Emergent Three-Dimensional Electric Dipole Sinewave in Bulk Perovskite Oxides.

The magnetic and electric dipoles of ferroics play a central role in their fascinating properties. In particular, topological configurations have shown promising potential for use in novel electromechanical and electronic devices. Magnetic configurations from simple collinear to complex topological are well-documented. In contrast, many complex topological features in the electric counterpart remain unexplored. Here, we report the first example of three-dimensional electric dipole sinewave topological structure in a PbZrO3-based bulk perovskite, which presents an interesting triple-hysteresis loop macroscopically. This polar configuration consists of two orthogonal sinewave electric dipole modulations decoded from a polar incommensurate phase by advanced diffraction and atomic-resolution imaging techniques. The resulting topology is unraveled to be the competition between the antiferroelectric and ferroelectric states, stabilized by the modulation of the Pb 6s2 lone pair and the antiferrodistortive effect. These findings further reinforce the similarity of the magnetic and electric topologies.

Antiferroelectricity-Induced Negative Thermal Expansion in Double Perovskite Pb2 CoMoO6.

Materials with negative thermal expansion (NTE) attract significant research attention owing to their unique physical properties and promising applications. Although ferroelectric phase transitions leading to NTE are widely investigated, information on antiferroelectricity-induced NTE remains limited. In this study, single-crystal and polycrystalline Pb2 CoMoO6 samples are prepared at high pressure and temperature conditions. The compound crystallizes into an antiferroelectric Pnma orthorhombic double perovskite structure at room temperature owing to the opposite displacements dominated by Pb2+ ions. With increasing temperature to 400 K, a structural phase transition to cubic Fm-3m paraelectric phase occurs, accompanied by a sharp volume contraction of 0.41%. This is the first report of an antiferroelectric-to-paraelectric transition-induced NTE in Pb2 CoMoO6 . Moreover, the compound also exhibits remarkable NTE with an average volumetric coefficient of thermal expansion αV = -1.33 × 10-5 K-1 in a wide temperature range of 30-420 K. The as-prepared Pb2 CoMoO6 thus serves as a prototype material system for studying antiferroelectricity-induced NTE.

Synthesis and Structure of Vacancy-Ordered Perovskite Ba6Ta2Na2X2O17 (X = P, V): Significance of Structural Model Selection on Discovered Compounds.

Vacancy-ordered 12H-type hexagonal perovskites Ba6Ru2Na2X2O17 (X = P, V) with a (c'cchcc)2 stacking sequence of [BaO3]c, [BaO3]h, and [BaO2]c' layers, where c and h represent a cubic and hexagonal stacking sequence, were previously reported by Quarez et al. in 2003. They also synthesized Ba6Ta2Na2V2O17, but structural refinement was absent. Very recently, Szymanski et al. reported 43 new compounds, including 12H-type Ba6Ta2Na2V2O17, using large-scale ab initio phase-stability data from the Materials Project and Google DeepMind with the assistance of an autonomous laboratory. But their structural refinement was very poor. Here, we report the synthesis and structure of Ba6Ta2Na2V2O17, which does not have 12H-type structure but has a vacancy-ordered 6C-type perovskite with a (c'ccccc) stacking sequence of [BaO3]c and [BaO2]c' layers. We also report the phosphite analogue Ba6Ta2Na2P2O17 as a new compound. We claim an importance of careful structural characterization on newly discovered compounds; otherwise, the database constructed will lose credibility.

Morphological and functional changes of the geniohyoid muscle in elderly patients after hip fracture surgery: Comparison of ultrasound images with a focus on swallowing function.

OBJECTIVE: The purpose of this study was to clarify, using ultrasound imaging, (1) whether the area and contraction of GH change in elderly patients after hip fracture surgery and (2) whether the changes in the area and contraction of GH are related to decline in swallowing function. METHODS: The participants were 21 female patients over 65 years of age who underwent hip fracture surgery. The patients were divided into two groups based on the results of swallowing assessment by water drinking: One with normal swallowing function (NSF) and the other with suspected decline in swallowing function (DSF). Sagittal cross-sectional area (SA) of GH at rest and the shortening rate (SR) of GH upon contraction during swallowing were compared at two time points: immediately and 2 weeks after surgery. Wilcoxon signed-rank test was used for intra-group comparisons, and Mann-Whitney U-test was used for between-group comparisons. RESULT: SA of GH decreased significantly at 2 weeks after surgery in both groups, regardless of their swallowing function. In the intra-group comparison, SR significantly decreased (worsened) only in DSF group. SR at 2 weeks after surgery was significantly higher in NSF than in the DSF. In the inter-group comparison, DSF showed a significantly smaller (worse) change of SR than NSF in 2 weeks after surgery. CONCLUSION: Decrease in muscle mass, or atrophy, of GH observed in both NSF and DSF, did not coincide with the post-operative change in GH contraction of the two groups. The results suggest the importance of continuous swallowing assessment in the elderly individuals during their perioperative period.

Thiocyanate-Stabilized Pseudo-cubic Perovskite CH(NH2)2PbI3 from Coincident Columnar Defect Lattices.

α-FAPbI3 (FA+ = CH(NH2)2+) with a cubic perovskite structure is promising for photophysical applications. However, α-FAPbI3 is metastable at room temperature, and it transforms to the δ-phase at a certain period of time at room temperature. Herein, we report a thiocyanate-stabilized pseudo-cubic perovskite FAPbI3 with ordered columnar defects (α'-phase). This compound has a √5ap × âˆš5ap × ap tetragonal unit cell (ap: cell parameter of primitive perovskite cell) with a band gap of 1.91 eV. It is stable at room temperature in a dry atmosphere. Furthermore, the presence of the α'-phase in a mixed sample with the δ-phase drastically reduces the δ-to-α transition temperature measured on heating, suggesting the reduction of the nucleation energy of the α-phase or thermodynamic stabilization of the α-phase through epitaxy. The defect-ordered pattern in the α'-phase forms a coincidence-site lattice at the twinned boundary of the single crystals, thus hinting at an epitaxy- or strain-based mechanism of α-phase formation and/or stabilization. In this study, we developed a new strategy to control defects in halide perovskites and provided new insight into the stabilization of α-FAPbI3 by pseudo-halide and grain boundary engineering.

Selective Synthesis of Perovskite Oxyhydrides Using a High-Pressure Flux Method.

Oxyhydrides with multi-anions (O2- and H-) are a recently developed material family and have attracted attention as catalysts and hydride ion conductors. High-pressure and high-temperature reactions are effective in synthesizing oxyhydrides, but the reactions sometimes result in inhomogeneous products due to insufficient diffusion of the solid components. Here, we synthesized new perovskite oxyhydrides SrVO2.4H0.6 and Sr3V2O6.2H0.8. We demonstrated that the addition of SrCl2 flux promotes diffusion during high-pressure and high-temperature reactions, and can be used for selective synthesis of the oxyhydride phases. We conducted in-situ synchrotron X-ray diffraction measurements to reveal the role of this flux and reaction pathways. We also demonstrated the electronic and magnetic properties of the newly synthesized oxyhydrides and that they work as anode materials for Li-ion batteries with excellent reversibility and high-rate characteristics, the first case with an oxyhydride. Our synthesis approach would also be effective in synthesizing various types of multi-component systems.

A-Site Columnar-Ordered Perovskite CaZnV2O6 as a Pauli-Paramagnetic Metal.

In this study, we successfully synthesized a novel A-site columnar-ordered perovskite CaZnV2O6. This compound features a square-planar-coordinated Zn2+ disorder, which is the same characteristic as the centrosymmetric paraelectric CaMnTi2O6. Unlike CaMnTi2O6, which shows a centrosymmetric paraelectric-noncentrosymmetric ferroelectric transition, CaZnV2O6 retains Pauli-paramagnetic metallicity arising from itinerant V4+ d1 electrons and centrosymmetry down to 5 K. Based on analogous compounds, we expect CaZnV2O6 to provide a new playground for the electronic and magnetic states of V4+.

Realization of Negative Thermal Expansion in Lead-Free Bi0.5K0.5VO3 by the Suppression of Tetragonality.

Bi1/2K1/2VO3 is a lead-free PbTiO3-type compound with a tetragonality (c/a = 1.054) comparable to that of typical ferroelectric PbTiO3 (c/a = 1.064) with negative thermal expansion (NTE) during the tetragonal-to-cubic phase transition; therefore, Bi1/2K1/2VO3 is a potential lead-free NTE material if its metastable perovskite structure can be maintained at high temperatures. In the present experiment, electron doping in Bi1/2K1/2VO3 was conducted through substituting K+ with La3+ to suppress the tetragonality and achieve NTE. La substitution successfully suppressed the tetragonality of Bi1/2K1/2VO3 and also improved its thermal stability. Moreover, both composition- and temperature-induced tetragonal-to-cubic phase transitions occurred. In particular, a large volume shrinkage with a large negative thermal coefficient of expansion (CTE) was obtained for Bi0.5K0.46La0.04VO3 during the tetragonal-to-cubic phase transition (ΔV = -0.66%). Hence, this study extends the NTE family and also sheds light on the exploration of lead-free piezoelectric materials with controllable thermal expansion.

Negative Thermal Expansion in Fluoroapatite Pb5(VO4)3F Enhanced by the Steric Effect of Pb2.

Negative thermal expansion (NTE) is an unusual thermophysical phenomenon and has gained attention as a way of controlling thermal expansion. Here, we report a substantial NTE in fluoroapatite Pb5(VO4)3F in a limited temperature range. The dilatometric study revealed volume shrinkage below 150 K, giving a linear thermal expansion coefficient of αL = -44 ppm/K in the temperature range from 140 to 120 K upon heating. The NTE behavior is associated with a structural transition from the hexagonal (P63/m) phase to the monoclinic (P21/b) phase. Such a structural transition has been found in other apatite-type compounds, but the magnitude of the volume change in Pb5(VO4)3F is remarkable. Our structural analysis revealed that the structural transition is classified as an antiferroelectric-to-paraelectric transition and the volume change during the transition is enhanced by the steric effect of 6s2 lone-pair electrons of Pb2+.

V-V Dimerization and Magnetic State of Cobalt Ions in Ilmenite-Type CoVO3.

The nature of chemical bonds determines the electronic and magnetic properties of compounds. A metal-metal bonding (V-V dimer) and its effect on the magnetism of ilmenite-type CoVO3 were studied. Polycrystalline CoVO3 samples were synthesized using a high-pressure synthesis method. Crystal structure refinement revealed that V-V dimers exist at temperatures below 550 K in the vanadium layers. Co2+ in CoVO3 exhibits an S = 3/2 state, whereas a Jeff = 1/2 state was reported in ilmenite-type CoTiO3. The existence of V-V dimers reduces the structural symmetry (from R3 to P1), which can change the magnetic ground state.

Sequential Pressure-Induced B1-B2 Transitions in the Anion-Ordered Oxyhydride Ba2YHO3.

We present a detailed experimental and computational investigation of the influence of pressure on the mixed-anion oxyhydride phase Ba2YHO3, which has recently been shown to support hydride conductivity. The unique feature of this layered perovskite is that the oxide and hydride anions are segregated into distinct regions of the unit cell, in contrast to the disordered arrangement in closely related Ba2ScHO3. Density functional theory (DFT) calculations reveal that the application of pressure drives two sequential B1-B2 transitions in the interlayer regions from rock salt to CsCl-type ordering, one in the hydride-rich layer at approximately 10 GPa and another in the oxide-rich layer at 35-40 GPa. To verify the theoretical predictions, we experimentally observe the structural transition at 10 GPa using high-pressure X-ray diffraction (XRD), but the details of the structure cannot be solved due to peak broadening of the XRD patterns. We use DFT to explore the structural impact of pressure on the atomic scale and show how the pressure-dependent properties can be understood in terms of simple electrostatic engineering.

Suppression of Pressure-Induced Phase Transitions in a Monoclinically Distorted LiNbO3-Type CuNbO3 by Preference for a CuO3 Triangular Coordination Environment.

Pressure-induced phase transitions in a monoclinically distorted LiNbO3-type CuNbO3 with triangularly coordinated Cu and octahedrally coordinated Nb were experimentally and computationally investigated. Phase transitions into GdFeO3-type or NaIO3-type structures generally observed in LiNbO3-type compounds below 30 GPa were not detected in CuNbO3 even at the maximum experimental pressure, 32.4 GPa. Our density functional theory calculations revealed that the phase transition is suppressed by the preference for the CuO3 triangular coordination environment, which reduces the total internal energy. This study clarifies that the change in the coordination environment of given ions can affect the pressure-induced phase transition.

Bi0.5Pb0.5FeO3 with Unusual Pb Charge Disproportionation: Indication of a Systematic Charge Distribution Change in Bi0.5Pb0.5MO3 (M: 3d Transition Metal).

Bi0.5Pb0.5FeO3 with 1:1 mixture of Bi and Pb having charge degrees of freedom at the A-site of perovskite oxide ABO3 is obtained for the first time by high-pressure synthesis. Comprehensive synchrotron X-ray powder diffraction, optical second harmonic generation, Mössbauer spectroscopy, and hard X-ray photoemission spectroscopy measurements revealed that Bi0.5Pb0.5FeO3 is a canted antiferromagnetic insulator crystalizing in a nonpolar tetragonal I4/mcm structure with √2a × âˆš2a × 2a unit cell and has unusually Pb charge disproportionated Bi3+0.5Pb2+0.25Pb4+0.25Fe3+O3 charge distribution. The valence of transition metal M in Bi0.5Pb0.5MO3 changes from 3.5+ to 3+ and finally to 2+ from Mn to Fe and to Ni, from left to right in the periodic table as the 3d-level becomes deeper. The valences of Bi and Pb increase to compensate for the decrease in the M's valence, and Pb changes from 6s2 (2+) to 6s0 (4+) before Bi changes.

SrV0.3Fe0.7O2.8: A Vacancy-Ordered Fe-Based Perovskite Exhibiting Room-Temperature Magnetoresistance.

We report room-temperature (RT) magnetoresistance (MR) in a novel Fe-based perovskite, SrV0.3Fe0.7O2.8. This compound contains ordered oxygen vacancies in every fifth primitive perovskite (111)p plane, leading to a layered structure consisting of triple-octahedral and double-tetrahedral layers. Along with the oxygen vacancies, the transition-metal ions are also ordered: the octahedral sites are occupied by 100% of Fe ions, while the tetrahedral sites are occupied by 25% of Fe ions and 75% of V ions. As a result, SrV0.3Fe0.7O2.8 forms a magnetically striped lattice in which the octahedral layers with 100% of magnetic Fe ions are separated by the diluted magnetic layer. The compound exhibits weak ferromagnetism and shows a large negative MR (-5% at 3 T) at RT, despite the small saturation moment (0.4 µB/Fe atom). Thus, this type of layered compound is promising for further large MR by an increase of magnetization through chemical substitution.

Two Distinct Cu(II)-V(IV) Superexchange Interactions with Similar Bond Angles in a Triangular "CuV2" Fragment.

The strength and sign of superexchange interactions are often predicted on the basis of the bond angles between magnetic ions, but complications may arise in situations with a nontrivial arrangement of the magnetic orbitals. We report on a novel molecular tetramer compound [Cu(H2O)dmbpy]2[V2O2F8] (dmbpy = 4,4'-dimethyl-2,2'-bipyridyl) that is composed of triangular "CuV2" fragments and displays a spin gap behavior. By combining first-principles calculations and electronic models, we reveal that superexchange Cu-V interactions carry drastically different coupling strengths along two Cu-F-V pathways with comparable bond angles in the triangular "CuV2" fragment. Counterintuitively, their strong disparity is found to originate from the restricted symmetry of the half-filled Cu dx2-y2 orbital stabilized by the crystal field, leading to one dominating antiferromagnetic Cu-V coupling in each fragment. We revisit the magnetic properties of the reported spin-gapped chain compound [enH2]Cu(H2O)2[V2O2F8] (enH2 = ethylene diammonium) containing similar triangular "CuV2" fragments, and the magnetic behavior of the molecular tetramer and the chain compounds is rationalized as that of weakly coupled spin dimers and spin trimers, respectively. This work demonstrates that fundamentally different magnetic couplings can be observed between magnetic ions with similar bond angles in a single spin motif, thus providing a strategy to introduce various exchange interactions combined with low dimensionality in heterometallic Cu(II)-V(IV) compounds.

Intermetallic Charge Transfer in V-Substituted PbCrO3.

PbCrO3 features an unusual charge distribution Pb0.52+Pb0.54+Cr3+O3 with Pb charge disproportionation at ambient pressure. A charge transfer between Pb and Cr is induced by the application of pressure resulting in Pb2+Cr4+O3 charge distribution and a large volume collapse. Here, structural and charge distribution changes in PbCr1-xVxO3 are investigated. Despite a cubic crystal structure in 0 ≤ x ≤ 0.60, discontinuous reduction in the unit cell volume was observed between x = 0.35 and 0.40. Hard X-ray photoemission spectroscopy confirmed the change in Pb charge state from the coexisting Pb2+ and Pb4+ at x = 0.35 to single Pb2+ at x = 0.40. This indicates that V substitution stabilizes the high pressure cubic Pb2+Cr4+O3-type phase. With further increase in the V substitution, the PbVO3-type polar tetragonal phase appeared at x = 0.80.

High-Pressure and High-Temperature Synthesis of Anion-Disordered Vanadium Perovskite Oxyhydrides.

Crystallographic order-disorder phenomena in solid state compounds are of fundamental interest due to intimate relationship between the structure and properties. Here, by using high-pressure and high-temperature synthesis, we obtained vanadium perovskite oxyhydrides Sr1-xNaxVO3-yHy (x = 0, 0.05, 0.1, 0.2) with an anion-disordered structure, which is different from anion-ordered SrVO2H synthesized by topochemical reduction. High-pressure and high-temperature synthesis from nominal composition SrVO2H yielded the anion-disordered perovskite SrVO3-yHy (y ∼ 0.4) with a significant amount of byproducts, while Na substitution resulted in the almost pure anion-disordered perovskite Sr1-xNaxVO3-yHy with an increased amount of hydride anion (y ∼ 0.7 for x = 0.2). The obtained disordered phases for x = 0.1 and 0.2 are paramagnetic with almost temperature-independent electronic conductivity, whereas anion-ordered SrVO2H is an antiferromagnetic insulator. Although we obtained the anion-disordered perovskite under high pressure, a first-principles calculation revealed that the application of pressure stabilizes the ordered phase due to a reduced volume in the ordered structure, suggesting that a further increase of the pressure or reduction of the reaction temperature leads to the anion ordering. This study shows that anion ordering in oxyhydrides can be controlled by changing synthetic pressure and temperature.

Sequential Spin State Transition and Intermetallic Charge Transfer in PbCoO3.

Spin state transitions and intermetallic charge transfers can essentially change material structural and physical properties while excluding external chemical doping. However, these two effects have rarely been found to occur sequentially in a specific material. In this article, we show the realization of these two phenomena in a perovskite oxide PbCoO3 with a simple ABO3 composition under high pressure. PbCoO3 possesses a peculiar A- and B-site ordered charge distribution Pb2+Pb4+3Co2+2Co3+2O12 with insulating behavior at ambient conditions. The high spin Co2+ gradually changes to low spin with increasing pressure up to about 15 GPa, leading to an anomalous increase of resistance magnitude. Between 15 and 30 GPa, the intermetallic charge transfer occurs between Pb4+ and Co2+ cations. The accumulated charge-transfer effect triggers a metal-insulator transition as well as a first-order structural phase transition toward a Tetra.-I phase at the onset of ∼20 GPa near room temperature. On further compression over 30 GPa, the charge transfer completes, giving rise to another first-order structural transformation toward a Tetra.-II phase and the reentrant electrical insulating behavior.

Stabilized Charge, Spin, and Orbital Ordering by the 6s2 Lone Pair in Bi0.5Pb0.5MnO3.

Bi and Pb ions with charge degree of freedom depending on 6s2 and 6s0 electronic configurations were combined with the Mn ion in a perovskite oxide. Comprehensive theoretical and experimental investigations revealed the Bi3+0.5Pb2+0.5Mn3+0.5Mn4+0.5O3 charge ordered state with CE-type spin and dz2 orbital orderings as observed in La0.5Ca0.5MnO3, Nd0.5Sr0.5MnO3, and Bi0.5Sr0.5MnO3. The charge and orbital orderings were preserved above 500 K owing to the stereochemical activity of Bi3+ and Pb2+ ions which stabilized the structural distortion.

Enhanced Spontaneous Polarization by V4+ Substitution in a Lead-Free Perovskite CaMnTi2O6.

Spontaneous polarization (Ps) of novel order-disorder type lead-free ferroelectric CaMnTi2O6 was successfully enhanced by partial V4+ substitution for Ti4+. A synchrotron X-ray diffraction study revealed that the polar displacement of octahedrally coordinated (Ti, V) in CaMn(Ti1-xVx)2O6 (0 ≤ x ≤ 0.4) increases with V4+ substitution having Jahn-Teller activity owing to the d1 electronic configuration. Our magnetic study suggested the presence of antisite disorder between Ca2+ and square planar coordinated Mn2+ associated with Mn-V intermetallic charge transfer for x ≥ 0.4, resulting in decreases in spontaneous polarization and the ferroelectric-paraelectric transition temperature. This is the first report on the enhanced polarization owing to the Jahn-Teller distortion of V4+ without stereochemical Pb2+ or Bi3+.