Behaviour of Jahn-Teller distorted NiO6 octahedra in NaNiO2 under pressure

Many transition metal (TM) oxides exhibit a Jahn-Teller (JT) structural distortion, and these are often susceptible to external pressure. In some materials, pressure-tuning has supressed the JT distortion entirely. XNiO2 (X=Li,Na) solid solutions, often doped with other TM ions, are important in battery research as cathode materials, and their crystal structures are heavily influenced by JT distortions in the NiO6 octahedra. NaNiO2 exhibits a cooperative JT distortion below 480 K, which elongates these octahedra and reduces the crystal symmetry from rhombohedral to monoclinic. We propose a set of diffraction measurements on NaNiO2 up to 6 GPa and 480 K, to map out the P-T phase diagram. We will be looking to see if the JT effect is quenched at high pressure, and whether this occurs by reduction in TJT towards 0 K, or by the long and short bonds in NiO6 octahedra tending towards equality.

许多过渡金属（transition metal, TM）氧化物会出现姜-泰勒（Jahn-Teller, JT）结构畸变，且这类材料通常对外压较为敏感。部分材料可通过压力调控完全抑制姜-泰勒畸变。XNiO₂（X=Li、Na）固溶体常掺杂其他过渡金属离子，作为正极材料在电池研究中具有重要价值，其晶体结构受NiO₆八面体的姜-泰勒畸变影响显著。NaNiO₂在480 K以下会发生协同姜-泰勒畸变，使该八面体发生伸长畸变，并将晶体对称性从三方晶系降至单斜晶系。本研究拟对NaNiO₂开展最高6 GPa、480 K条件下的衍射测试，以绘制其压力-温度（P-T）相图。我们将重点考察高压下姜-泰勒效应是否被淬灭，以及该淬灭是通过将JT临界温度（TJT）降至0 K实现，还是通过使NiO₆八面体的长短键趋于均等达成。