MnTaO2N: Polar LiNbO3-type Oxynitride with a Helical Spin Order

Highly Electron-Doped TaON Single-Crystal Growth by a High-Pressure Flux Method

Transition-metal oxynitrides have a variety of functions such as visible light-responsive catalysts and dielectric materials, but acquiring single crystals necessary to understand inherent properties is difficult and is limited to relatively small sizes (<10 μm) because they easily decompose at high temperatures. Here, we have succeeded in growing platelet single crystals of TaON with a typical size of 50 × 100 × 10 μm³ under a high pressure and high temperature (6 GPa and 1400 °C) using a LiCl flux. Such a harsh condition, in contrast to powder samples synthesized under mild conditions, resulted in the introduction of a large amount of oxygen vacancies (x = 0.06 in TaO_1-xN) into the crystal, providing a metallic behavior with a large anisotropy of ρ_c/ρ_ab ∼ 10³. Low-temperature oxygen annealing allows for a single-crystal-to-single-crystal transformation to obtain fully oxidized TaON (yellow) crystals. Needle-like crystals can be obtained when NH₄Cl is used as a flux. Furthermore, black Hf₂ON₂ single crystals are also grown, suggesting that the high-pressure flux method is widely applicable to other transition-metal oxynitrides, with extensive carrier control.

Chemical design of a new displacive-type ferroelectric

Since the discovery of the ferroelectric perovskite-type oxide BaTiO₃ in 1943, numerous materials have been surveyed as candidates for new ferroelectrics. Perovskite-type materials have played a leading role in basic research and applications of ferroelectric materials since the last century. Experimentalists and theoreticians have developed a new materials design stream for post-perovskite materials. In this stream, we have mainly focused on the role of covalency in the evolution of ferroelectricity for displacive-type ferroelectrics in oxides. This perspective surveys the following topics: (1) crossover from quantum paraelectric to ferroelectric through a ferroelectric quantum critical point, (2) the role of cation-oxygen covalency in ferroelectricity and the crossover to quantum paraelectric in perovskite-type compounds, (3) off-center-induced ferroelectricity in perovskites, (4) second-order Jahn-Teller effect enhancement of ferroelectricity in lithium-niobate-type oxides, (5) the presence of four ferroelectric phases and structural transitions of phases of AFeO₃ with decreasing radius of A (A = La-Al), (6) tetrahedral ferroelectrics of perovskite-related Bi₂SiO₅ and wurtzites, (7) a rare type of polarization switching system in which the coordination number of ions in κ-Al₂O₃ systems changes between 4 and 6, and (8) lone-pair-electron-induced ferroelectrics in langasite-type compounds.

Engineering Polar Oxynitrides: Hexagonal Perovskite BaWON2

Non-centrosymmetric polar compounds have important technological properties. Reported perovskite oxynitrides show centrosymmetric structures, and for some of them high permittivities have been observed and ascribed to local dipoles induced by partial order of nitride and oxide. Reported here is the first hexagonal perovskite oxynitride BaWON₂ , which shows a polar 6H polytype. Synchrotron X-ray and neutron powder diffraction, and annular bright-field in scanning transmission electron microscopy indicate that it crystalizes in the non-centrosymmetric space group P6₃ mc, with a total order of nitride and oxide at two distinct coordination environments in cubic and hexagonal packed BaX₃ layers. A synergetic second-order Jahn-Teller effect, supported by first principle calculations, anion order, and electrostatic repulsions between W⁶⁺ cations, induce large distortions at two inequivalent face-sharing octahedra that lead to long-range ordered dipoles and spontaneous polarization along the c axis. The new oxynitride is a semiconductor with a band gap of 1.1 eV and a large permittivity.

Universal A-Cation Splitting in LiNbO3-Type Structure Driven by Intrapositional Multivalent Coupling

Understanding the electric dipole switching in multiferroic materials requires deep insight of the atomic-scale local structure evolution to reveal the ferroelectric mechanism, which remains unclear and lacks a solid experimental indicator in high-pressure prepared LiNbO₃-type polar magnets. Here, we report the discovery of Zn-ion splitting in LiNbO₃-type Zn₂FeNbO₆ established by multiple diffraction techniques. The coexistence of a high-temperature paraelectric-like phase in the polar Zn₂FeNbO₆ lattice motivated us to revisit other high-pressure prepared LiNbO₃-type A₂BB'O₆ compounds. The A-site atomic splitting (∼1.0-1.2 Å between the split-atom pair) in B/B'-mixed Zn₂FeTaO₆ and O/N-mixed ZnTaO₂N is verified by both powder X-ray diffraction structural refinements and high angle annular dark field scanning transmission electron microscopy images, but is absent in single-B-site ZnSnO₃. Theoretical calculations are in good agreement with experimental results and suggest that this kind of A-site splitting also exists in the B-site mixed Mn-analogues, Mn₂FeMO₆ (M = Nb, Ta) and anion-mixed MnTaO₂N, where the smaller A-site splitting (∼0.2 Å atomic displacement) is attributed to magnetic interactions and bonding between A and B cations. These findings reveal universal A-site splitting in LiNbO₃-type structures with mixed multivalent B/B', or anionic sites, and the splitting-atomic displacement can be strongly suppressed by magnetic interactions and/or hybridization of valence bands between d electrons of the A- and B-site cations.

High-Pressure Synthesis of Non-Stoichiometric Li x WO3 (0.5 ≤ x ≤ 1.0) with LiNbO3 Structure

Compounds with the LiNbO₃-type structure are important for a variety of applications, such as piezoelectric sensors, while recent attention has been paid to magnetic and electronic properties. However, all the materials reported are stoichiometric. This work reports on the high-pressure synthesis of lithium tungsten bronze Li_xWO₃ with the LiNbO₃-type structure, with a substantial non-stoichiometry (0.5 ≤ x ≤ 1). Li_0.8WO₃ exhibit a metallic conductivity. This phase is related to an ambient-pressure perovskite phase (0 ≤ x ≤ 0.5) by the octahedral tilting switching between a⁻a⁻a⁻ and a⁺a⁺a⁺.