Noncollinear Electric Dipoles in a Polar Chiral Phase of CsSnBr3 Perovskite.

Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two phases of the 3D perovskite, CsSnBr3, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II (77 K < T < 85 K, space group P21/m) exhibits ferroaxial order driven by a noncollinear pattern of lone pair-driven distortions within the plane normal to the unique octahedral tilt axis, preserving the inversion symmetry observed at higher temperatures. Phase I (T < 77 K, space group P21) additionally exhibits ferroelectric order due to distortions along the unique tilt axis, breaking both inversion and mirror symmetries. This polar and chiral phase exhibits second harmonic generation from the bulk and pronounced electrostriction and negative thermal expansion along the polar axis (Q22 ≈ 1.1 m4 C-2; αb = -7.8 × 10-5 K-1) through the onset of polarization. The structures of phases I and II were predicted by recursively following harmonic phonon instabilities to generate a tree of candidate structures and subsequently corroborated by synchrotron X-ray powder diffraction and polarized Raman and 81Br nuclear quadrupole resonance spectroscopies. Preliminary attempts to suppress unintentional hole doping to allow for ferroelectric switching are described. Together, the polar symmetry, small band gap, large spin-orbit splitting of Sn 5p orbitals, and predicted strain sensitivity of the symmetry-breaking distortions suggest bulk samples and epitaxial films of CsSnBr3 or its neighboring solid solutions as candidates for bulk Rashba effects.

Superconductivity in (Ba,K)SbO3.

(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.

Influence of Disorder on the Bad Metal Behavior in Polar Amalgams.

The two new ternary amalgams K1-xRbxHg11 [x = 0.472(7)] and Cs3-xCaxHg20 [x = 0.20(3)] represent two different examples of how to create ternary compounds from binaries by statistical atom substitution. K1-xRbxHg11 is a Vegard-type mixed crystal of the isostructural binaries KHg11 and RbHg11 [cubic, BaHg11 structure type, space group Pm3̅m, a = 9.69143(3) Å, Rietveld refinement], whereas Cs3-xCaxHg20 is a substitution variant of the Rb3Hg20 structure type [cubic, space group Pm3̅n, a = 10.89553(14) Å, Rietveld refinement] for which a fully substituted isostructural binary Ca phase is unknown. In K1-xRbxHg11, the valence electron concentration (VEC) is not changed by the substitution, whereas in Cs3-xCaxHg20, the VEC increases with the Ca content. Amalgams of electropositive metals form polar metal bonds and show "bad metal" properties. By thermal analysis, magnetic susceptibility and resistivity measurements, and density functional theory calculations of the electronic structures, we investigate the effect of the structural disorder introduced by creating mixed-atom occupation on the physical properties of the two new polar amalgam systems.

High-Spin Orbital Interactions Across van der Waals Gaps Controlling the Interlayer Ferromagnetism in van der Waals Ferromagnets.

We examined what interactions control the sign and strength of the interlayer coupling in van der Waals ferromagnets such as Fe3-xGeTe2, Cr2Ge2Te6, CrI3, and VI3 to find that high-spin orbital interactions across the van der Waals gaps are a key to understanding their ferromagnetism. Interlayer ferromagnetic coupling in Fe3-xGeTe2, Cr2Ge2Te6, and CrI3 is governed by the high-spin two-orbital two-electron destabilization, but that in VI3 by the high-spin four-orbital two-electron stabilization. These interactions explain a number of seemingly puzzling observations in van der Waals ferromagnets.

Spin Exchanges Between Transition Metal Ions Governed by the Ligand p-Orbitals in Their Magnetic Orbitals.

In this review on spin exchanges, written to provide guidelines useful for finding the spin lattice relevant for any given magnetic solid, we discuss how the values of spin exchanges in transition metal magnetic compounds are quantitatively determined from electronic structure calculations, which electronic factors control whether a spin exchange is antiferromagnetic or ferromagnetic, and how these factors are related to the geometrical parameters of the spin exchange path. In an extended solid containing transition metal magnetic ions, each metal ion M is surrounded with main-group ligands L to form an MLn polyhedron (typically, n = 3-6), and the unpaired spins of M are represented by the singly-occupied d-states (i.e., the magnetic orbitals) of MLn. Each magnetic orbital has the metal d-orbital combined out-of-phase with the ligand p-orbitals; therefore, the spin exchanges between adjacent metal ions M lead not only to the M-L-M-type exchanges, but also to the M-L…L-M-type exchanges in which the two metal ions do not share a common ligand. The latter can be further modified by d0 cations A such as V5+ and W6+ to bridge the L…L contact generating M-L…A…L-M-type exchanges. We describe several qualitative rules for predicting whether the M-L…L-M and M-L…A…L-M-type exchanges are antiferromagnetic or ferromagnetic by analyzing how the ligand p-orbitals in their magnetic orbitals (the ligand p-orbital tails, for short) are arranged in the exchange paths. Finally, we illustrate how these rules work by analyzing the crystal structures and magnetic properties of four cuprates of current interest: -CuV2O6, LiCuVO4, (CuCl)LaNb2O7, and Cu3(CO3)2(OH)2.

Orbital Magnetic Moments of the High-Spin Co2+ Ions at Axially-Elongated Octahedral Sites: Unquenched as Reported from Experiment or Quenched as Predicted by Theory?

Neutron diffraction studies on magnetic solids composed of axially elongated CoO4X2 (X = Cl, Br, S, Se) octahedra show that the ordered magnetic moments of their high-spin Co2+ (d7, S = 3/2) ions are greater than 3 µB, i.e., the spin moment expected for S = 3/2 ions, and increase almost linearly from 3.22 to 4.45 µB as the bond-length ratio rCo-X/rCo-O increases from 1.347 to 1.659 where rCo-X and rCo-O are the Co-X and Co-O bond lengths, respectively. These observations imply that the orbital moments of the Co2+ ions increase linearly from 0.22 to 1.45 µB with increasing the rCo-X/rCo-O ratio from 1.347 to 1.659. We probed this implication by examining the condition for unquenched orbital moment and also by evaluating the magnetic moments of the Co2+ ions based on DFT+U+SOC calculations for those systems of the CoO4X2 octahedra. Our work shows that the orbital moments of the Co2+ ions are essentially quenched and, hence, that the observations of the neutron diffraction studies are not explained by the current theory of magnetic moments. This discrepancy between experiment and theory urges one to check the foundations of the current theory of magnetic moments as well as the current method of neutron diffraction refinements for ordered magnetic structures.

Superconductivity at 4.8 K and Violation of Pauli Limit in La2IRu2 Comprising Ru Honeycomb Layer.

We discovered superconductivity at 4.8 K in the hexagonal layered compound La2IRu2 comprising a triangular lattice of the La and a honeycomb lattice of the Ru atoms. First-principles calculations reveal a two-dimensional band structure made up of La 5d and Ru 4d electrons and formal oxidation states +1.5 for the La and the uncommon oxidation state -1 for the Ru atoms. The temperature dependence of the specific heat indicates fully gapped superconductivity. Nevertheless, the upper critical field of this compound violates the Pauli limit. We argue that the high upper critical field is ascribed to an antisymmetric spin-orbit coupling in the unique multilayer structure.

Field-induced States and Excitations in the Quasicritical Spin-1/2 Chain Linarite.

The mineral linarite, PbCuSO_{4}(OH)_{2}, is a spin-1/2 chain with frustrating nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic exchange interactions. Our inelastic neutron scattering experiments performed above the saturation field establish that the ratio between these exchanges is such that linarite is extremely close to the quantum critical point between spin-multipolar phases and the ferromagnetic state. We show that the predicted quantum multipolar phases are fragile and actually suppressed by a tiny orthorhombic exchange anisotropy and weak interchain interactions in favor of a dipolar fan phase. Including this anisotropy in classical simulations of a nearly critical model explains the field-dependent phase sequence of the phase diagram of linarite, its strong dependence of the magnetic field direction, and the measured variations of the wave vector as well as the staggered and the uniform magnetizations in an applied field.

Synthesis and Characterization of the Aurivillius Phase CoBi2O2F4.

The new CoBi2O2F4 compound was synthesized by a hydrothermal method at 230 °C. Single-crystal X-ray diffraction data were used to determine the crystal structure. The compound is layered and belongs to the Aurivillius family of compounds. The present compound is the first oxo-fluoride Aurivillius phase containing Co2+. Inclusion of a d-block cation with such a low oxidation state as 2+ was achieved by partially replacing O2- with F- ions. The crystal structure is best described in the tetragonal noncentrosymmetric space group I4̅ with unit-cell parameters a = 3.843(2) Å and c = 16.341(8) Å. The crystal structure consists of two main building units: [BiO4F4] distorted cubes and [CoF6] octahedra. Interestingly, since the octahedra [CoF6] tilt between four equivalent positions, the F atoms occupy a 4-fold split position at room temperature. For the investigation of the structural disorder, Raman scattering data were collected in the range from 10 K to room temperature. As the temperature decreases, sharper phonon peaks appear and several modes clearly appear, which indicates a reduction of the disorder. Magnetic susceptibility and heat capacity measurements evidence long-range antiferromagnetic ordering below the Néel temperature of ∼50 K. The magnetic susceptibility is in agreement with the Curie-Weiss law above 75 K with a Curie-Weiss temperature of θCW = -142(2) K.

Synthesis and Physical Properties of the Oxofluoride Cu2(SeO3)F2.

Single crystals of the new compound Cu2(SeO3)F2 were successfully synthesized via a hydrothermal method, and the crystal structure was determined from single-crystal X-ray diffraction data. The compound crystallizes in the orthorhombic space group Pnma with the unit cell parameters a = 7.066(4) Å, b = 9.590(4) Å, and c = 5.563(3) Å. Cu2(SeO3)F2 is isostructural with the previously described compounds Co2TeO3F2 and CoSeO3F2. The crystal structure comprises a framework of corner- and edge-sharing distorted [CuO3F3] octahedra, within which [SeO3] trigonal pyramids are present in voids and are connected to [CuO3F3] octahedra by corner sharing. The presence of a single local environment in both the 19F and 77Se solid-state MAS NMR spectra supports the hypothesis that O and F do not mix at the same crystallographic positions. Also the specific phonon modes observed with Raman scattering support the coordination around the cations. At high temperatures the magnetic susceptibility follows the Curie-Weiss law with Curie temperature of Θ = -173(2) K and an effective magnetic moment of µeff ∼ 2.2 µB. Antiferromagnetic ordering below ∼44 K is indicated by a peak in the magnetic susceptibility. A second though smaller peak at ∼16 K is tentatively ascribed to a magnetic reorientation transition. Both transitions are also confirmed by heat capacity measurements. Raman scattering experiments propose a structural phase instability in the temperature range 6-50 K based on phonon anomalies. Further changes in the Raman shift of modes at ∼46 K and ∼16 K arise from transitions of the magnetic lattice in accordance with the susceptibility and heat capacity measurements.

Hydrothermal Synthesis of the Oxofluoride FeSbO2F2-An Anti-ferromagnetic Spin S = 5/2 Compound.

The new oxofluoride compound FeSbO2F2 was synthesized by hydrothermal techniques at 230 °C. Its crystal structure was determined from single-crystal X-ray diffraction data. The compound crystallizes in the monoclinic space group C2/c with one crystallographic site for Fe3+ and Sb3+, respectively. The crystal structure is made of [FeO2F4] octahedra and seesaw [SbO4] building blocks. These are connected to form [FeO2F2]n layers and [SbO2]n chains that bond together via the oxygen atoms to form the three-dimensional framework structure. Magnetic susceptibility and heat capacity measurements indicate long-range anti-ferromagnetic ordering below a Néel temperature of ∼175 K. Two-dimensional anti-ferromagnetic short-range order in the square planar net of the Fe3+ cations extends to temperatures far above the Néel temperature.

Structural Stability Diagram of ALnP2S6 Compounds (A = Na, K, Rb, Cs; Ln = Lanthanide).

Thiophosphate compounds have been studied extensively in the past for their rich structural variations and for a large variety of interesting properties. Here, we report 11 new phases with the composition ALnP2S6 (A = Na, K, Rb, Cs; Ln = lanthanide). These new thiophosphates crystallize in four different structure types, with the space groups Fdd2, P1̅, P21, and P21/c, respectively. All phases are insulating and the calculated band gaps range between 3 eV and 3.5 eV. Magnetic measurements on the compounds with open f-shells show paramagnetic behavior and magnetic moments that match the expected free ion values of the respective lanthanide cations. We present a structural stability phase diagram for the ALnP2S6 family of compounds, which reveals a clear relationship between ionic radii and the preferred crystal structure, as well as stability regions to form ALnP2S6-type phases.

Structural and Magnetic Properties of the Trirutile-type 1D-Heisenberg Anti-Ferromagnet CuTa2O6.

We prepared trirutile-type polycrystalline samples of CuTa2O6 by low-temperature decomposition of a Cu-Ta-oxalate precursor. Diffraction studies at room temperature identified a slight monoclinic distortion of the hitherto surmised tetragonal trirutile crystal structure. Detailed high-temperature X-ray and neutron powder diffraction investigations as well as Raman scattering spectroscopy revealed a structural phase transition at 503(3) K from the monoclinic structure to the tetragonal trirutile structure. GGA+U density functional calculations of the spin-exchange parameters as well as magnetic susceptibility and isothermal magnetization measurements reveal that CuTa2O6 is a new 1D Heisenberg magnet with predominant anti-ferromagnetic nearest-neighbor intrachain spin-exchange interaction of ∼50 K. Interchain exchange is a factor of ∼5 smaller. Heat capacity and low-temperature high-intensity neutron powder diffraction studies could not detect long-range order down to 0.45 K.

Hydrothermal Synthesis and Magnetic Characterization of the Quaternary Oxide CoMo2Sb2O10.

The new quaternary layered oxide CoMo2Sb2O10 was synthesized by hydrothermal synthesis techniques, and its structure was determined from single-crystal X-ray diffraction data. CoMo2Sb2O10 crystallizes in the monoclinic space group C2/c with one Sb3+, Mo6+, and Co2+ atom site per unit cell, respectively. The crystal structure contains building units consisting of [Co2O8]n, [Mo2O8]n, and [SbO2]n chains. These are connected through corner sharing to form charge-neutral [CoMo2Sb2O10]n layers. Thermal decomposition of CoMo2Sb2O10 starts at 550 °C. The magnetic susceptibility follows a Curie-Weiss law above 50 K with a Curie constant of C = 3.46 emu·K·mol-1 corresponding to an effective moment of µeff = 5.26 µB per cobalt atom and a Curie-Weiss temperature θ = -13.2 K. Short-range anti-ferromagnetic ordering dominates below 5 K. Magnetic susceptibility and heat capacity data can be successfully modeled by the predictions from an Ising linear chain with an intrachain spin exchange of ca. -7.8 K.

Copper Selenidophosphates Cu4P2Se6, Cu4P3Se4, Cu4P4Se3, and CuP2Se, Featuring Zero-, One-, and Two-Dimensional Anions.

Five new compounds in the Cu/P/Se phase diagram have been synthesized, and their crystal structures have been determined. The crystal structures of these compounds comprise four previously unreported zero-, one-, and two-dimensional selenidophosphate anions containing low-valent phosphorus. In addition to two new modifications of Cu4P2Se6 featuring the well-known hexaselenidohypodiphosphate(IV) ion, there are three copper selenidophosphates with low-valent P: Cu4P3Se4 contains two different new anions, (i) a monomeric (zero-dimensional) selenidophosphate anion [P2Se4](4-) and (ii) a one-dimensional selenidophosphate anion [Formula: see text], which is related to the well-known gray-Se-like [Formula: see text] Zintl anion. Cu4P4Se3 contains one-dimensional [Formula: see text] polyanions, whereas CuP2Se contains the 2D selenidophosphate [Formula: see text] polyanion. It consists of charge-neutral CuP2Se layers separated by a van der Waals gap which is very rare for a Zintl-type phase. Hence, besides black P, CuP2Se constitutes a new possible source of 2D oxidized phosphorus containing layers for intercalation or exfoliation experiments. Additionally, the electronic structures and some fundamental physical properties of the new compounds are reported. All compounds are semiconducting with indirect band gaps of the orders of around 1 eV. The phases reported here add to the structural diversity of chalcogenido phosphates. The structural variety of this family of compounds may translate into a variety of tunable physical properties.

Structure and Composition of the 200†K-Superconducting Phase of H2 S at Ultrahigh Pressure: The Perovskite (SH(-) )(H3 S(+) ).

At ultrahigh pressure (>110 GPa), H2 S is converted into a metallic phase that becomes superconducting with a record Tc of approximately 200 K. It has been proposed that the superconducting phase is body-centered cubic H3 S (Im3‾ m, a=3.089 Å) resulting from the decomposition reaction 3 H2 S→2 H3 S+S. The analogy between H2 S and H2 O led us to a very different conclusion. The well-known dissociation of water into H3 O(+) and OH(-) increases by orders of magnitude under pressure. H2 S is anticipated to behave similarly under pressure, with the dissociation process 2 H2 S→H3 S(+) +SH(-) leading to the perovskite structure (SH(-) )(H3 S(+) ). This phase consists of corner-sharing SH6 octahedra with SH(-) ions at each A site (the centers of the S8 cubes). DFT calculations show that the perovskite (SH(-) )(H3 S(+) ) is thermodynamically more stable than the Im3‾ m structure of H3 S, and suggest that the A site hydrogen atoms are most likely fluxional even at Tc .

High-Pressure Phase Transformations in TiPO4 : A Route to Pentacoordinated Phosphorus.

Titanium(III) phosphate, TiPO4 , is a typical example of an oxyphosphorus compound containing covalent P-O bonds. Single-crystal X-ray diffraction studies of TiPO4 reveal complex and unexpected structural and chemical behavior as a function of pressure at room temperature. A series of phase transitions lead to the high-pressure phase V, which is stable above 46 GPa and features an unusual oxygen coordination of the phosphorus atoms. TiPO4 -V is the first inorganic phosphorus-containing compound that exhibits fivefold coordination with oxygen. Up to the highest studied pressure of 56 GPa, TiPO4 -V coexists with TiPO4 -IV, which is less dense and might be kinetically stabilized. Above a pressure of about 6 GPa, TiPO4 -II is found to be an incommensurately modulated phase whereas a lock-in transition at about 7 GPa leads to TiPO4 -III with a fourfold superstructure compared to the structure of TiPO4 -I at ambient conditions. TiPO4 -II and TiPO4 -III are similar to the corresponding low-temperature incommensurate and commensurate magnetic phases and reflect the strong pressure dependence of the spin-Peierls interactions.

Crystal structure and magnetic properties of FeSeO(3)F--alternating antiferromagnetic S = 5/2 chains.

The new oxofluoride FeSeO3F, which is isostructural with FeTeO3F and GaTeO3F, was prepared by hydrothermal synthesis, and its structure was determined by X-ray diffraction. The magnetic properties of FeSeO3F were characterized by magnetic susceptibility and specific heat measurements, by evaluating its spin exchanges on the basis of density functional theory (DFT) calculations, and by performing a quantum Monte Carlo simulation of the magnetic susceptibility. FeSeO3F crystallizes in the monoclinic space group P21/n and has one unique Se(4+) ion and one unique Fe(3+) ion. The building blocks of FeSeO3F are [SeO3] trigonal pyramids and cis-[FeO4F2] distorted octahedra. The cis-[FeO4F2] octahedra are condensed by sharing the O-O and F-F edges alternatingly to form [FeO3F]∞ chains, which are interconnected via the [SeO3] pyramids by corner-sharing. The magnetic susceptibility of FeSeO3F is characterized by a broad maximum at 75(2) K and a long-range antiferromagnetic order below ∼45 K. The latter is observed by magnetic susceptibility and specific heat measurements. DFT calculations show that the Fe-F-Fe spin exchange is stronger than the Fe-O-Fe exchange, so each [FeO3F]∞ chain is a Heisenberg antiferromagnetic chain with alternating antiferromagnetic spin exchanges. The temperature dependence of the magnetic susceptibility is well-reproduced by a quantum-Monte Carlo simulation.

Most spin-1/2 transition-metal ions do have single ion anisotropy.

The cause for the preferred spin orientation in magnetic systems containing spin-1/2 transition-metal ions was explored by studying the origin of the easy-plane anisotropy of the spin-1/2 Cu(2+) ions in CuCl2·2H2O, LiCuVO4, CuCl2, and CuBr2 on the basis of density functional theory and magnetic dipole-dipole energy calculations as well as a perturbation theory treatment of the spin-orbit coupling. We find that the spin orientation observed for these spin-1/2 ions is not caused by their anisotropic spin exchange interactions, nor by their magnetic dipole-dipole interactions, but by the spin-orbit coupling associated with their crystal-field split d-states. Our study also predicts in-plane anisotropy for the Cu(2+) ions of Bi2CuO4 and Li2CuO2. The results of our investigations dispel the mistaken belief that magnetic systems with spin-1/2 ions have no magnetic anisotropy induced by spin-orbit coupling.

Vibrational states of a water molecule in a nano-cavity of beryl crystal lattice.

Low-energy excitations of a single water molecule are studied when confined within a nano-size cavity formed by the ionic crystal lattice. Optical spectra are measured of manganese doped beryl single crystal Mn:Be3Al2Si6O18, that contains water molecules individually isolated in 0.51 nm diameter voids within the crystal lattice. Two types of orientation are distinguished: water-I molecules have their dipole moments aligned perpendicular to the c axis and dipole moments of water-II molecules are parallel to the c-axis. The optical conductivity σ(ν) and permittivity ɛ'(ν) spectra are recorded in terahertz and infrared ranges, at frequencies from several wavenumbers up to ν = 7000 cm(-1), at temperatures 5-300 K and for two polarizations, when the electric vector E of the radiation is parallel and perpendicular to the c-axis. Comparative experiments on as-grown and on dehydrated samples allow to identify the spectra of σ(ν) and ɛ'(ν) caused exclusively by water molecules. In the infrared range, well-known internal modes ν1, ν2, and ν3 of the H2O molecule are observed for both polarizations, indicating the presence of water-I and water-II molecules in the crystal. Spectra recorded below 1000 cm(-1) reveal a rich set of highly anisotropic features in the low-energy response of H2O molecule in a crystalline nano-cavity. While for E∥c only two absorption peaks are detected, at ~90 cm(-1) and ~160 cm(-1), several absorption bands are discovered for E⊥c, each consisting of narrower resonances. The bands are assigned to librational (400-500 cm(-1)) and translational (150-200 cm(-1)) vibrations of water-I molecule that is weakly coupled to the nano-cavity "walls." A model is presented that explains the "fine structure" of the bands by a splitting of the energy levels due to quantum tunneling between the minima in a six-well potential relief felt by a molecule within the cavity.