Crystal structure and properties of perovskite-type rubidium niobate, a high-pressure phase of RbNbO3.

We synthesized a perovskite-type RbNbO3 at 1173 K and 4 GPa from non-perovskite RbNbO3 and investigated its crystal structure and properties towards ferroelectric material design. Single-crystal X-ray diffraction analysis revealed an orthorhombic cell in the perovskite-type structure (space group Amm2, no. 38) with a = 3.9937(2) Å, b = 5.8217(3) Å, and c = 5.8647(2) Å. This non-centrosymmetric space group is the same as the ferroelectric BaTiO3 and KNbO3 but with enhanced distortion. Structural transition from orthorhombic to two successive tetragonal phases (Tetra1 at 493 K, Tetra2 at 573 K) was observed, maintaining the perovskite framework before reverting to the triclinic ambient phase at 693 K, with no structural changes between 4 and 300 K. The first transition is similar to that of KNbO3, whereas the second to Tetra2, marked by c-axis elongation and a significant cp/ap ratio jump (from 1.07 to 1.43), is unique. This distortion suggests a transition similar to that of PbVO3, where an octahedron's oxygen separates along the c-axis, forming a pyramid. Ab initio calculations simulating negative pressure like thermal expansion predicted this phase transition (cp/ap = 1.47 at -1.2 GPa), aligning with experimental findings. Thermal analysis revealed two endothermic peaks, with the second transition entailing a greater enthalpy change and volume alteration. Strong second harmonic generation signals were observed across Ortho, Tetra1, and Tetra2 phases, similar to BaTiO3 and KNbO3. Permittivity increased during the first transition, although the second transition's effects were limited by thermal expansion-induced bulk sample collapse. Perovskite-type RbNbO3 emerges as a promising ferroelectric material.

Strongly Enhanced Polarization in a Ferroelectric Crystal by Conduction-Proton Flow.

Ion conductors comprising noncentrosymmetric frameworks have emerged as new functional materials. However, strongly correlated polarity functionality and ion transport have not been achieved. Herein, we report a ferroelectric proton conductor, K2MnN(CN)4·H2O (1·H2O), exhibiting the strong correlation between its polar skeleton and conductive ions that generate anomalous ferroelectricity via the proton-bias phenomenon. The application of an electric field of ±1 kV/cm (0.1 Hz) on 1·H2O at 298 K produced the ferroelectricity (polarization = 1.5 × 104 µC/cm2), which was enhanced by the ferroelectric-skeleton-trapped conductive protons. Furthermore, the strong polarity-proton transport coupling of 1·H2O induced a proton-rectification-like directional ion-conductive behavior that could be adjusted by the magnitude and direction of DC electric fields. Moreover, 1·H2O exhibited reversible polarity switching between the polar 1·H2O and its dehydrated form, 1, with a centrosymmetric structure comprising an order-disorder-type transition of the nitrido-bridged chains.

Mechanisms of point defect formation and ionic conduction in divalent cation-doped lanthanum oxybromide: first-principles and experimental study.

The ionic conduction mechanism in M2+-doped (M: Mg, Ca, Zn, and Sr) lanthanum oxybromide (LaOBr) was investigated theoretically and experimentally. Formation energy calculations of point defects revealed that Br- ion vacancies and substitutional M2+ ions were the major point defects in M2+-doped LaOBr, while Br- ion vacancies and antisite O2- ions at Br sites were the major defect types in pure LaOBr. In the relaxed point defect models, doped Mg2+ and Zn2+ ions were displaced from the initial positions of the La3+ ions, and this was experimentally supported by crystal structural analysis. These significant atomic shifts were probably due to the strong interactions between Br- and the dopant ions. First-principles calculations and experimental analyses using X-ray photoelectron spectroscopy and X-ray absorption fine-structure spectroscopy also suggested the existence of strong interactions. The migration energy of Br- ions was calculated to be 0.53 eV, while the migration energy of O2- ions was 0.92 eV, implying that Br- ion migration via a vacancy system was more probable than O2- ion migration. The calculated association energies between MLa and VBr were 0.4-0.6 eV, suggesting that the association needed to be disrupted for Br- ion conduction. The sum of the association and migration energies was comparable to the experimental association energies of M2+-doped LaOBr.

Anion ordering enables fast H- conduction at low temperatures.

The introduction of chemical disorder by substitutional chemistry into ionic conductors is the most commonly used strategy to stabilize high-symmetric phases while maintaining ionic conductivity at lower temperatures. In recent years, hydride materials have received much attention owing to their potential for new energy applications, but there remains room for development in ionic conductivity below 300°C. Here, we show that layered anion-ordered Ba2-δH3-2δ X (X = Cl, Br, and I) exhibit a remarkable conductivity, reaching 1 mS cm-1 at 200°C, with low activation barriers allowing H- conduction even at room temperature. In contrast to structurally related BaH2 (i.e., Ba2H4), the layered anion order in Ba2-δH3-2δ X, along with Schottky defects, likely suppresses a structural transition, rather than the traditional chemical disorder, while retaining a highly symmetric hexagonal lattice. This discovery could open a new direction in electrochemical use of hydrogen in synthetic processes and energy devices.

Kinetically Stabilized Cation Arrangement in Li3 YCl6 Superionic Conductor during Solid-State Reaction.

The main approach for exploring metastable materials is via trial-and-error synthesis, and there is limited understanding of how metastable materials are kinetically stabilized. In this study, a metastable phase superionic conductor, ß-Li3 YCl6 , is discovered through in situ X-ray diffraction after heating a mixture of LiCl and YCl3 powders. While Cl- arrangement is represented as a hexagonal close packed structure in both metastable ß-Li3 YCl6 synthesized below 600 K and stable α-Li3 YCl6 above 600 K, the arrangement of Li+ and Y3+ in ß-Li3 YCl6 determined by neutron diffraction brought about the cell with a 1/√3 a-axis and a similar c-axis of stable α-Li3 YCl6 . Higher Li+ ion conductivity and lower activation energy for Li+ transport are observed in comparison with α-Li3 YCl6 . The computationally calculated low migration barrier of Li+ supports the low activation energy for Li+ conduction, and the calculated high migration barrier of Y3+ kinetically stabilizes this metastable phase by impeding phase transformation to α-Li3 YCl6 . This work shows that the combination of in situ observation of solid-state reactions and computation of the migration energy can facilitate the comprehension of the solid-state reactions allowing kinetic stabilization of metastable materials, and can enable the discovery of new metastable materials in a short time.

Ionic conduction mechanism in Ca-doped lanthanum oxychloride.

The mechanism of ionic conduction in Ca-doped lanthanum oxychloride (LaOCl) was investigated using first-principles calculations based on density functional theory. The calculations of the point defect formation energies suggest that Cl- ion vacancies and substituted Ca2+ ions at La sites were dominant point defects. Although the migration energy of an O2- ion is 0.95 eV, the migration energy of a Cl- ion was calculated to be 0.44 eV, which is consistent with the reported experimental value. These results imply that the main carrier in Ca-doped LaOCl is Cl- ions and ionic conduction occurs by a Cl- ion vacancy mechanism.

Chemical Pressure-Induced Anion Order-Disorder Transition in LnHO Enabled by Hydride Size Flexibility.

While cation order-disorder transitions have been achieved in a wide range of materials and provide crucial effects in various physical and chemical properties, anion analogues are scarce. Here we have expanded the number of known lanthanide oxyhydrides, LnHO (Ln = La, Ce, Pr, Nd), to include Ln = Sm, Gd, Tb, Dy, Ho, and Er, which has allowed the observation of an anion order-disorder transition from the anion-ordered fluorite structure ( P4/ nmm) for larger Ln3+ ions (La-Nd) to a disordered arrangement ( Fm3̅ m) for smaller Ln3+ (Sm-Er). Structural analysis reveals that with the increase of Ln3+ radius (application of negative chemical pressure), the oxide anion in the disordered phase becomes too under-bonded, which drives a change to an anion-ordered structure, with smaller OLn4 and larger HLn4 tetrahedra, demonstrating that the size flexibility of hydride anions drives this transition. Such anion ordering control is crucial regarding applications that involve hydride diffusion such as catalysis and electrochemical solid devices.

ZnTaO2N: Stabilized High-Temperature LiNbO3-type Structure.

By using a high-pressure reaction, we prepared a new oxynitride ZnTaO2N that crystallizes in a centrosymmetric (R3̅c) high-temperature LiNbO3-type structure (HTLN-type). The stabilization of the HTLN-type structure down to low temperatures (at least 20 K) makes it possible to investigate not only the stability of this phase, but also the phase transition to a noncentrosymmetric (R3c) LiNbO3-type structure (LN-type) which is yet to be clarified. Synchrotron and neutron diffraction studies in combination with transmission electron microscopy show that Zn is located at a disordered 12c site instead of 6a, implying an order-disorder mechanism of the phase transition. It is found that the closed d-shell of Zn2+, as well as the high-valent Ta5+ ion, is responsible for the stabilization of the HTLN-type structure, affording a novel quasitriangular ZnO2N coordination. Interestingly, only 3% Zn substitution for MnTaO2N induces a phase transition from LN- to HTLN-type structure, implying the proximity in energy between the two structural types, which is supported by the first-principles calculations.

Theoretical calculations of the thermodynamic stability of ionic substitutions in hydroxyapatite under an aqueous solution environment.

Defect formation energies in materials generally depend on chemical potentials determined by a chemical equilibrium condition. In particular, an aqueous solution environment is important for biomaterials such as hydroxyapatite studied here. Therefore, a methodology to obtain ionic chemical potentials under chemical equilibrium between solid and aqueous solution was introduced, and was applied to substitutional divalent cations formed via ion exchange with Ca(2+) in hydroxyapatite. The calculated ranking of the stability of substitutional cations in HAp was in good agreement with the experimentally observed trend. The present theoretical approach would be useful to explore the thermodynamic stability of defects in materials subjected to an aqueous solution environment.

First-principles calculations of Zn-K XANES in Ca-deficient hydroxyapatite.

The local environment of substitutional Zn(2+) in Ca-deficient hydroxyapatite (HAp) was investigated using experimental and theoretical analyses of the x-ray absorption near edge structure (XANES). For Zn-K XANES calculations, two situations of Zn(2+) were considered. One was Zn(2+) substituted for Ca sites in perfect HAp, and the other was a Ca-deficient HAp model of substitutional Zn(2+) associated with a Ca(2+) vacancy charge compensated by two protons. The model of Zn(2+) in perfect HAp did not reproduce the experimental Zn-K XANES spectrum. In contrast, the Ca-deficient HAp model agreed well with the experimental spectrum. This indicates that substitutional Zn(2+) in Ca-deficient HAp is associated with the Ca(2+) vacancy complex in HAp.