Solid ${\mathrm{NH}}_4\mathrm{F}$  displays intriguing parallels with ice despite its apparently ionic character. Here we investigate its phase diagram in low-temperature and high-pressure regions using Raman spectroscopy, x-ray diffraction, and ab initio structure search calculations. We focus on the high-pressure cubic phase which resembles that found in ice under pressure and is also the ambient pressure phase of other ammonium halides. We detect a disorder-order transition above 10 GPa, recalling those found both in other ammonium halides and in ice. The transition reveals itself in the pressure dependence of several Raman modes as well as through the progressive splitting of lattice and bending modes of the cubic phase at low temperatures. An in-depth analysis of the Raman modes and their evolution is made.

中文翻译：

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氟化铵在高压下的无序相变

坚硬的 ${\mathrm{NH}}_4\mathrm{F}$  尽管具有明显的离子特性，但它与冰的相似之处令人着迷。在这里，我们使用拉曼光谱，x射线衍射和从头算结构搜索计算研究其在低温和高压区域的相图。我们专注于高压立方相，类似于在压力下在冰中发现的高压相，也是其他卤化铵的环境压力相。我们检测到高于10 GPa的无序变化，并回忆起在其他卤化铵和冰中发现的无序变化。该转变通过几种拉曼模式的压力依赖性以及在低温下立方相的晶格和弯曲模式的逐步分裂而展现出来。对拉曼模式及其发展进行了深入的分析。