The phase transition sequence of perovskite materials is an important subject in numerous fields ranging from geophysics to material science. The fluoride NaZnF, adopting the distorted perovskite (Pv) structure (GdFeO-type), is a model system for studying such transitions. Up to date compression experiments on this compound, limited to pressures of 40 GPa, have identified a perovskite to post-perovskite (pPv, CaIrO-type structure) transition to occur above 11 GPa followed by yet another transition commencing at 25 GPa and leading to a post-post-perovskite (ppPv) phase whose structure is yet to be identified. Here we present high-pressure Raman spectroscopic study aided by Density Functional Theory (DFT) calculations to explore the phase transition sequence of NaZnF at pressures reaching 70 GPa. We confirm that NaZnF undergoes a gradual phase transition between the GdFeO and CaIrO phases starting at 12 GPa. Our results also indicate that above 31 GPa, the CaIrO polymorph slowly transforms to the ppPv phase which we unambiguously confirm to adopt the metastable SbS-type structure despite the ground state structure above 23 GPa being LaS. The transition into SbS-type structure type instead of the LaS polytype is most probably due to kinetic effects which also hinder the predicted decomposition of NaZnF.

中文翻译：

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拉曼光谱洞察 NaZnF3 向后后钙钛矿相的转变

钙钛矿材料的相变序列是从地球物理学到材料科学等众多领域的重要课题。氟化物NaZnF采用扭曲的钙钛矿（Pv）结构（GdFeO型），是研究此类转变的模型系统。对该化合物的最新压缩实验（仅限于 40 GPa 的压力）已发现，在 11 GPa 以上时会发生钙钛矿到后钙钛矿（pPv，CaIrO 型结构）的转变，随后又在 25 GPa 下开始发生另一个转变，并导致后后钙钛矿（ppPv）相，其结构尚未确定。在这里，我们提出了借助密度泛函理论 (DFT) 计算的高压拉曼光谱研究，以探索 NaZnF 在压力达到 70 GPa 时的相变序列。我们确认 NaZnF 从 12 GPa 开始在 GdFeO 和 CaIrO 相之间经历逐渐的相变。我们的结果还表明，在 31 GPa 以上，CaIrO 多晶型物缓慢转变为 ppPv 相，尽管 23 GPa 以上的基态结构是 LaS，但我们明确证实其采用亚稳态 SbS 型结构。转变为 SbS 型结构类型而不是 LaS 多型体很可能是由于动力学效应，这也阻碍了 NaZnF 的预测分解。