PDF

Two new lithium-magnesium phosphates LiMg6(PO4)3(P2O7) and Li(Mg5.62Sc0.19Li0.19)(PO4)3(P2O7) were synthesized by a solid-phase method. Using high-resolution time-of-flight neutron powder diffraction (TOF NPD) and X-ray powder diffraction (XRPD), we established that these phosphates have a Pnma orthorhombic structure with the cell parameters a = 9.14664(5) A, b = 18.83773(8) A, c = 8.27450(4) A, and V = 1425.71(1) A3 and a = 9.14516(5) A, b = 18.84222(9) A, c = 8.28204(4) A, and V = 1427.12(1) A3, respectively. The crystal structures can be described by stacking of the [Mg6O18]∞ or [Mg5.62Sc0.19Li0.19O18]∞ wavy layers, which are parallel to the (100) direction and interconnected through PO4 tetrahedra and P2O7 groups to form a 3D-framework. The Li atoms are located in large tunnels formed in a 3D lattice, which contributes to lithium diffusion. AC impedance spectroscopy analysis shows that LiMg6(PO4)3(P2O7) and Li(Mg5.62Sc0.19Li0.19)(PO4)3(P2O7) have a Li ion conductivity of 3.6 × 10-4 S cm-1 and 1.7 × 10-4 S cm-1 at 950 °C, with an activation energy of 1.28 eV and 1.55 eV, respectively. NMR MAS studies confirmed the coexistence of pyro- and orthogroups in the structure of both phases and two lithium positions in Li(Mg5.62Sc0.19Li0.19)(PO4)3(P2O7). The first-principles method was used to study the electronic structure and stability of the two phases. The calculated formation enthalpies demonstrated that Sc is a stabilizing impurity in LiMg6(PO4)3(P2O7), while a strong destabilization of olivine LiMgPO4 is observed upon doping with Sc. This explains the failure to synthesize Sc-doped olivine. The new phosphate LiMg6(PO4)3(P2O7) is a dielectric with a band gap of 5.35 eV, which decreases to 4.85 eV due to the appearance of a localized Sc 3d peak upon doping with Sc. These findings are consistent with the results obtained by UV-Vis spectroscopy. The new phase may be a good optical matrix similar to LiMgPO4.

The high-temperature phase behaviour of the ferroelectric layered perovskite Bi4Ti3O12 has been re-examined by high-resolution powder neutron diffraction. Previous studies, both experimental and theoretical, had suggested conflicting structural models and phase transition sequences, exacerbated by the complex interplay of several competing structural instabilities. This study confirms that Bi4Ti3O12 undergoes two separate structural transitions from the aristotype tetragonal phase (space group I4/mmm) to the ambient-temperature ferroelectric phase (confirmed as monoclinic, B1a1). An unusual, and previously unconsidered, intermediate paraelectric phase is suggested to exist above T C with tetragonal symmetry, space group P4/mbm. This phase is peculiar in displaying a unique type of octahedral tilting, in which the triple perovskite blocks of the layered structure alternate between tilted and untilted. This is rationalized in terms of the bonding requirements of the Bi3+ cations within the perovskite blocks.

We have mapped the phase diagram of BaTiO3 more extensively than previous attempts using high-pressure neutron-powder diffraction. The mapping of the phase diagram has been performed using isothermal compression at fixed temperatures (175, 225, 290, 480 K) within each of the known crystallographic phases, up to ∼6 GPa using a large volume press. The crystallographic structure of each phase has been measured, and the determined absolute atomic displacements of all atoms within the cell have provided detailed information on the order of the phase transitions observed, and the behaviour of the ferroelectric dipole moment. Publisher PDF

Super Invar (SIV), i.e., zero thermal expansion of metallic materials underpinned by magnetic ordering, is of great practical merit for a wide range of high precision engineering. However, the relatively narrow temperature window of SIV in most materials restricts its potential applications in many critical fields. Here, we demonstrate the controlled design of thermal expansion in a family of ${R}_{2}{(\mathrm{Fe},\mathrm{Co})}_{17}$ materials ($R=\text{rare}$ Earth). We find that adjusting the Fe-Co content tunes the thermal expansion behavior and its optimization leads to a record-wide SIV with good cyclic stability from 3--461 K, almost twice the range of currently known SIV. In situ neutron diffraction, M\"ossbauer spectra and first-principles calculations reveal the $3d$ bonding state transition of the Fe-sublattice favors extra lattice stress upon magnetic ordering. On the other hand, Co content induces a dramatic enhancement of the internal molecular field, which can be manipulated to achieve ``ultrawide'' SIV over broad temperature, composition and magnetic field windows. These findings pave the way for exploiting thermal-expansion-control engineering and related functional materials.

(Ba,K)BiO

C.M.Y., S.N.P., A.W.R., and P.W. acknowledge support from EPSRC through EP/S005005/1, and C.To. and A.W.R. through EP/P024564/1. C.M.Y. acknowledges additional support from a Shanghai talent program and funding through the Shanghai Pujiang Program (20PJ1408200). C.H. acknowledges support from the Austrian Science Fund (FWF), project no. P 32144-N36, and the VSC4 of the Vienna University of Technology. A key property of many quantum materials is that their ground state depends sensitively on small changes of an external tuning parameter, e.g., doping, magnetic field, or pressure, creating opportunities for potential technological applications. Here, we explore tuning of the ground state of the nonsuperconducting parent compound, Fe1+xTe, of the iron chalcogenides by uniaxial strain. Iron telluride exhibits a peculiar (π, 0) antiferromagnetic order unlike the (π, π) order observed in the Fe-pnictide superconductors. The (π, 0) order is accompanied by a significant monoclinic distortion. We explore tuning of the ground state by uniaxial strain combined with low-temperature scanning tunneling microscopy. We demonstrate that, indeed under strain, the surface of Fe1.1Te undergoes a transition to a (π, π)-charge-ordered state. Comparison with transport experiments on uniaxially strained samples shows that this is a surface phase, demonstrating the opportunities afforded by 2D correlated phases stabilized near surfaces and interfaces. Publisher PDF

A key property of many quantum materials is that their ground state depends sensitively on small changes of an external tuning parameter, e.g., doping, magnetic field, or pressure, creating opportunities for potential technological applications. Here, we explore tuning of the ground state of the nonsuperconducting parent compound, Fe1+xTe, of the iron chalcogenides by uniaxial strain. Iron telluride exhibits a peculiar (π, 0) antiferromagnetic order unlike the (π, π) order observed in the Fe-pnictide superconductors. The (π, 0) order is accompanied by a significant monoclinic distortion. We explore tuning of the ground state by uniaxial strain combined with low-temperature scanning tunneling microscopy. We demonstrate that, indeed under strain, the surface of Fe1.1Te undergoes a transition to a (π, π)-charge-ordered state. Comparison with transport experiments on uniaxially strained samples shows that this is a surface phase, demonstrating the opportunities afforded by 2D correlated phases stabilized near surfaces and interfaces.

A key question regarding the unconventional superconductivity of Sr$_2$RuO$_4$ remains whether the order parameter is single- or two-component. Under a hypothesis of two-component superconductivity, uniaxial pressure is expected to lift their degeneracy, resulting in a split transition. The most direct and fundamental probe of a split transition is heat capacity. Here, we report measurement of heat capacity of samples subject to large and highly homogeneous uniaxial pressure. We place an upper limit on the heat-capacity signature of any second transition of a few per cent of that of the primary superconducting transition. The normalized jump in heat capacity, $\Delta C/C$, grows smoothly as a function of uniaxial pressure, favouring order parameters which are allowed to maximize in the same part of the Brillouin zone as the well-studied van Hove singularity. Thanks to the high precision of our measurements, these findings place stringent constraints on theories of the superconductivity of Sr$_2$RuO$_4$., Comment: This manuscript is an extended version of the scientific part of arXiv:1906.07597, which is being split into two separate papers. An extended discussion on the experimental methods can be found at arXiv:2009.03125. It includes supplemental material

Funding: School of Chemistry, University of St Andrews for funding of a studentship to GWH through the EPSRC doctoral training grant (Grant No. EP/N509759/1). This work was also facilitated by funding provided by the EPSRC (Grant No. EP/P024637/1). K.K.M. and R.S.K. acknowledge financial support from the Department of Defense, USA (DoD Grant No. FA9550-20-1-0064). The RUS component of this work was funded by EPSRC Grant No. EP/P024904/1. The recent report of an intermediate incommensurately modulated orthorhombic phase in LaTaO4 has prompted a re-examination of the phase transition sequence in LaTaO4 as a function of temperature. With falling temperature, the sequence of phases examined is (orthorhombic) Cmc21(C)↔Cmc21(IC)↔(monoclinic)P21/c, with C and IC denoting commensurate and incommensurate phases, respectively. The orthorhombic to monoclinic transition, Tm-o, is a first order reconstructive transition occurring at 440 K and TIC-C is a first-order displacive transition occurring at 500-530 K. Strain and elasticity data confirm a first-order transition between the basic and modulated Cmc21 phases, with similarities to the isostructural fluoride BaMnF4. A Raman spectroscopic study of the LaTaO4 phase transition indicates that the IC-C phase transition is driven by a soft zone-boundary phonon (unstable) of the commensurate orthorhombic (Cmc21) phase. The soft phonon is found to appear (underdamped) above 443 K and vanishes (overdamped) around 528 K. A large supercell of the monoclinic phase below Tm-o is proposed based on the Raman spectroscopic results. Publisher PDF

The layered delafossite PdCoO2 has been predicted to be one of very few materials with a thermopower that is highly anisotropic and switches sign between different crystallographic directions. These properties are of interest for various applications, but have been difficult to verify because sufficiently large crystals have not been available. We report measurements of the high-temperature electrical resistivity, thermal conductivity, and thermopower of phase-pure PdCoO2 powder compacts prepared by a highly Pd-efficient synthesis route. While the electronic transport of the polycrystalline samples is dominated by that of the Pd planes, the thermopower exhibits a well-defined deviation from the in-plane character at temperatures above 600K, which is indicative of opposing trends in the Seebeck coefficients within and perpendicular to the delafossite layers. The experimental data are consistently described by a combination of effective-medium models based on the main axes transport quantities. The results support the predicted ambipolar thermopower anisotropy in PdCoO2.

We report the crystal structures and magnetic properties of two pseudopolymorphs of the $S=1/2 {\mathrm{Ti}}^{3+}$ coordination framework, $\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$. Single-crystal x-ray and powder neutron diffraction measurements on $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$ confirm its structure in the tetragonal $I4/mcm$ space group with a square planar arrangement of ${\mathrm{Ti}}^{3+}$ ions. Magnetometry and specific heat measurements reveal weak antiferromagnetic interactions, with ${J}_{1}\ensuremath{\approx}7$ K and ${J}_{2}/{J}_{1}=0.11$ indicating a slight frustration of nearest- and next-nearest-neighbor interactions. Below 1.8 K, $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$ undergoes a transition to G-type antiferromagnetic order with magnetic moments aligned along the $c$ axis of the tetragonal structure. The estimated ordered moment of ${\mathrm{Ti}}^{3+}$ in $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$ is suppressed from its spin-only value to $0.62(3)\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$, thus verifying the two-dimensional nature of the magnetic interactions within the system. $\ensuremath{\beta}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{H}}_{2}\mathrm{O}$, on the other hand, realizes a three-dimensional diamondlike magnetic network of ${\mathrm{Ti}}^{3+}$ moments within a hexagonal $P{6}_{2}22$ structure. An antiferromagnetic exchange coupling of $J\ensuremath{\approx}54$ K---an order of magnitude larger than in $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$---is extracted from magnetometry and specific heat data. $\ensuremath{\beta}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{H}}_{2}\mathrm{O}$ undergoes N\'eel ordering at ${T}_{N}=28$ K, with the magnetic moments aligned within the $ab$ plane and a slightly reduced ordered moment of $0.79\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ per ${\mathrm{Ti}}^{3+}$. Through density-functional theory calculations, we address the origin of the large difference in the exchange parameters between the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ pseudopolymorphs. Given their observed magnetic behaviors, we propose $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$ and $\ensuremath{\beta}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{H}}_{2}\mathrm{O}$ as close to ideal model $S=1/2$ Heisenberg square and diamond lattice antiferromagnets, respectively.

Funding: UK Engineering and Physical Sciences Research Council (Grant Number(s): EP/P022637/1, EP/K503162/1), Science and Technology Facilities Council (Grant Number(s): RB1820307), China Scholarship Council (CN). The layered perovskite LaTaO4 has been synthesized to be stable in both (polar) orthorhombic and (nonpolar) monoclinic polymorphs at ambient conditions. Although the structural transition between monoclinic and orthorhombic phases has been well established, there is some controversy regarding a further, unidentified transition around 500 K. Here this is identified as an incommensurate–commensurate first‐order transition between incommensurate Cmc21(α00)0s0 and commensurate Cmc21 orthorhombic phases. Transmission electron microscopy indicates partially ordered stacking of different structural units in a, identifying the local cause for the modulation, whereas variable temperature powder neutron diffraction has shown the overall macroscopic modulation vector, q ≈ (0.456, 0, 0)—roughly a 2.2 × expansion in a, corresponding to an approximate 11a commensurate superunit cell dimension. The modulation shows a continuous temperature dependence until transitioning to the basic (commensurate) cell at TIC‐C. Doping the interlayer La sites with smaller Nd cations stabilizes the incommensuration to higher temperature, suggesting the modulation is geometrically driven at the A site. Publisher PDF

The impact of Li doping on the temperature-induced phase transitions in silver niobates Ag

A hyperkagome iridate ${\mathrm{Li}}_{3}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$ is synthesized by an ion-exchange reaction from ${\mathrm{Na}}_{4}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$. The transport, magnetic, and thermodynamic measurements suggest a metallic state. The electronic structure calculation shows that ${\mathrm{Li}}_{3}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$ hosts a semimetallic electronic structure produced by a competition between the formation of a localized molecular orbital of Ir $d$ electrons on the ${\mathrm{Ir}}_{3}$ triangles and the strong spin-orbit coupling as in the sister compound ${\mathrm{Na}}_{3}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$. The semimetallic state induced by spin-orbit coupling is quite robust against the moderate change of lattice distortion associated with the different ionic radii of ${\mathrm{Li}}^{+}$ and ${\mathrm{Na}}^{+}$.

Hybrid improper ferroelectricity, which utilizes nonpolar but ubiquitous rotational/tilting distortions to create polarization, offers an attractive route to the discovery of new ferroelectric and multiferroic materials because its activity derives from geometric rather than electronic origins. Design approaches blending group theory and first principles can be utilized to explore the crystal symmetries of ferroelectric ground states, but in general, they do not make accurate predictions for some important parameters of ferroelectrics, such as Curie temperature (TC). Here, we establish a predictive and quantitative relationship between TC and the Goldschmidt tolerance factor, t, by employing n = 2 Ruddlesden–Popper (RP) A3B2O7 as a prototypical example of hybrid improper ferroelectrics. The focus is placed on an RP system, (Sr1–xCax)3Sn2O7 (x = 0, 0.1, and 0.2), which allows for the investigation of the purely geometric (ionic size) effect on ferroelectric transitions, due to the absence of the second-ord...

A new method for the measurement of bound coherent neutron scattering lengths is reported. It is shown that a relative measurement of the neutron scattering length, {\overline b}, of an element can be made by analysis of the neutron correlation function of a suitable oxide crystal powder. For this analysis, it is essential to take into account the average density contribution to the correlation function, as well as the contributions arising from distances between atoms in the crystal. The method is demonstrated and verified by analysis of the neutron correlation function for the corundum form of Al2O3, yielding a value {\overline b} = 3.44 (1) fm for Al, in good agreement with the literature. The method is then applied to the isotopes of iridium, for which the values of the scattering lengths were unknown, and which are difficult to investigate by other methods owing to the large cross sections for the absorption of neutrons. The neutron correlation function of a sample of Sr2IrO4 enriched in 193Ir is used to determine values {\overline b} = 9.71 (18) fm and {\overline b} = 12.1 (9) fm for 193Ir and 191Ir, respectively, and these are consistent with the tabulated scattering length and cross sections of natural Ir. These values are of potential application for obtaining improved neutron diffraction results on iridates by the use of samples enriched in 193Ir, so that the severe absorption problems associated with 191Ir are avoided. Rietveld refinement of the neutron diffraction pattern of isotopically enriched Sr2IrO4 is used to yield a similar result for Ir. However, in practice the Rietveld result is shown to be less reliable because of correlation between the parameters of the fit.

Since being discovered initially in mixed-cation systems, a method of forming end-member NiSO4·9H2O and NiSO4·8H2O has been found. We have obtained powder diffraction data from protonated analogues (with X-rays) and deuterated analogues (using neutrons) of these compounds over a range of temperatures, allowing us to determine their crystal structures—including all H-atoms—and to characterise the transitions on warming from 220 to 278 K; glass → 9-hydrate → 8-hydrate + ice → 7-hydrate + ice → partial melt (7-hydrate + liquid). NiSO4·8D2O is triclinic, space-group $$P\bar {1}$$ , Z = 2, with unit cell parameters at 150 K, a = 6.12463(8) A, b = 6.8401(1) A, c = 12.5339(2) A, α = 92.846(1)°, β = 97.822(1)°, γ = 96.627(1)° and V = 515.58(1) A3. The structure consists of two symmetry-inequivalent Ni(D2O)6 octahedra on sites of $$\bar {1}$$ symmetry. These are directly joined by a water–water H-bond to form chains of octahedra parallel with the c-axis at x = 0. Two interstitial water molecules serve both to bridge the Ni(D2O)6 octahedral chains in the b–c plane and also to connect with the SO42− tetrahedral oxyanion. These tetrahedra are linked by the two interstitial water molecules in a reticular motif to form sheets perpendicular to c. NiSO4·9D2O is monoclinic, space-group P21/c, Z = 4, with unit-cell parameters at 150 K, a = 6.69739(6) A, b = 11.8628(1) A, c = 14.5667(1) A, β = 94.9739(8)° and V = 1152.96(1) A3. The structure is isotypic with the Mg analogue described elsewhere (Fortes et al., Acta Cryst B 73:47‒64, 2017b). It shares the motif of H-bonded octahedral chains with NiSO4·8D2O, although in the enneahydrate these run parallel with the b-axis at x = 0. Three interstitial water molecules bridge the Ni(D2O)6 octahedra to the SO42− tetrahedral oxyanion. The tetrahedra sit at x ≈ 0.5 and are linked by two of the three interstitial water molecules in a pentagonal motif to form ribbons parallel with b. A solid-solution series exists between Mg and Ni enneahydrate end-members where we observe preferential partitioning of Ni2+ into the octahedral sites on the 2c Wyckoff positions rather than the 2a sites. The solution is slightly non-ideal, as indicated by the small positive excess volume of mixing. Measurements of the DC magnetisation of quenched NiSO4 solutions reveal anomalies in the molar susceptibility on warming through the region from 221 to 225 K, probably due to devitrification of the (assumed) glassy specimen into a mixture of NiSO4·9H2O + ice Ih. Further temperature-dependent measurements on repeated warming and cooling provide no evidence of magnetic ordering and indicate a weak ferromagnetic coupling between neighbouring Ni2+ ions, likely via super-exchange through the H-bond between neighbouring Ni(H2O)6 octahedra.

In this paper we report the synthesis of the new layered perovskite oxide carbonate, Ba3Yb2O5CO3. This phase is formed when 3BaCO3 : 1Yb2O3 mixtures are heated in air at temperatures ≤1000 °C, while above this temperature the carbonate is lost and the simple oxide phase Ba3Yb4O9 is observed. The structure of Ba3Yb2O5CO3 was determined from neutron diffraction studies and consists of a tripled perovskite with double Yb–O layers separated by carbonate layers, the first example of a material with such a structure. Further studies showed that analogous Ba3Ln2O5CO3 phases could be formed for other rare earths (Ln = Y, Dy, Ho, Er, Tm and Lu). The results highlight the ability of the perovskite structure to accommodate carbonate groups, and emphasise the need to consider their potential presence particularly for perovskite systems prepared in lower temperature synthesis routes.

The crystallisation of methyl stearate under a range of crystallisation conditions has been studied and three new polymorphs have been identified and structurally characterised. Form III (monoclinic, space group Cc, Z = 8) was obtained at room temperature by slow evaporation of a saturated solution in CS2. Form IV (monoclinic, space group C2/c, Z = 4) was obtained by slow cooling of the melt. Both structures were characterised by single crystal X-ray diffraction. Form V (monoclinic, space group Cc, Z = 4) was obtained from the melt by rapid cooling. X-ray and neutron powder diffraction methods were employed in the determination of this structure. Form V shows highly anisotropic thermal expansion, with expansion along the crystallographic b-axis being substantially greater than along the other two axes.

Funding: UK Engineering and Physical Sciences Research Council (Grant Number(s): EP/R023522, EP/R023751, EP/L017008, EP/P007821, EP/L000202, EP/R029431); Diamond Light Source (Grant Number(s): SP17198-8); Rutherford Appleton Laboratory (Grant Number(s): RB1920629). The magnitude of ionic conductivity is known to depend upon both mobility and number of available carriers. For proton conductors, hydration is a key factor in determining the charge–carrier concentration in ABO3 perovskite oxides. Despite the high reported proton mobility of calcium titanate (CaTiO3), this titanate perovskite has thus far been regarded as a poor proton conductor due to the low hydration capability. Here, the enhanced proton conductivity of the defective calcium titanate Ca0.92TiO2.84(OH)0.16 prepared by replacing lattice oxygens with hydroxyl groups via a solvothermal route is shown. Conductivity measurements in a humidified Ar atmosphere reveal that, remarkably, this material exhibits one order of magnitude higher bulk conductivity (10−4 Scm−1 at 200 °C) than hydrated stoichiometric CaTiO3 prepared by traditional solid-state synthesis due to the higher concentration of protonic defects and variation in the crystal structure. The replacement of Ca2+ by Ni2+ in the Ca1−xTi1O3−2x(OH)2x, which mostly exsolve metallic Ni nanoparticles along orthorhombic (100) planes upon reduction, is also demonstrated. These results suggest a new strategy by tailoring the defect chemistry via hydration or cation doping followed by exsolution for targeted energy applications. Publisher PDF

A flurry of recent theoretical studies have predicted the existence of new polar materials among several families of layered perovskites, including the double-layered Dion-Jacobson phases. These predictions have opened up exciting new opportunities for both fundamental studies of the crystal chemistry of Dion-Jacobson phases and their application as components in next-generation memories and low-power electronic devices. However, with some rare exceptions, all known double-layered Dion-Jacobson phases are nonpolar. We use an integrated theoretical-experimental approach to show that several Dion-Jacobson phases that have previously been synthesized and characterized as nonpolar are in fact polar. Additional theoretical calculations reveal that the polar phases of these materials emerge through a hybrid improper or trilinear coupling mechanism. Finally, our work has highlighted the critical role of neutron diffraction in characterizing the structures of double-layered Dion-Jacobson phases, which are typified by subtle oxygen atom displacements not easily resolved using even synchrotron X-ray diffraction.

We describe the capability to measure high-resolution neutron powder diffraction data to a pressure of at least 8 kbar. We have used the HRPD instrument at the ISIS neutron source and a piston-cyli...

We present an extensive experimental and theoretical study on the low-temperature magnetic properties of the monoclinic anhydrous alum compound BaMo(PO4)(2). The magnetic susceptibility reveals strong antiferromagnetic interactions theta(CW) = -167 K and long-range magnetic order at T-N = 22 K, in agreement with a recent report. Powder neutron diffraction furthermore shows that the order is collinear, with the moments near the ac plane. Neutron spectroscopy reveals a large excitation gap Delta = 15 meV in the low-temperature ordered phase, suggesting a much larger easy-axis spin anisotropy than anticipated. However, the large anisotropy justifies the relatively high ordered moment, Neel temperature, and collinear order observed experimentally and is furthermore reproduced in a first-principles calculations by using a new computational scheme. We therefore propose BaMo(PO4)(2) to host S = 1 antiferromagnetic chains with large easy-axis anisotropy, which has been theoretically predicted to realize novel excitation continua.

We present bulk and neutron scattering measurements performed on the isotopically enriched $^{154}\mathrm{Sm_2Ti_2O_7}$ and $^{154}\mathrm{Sm_2Sn_2O_7}$ samples. Both compounds display sharp heat capacity anomalies, at 350 mK and 440 mK, respectively. Inelastic neutron scattering measurements are employed to determine the crystalline electric field (CEF) level scheme, which includes transitions between the ground-state and first excited $J$ multiplets of the $\mathrm{Sm}^{3+}$ ion. To further validate those results, the single-ion magnetic susceptibility of the compounds is calculated and compared with the experimental DC-susceptibility measured in low applied magnetic fields. It is demonstrated that the inclusion of intermultiplet transitions in the CEF analysis is fundamental to the understanding of the intermediate and, more importantly, low temperature magnetic behaviour of the Sm-based pyrochlores. Finally, the heat capacity anomaly is shown to correspond to the onset of an all-in-all-out long-range order in the stannate sample, while in the titanate a dipolar long-range order can be only indirectly inferred., 13 pages, 10 Figures

We thank the Science and Technology Facilities Council (STFC) for the provision of neutron diffraction facilities at ISIS (HRPD experiment RB1710021, doi: 10.5286/ISIS.E.RB1710021) and the School of Chemistry, University of St Andrews for funding of a studentship to JAM through the EPSRC doctoral training grant (EP/K503162/1). The ambient temperature structures (orthorhombic, space group, Cmc21) of the polar hexagonal tungsten bronzes RbNbW2O9 and KNbW2O9 have been determined by high-resolution powder neutron diffraction. Displacement of the A-site cation in the polar c-axis, with concomitant octahedral tilting, occurs to optimise the A-cation bonding environment reducing the coordination from 18 to 16. This effect is more evident in KNbW2O9 due to decreased A-cation size. The octahedral tilting in both compositions results in a doubling of the c-axis that has not previously been reported, highlighting the importance of neutron diffraction as a complementary technique for structural determination of such systems. Publisher PDF

In the triangular layered magnet PdCrO2 the intralayer magnetic interactions are strong, however the lattice structure frustrates interlayer interactions. In spite of this, long-range, 120$^\circ$ antiferromagnetic order condenses at $T_N = 38$~K. We show here through neutron scattering measurements under in-plane uniaxial stress and in-plane magnetic field that this occurs through a spontaneous lifting of the three-fold rotational symmetry of the nonmagnetic lattice, which relieves the interlayer frustration. We also show through resistivity measurements that uniaxial stress can suppress thermal magnetic disorder within the antiferromagnetic phase., Comment: 9 pages, 9 figures

Funding: The experimental work has been supported by the European Research Council (ERC), the Scottish Funding Council, the UK EPSRC and the Swiss National Science Foundation (SNSF). Theoretical work was supported by the ERC grant ERC-319286-QMAC and by the SNSF (NCCR MARVEL).

The ambient-temperature structures (orthorhombic, space group Cmc2

Hyperhoneycomb iridate $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ is a three-dimensional analog of two-dimensional honeycomb iridates, such as $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$, which recently appeared as another playground for the physics of Kitaev-type spin liquid. $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ shows a noncollinear spiral ordering of spin-orbital-entangled ${J}_{\mathrm{eff}}=1/2$ moments at low temperatures below 38 K, which is known to be suppressed under a pressure of $\ensuremath{\sim}2$ GPa. In addition, a structural transition is observed at ${P}_{\mathrm{S}}\ensuremath{\sim}4$ GPa at room temperature. Using the neutron powder diffraction technique, the crystal structure in the high-pressure phase of $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ above ${P}_{\mathrm{S}}$ was refined, which indicates the formation of ${\mathrm{Ir}}_{2}$ dimers on the zigzag chains, with an Ir-Ir distance of $\ensuremath{\sim}2.66\phantom{\rule{0.16em}{0ex}}\AA{}$, even shorter than that of metallic Ir. We argue that the strong dimerization stabilizes the bonding molecular-orbital state comprising the two local ${d}_{zx}$ orbitals in the Ir-${\mathrm{O}}_{2}$-Ir bond plane, which conflicts with the equal superposition of ${d}_{xy}, {d}_{yz}$, and ${d}_{zx}$ orbitals in the ${J}_{\mathrm{eff}}=1/2$ wave function produced by strong spin-orbit coupling. The results of resonant inelastic x-ray scattering measurements and the electronic structure calculations are fully consistent with the collapse of the ${J}_{\mathrm{eff}}=1/2$ state. The competition between the spin-orbital-entangled ${J}_{\mathrm{eff}}=1/2$ state and molecular-orbital formation is most likely universal in honeycomb-based Kitaev materials.

Cu2ZnSnS4 (CZTS) is a technologically important and complex quaternary semiconductor and a highly promising material for the absorber layer in sustainable thin film solar cells. Its photovoltaic performance is currently limited by low open-circuit voltage, thought to be due to a range of point defects such as disorder between the copper and zinc lattice sites. This is the highest-resolution neutron diffraction study reported for CZTS, which unambiguously identifies the crystal symmetry and accurately quantifies precise values for the disorder on all cation symmetry sites as a function of temperature. Two samples of CZTS were fabricated by solid state reaction and their compositions were measured by inductively-coupled plasma mass spectroscopy, which identified significant tin loss during growth, leaving the samples Sn-poor, Cu-rich and Sn-poor, Zn-rich respectively. Both samples were found exclusively to adopt the tetragonal kesterite crystal structure with significant cation disorder, which is investigated in detail over the range 4–1275 K. Importantly, and in contrast to previous reports, the 2a Wyckoff site shows disorder equal to or greater than the 2c site. The order–disorder phase transition was observed at different temperatures for the two compositions, 489 and 501 K respectively, lower than previously reported. The kesterite–sphalerite transition was observed between 1250 and 1275 K for the Sn-poor, Cu-rich sample, significantly higher than previously reported. These results provide new insights into the high levels of disorder present in CZTS and confirm that composition and cation disorder have a significant effect on the phase transition mechanism. This work will enable the development of routes to the fabrication of higher-efficiency photovoltaic devices.