II族二氟化物高压相的多晶型和优越性
High-pressure phases of group II difluorides: polymorphism and superionicity
摘要：
采用从头算随机结构搜索和密度泛函理论（DFT）计算，研究了铍、镁和钙的二氟化物在0-70 GPa压力范围内的高压行为.氟化铍在低压下表现出广泛的多晶型，我们发现这种化合物的两个新相-二氧化硅moganite和CaCl 2结构-在12-57 GPa的宽压力范围内稳定。对于氟化镁，我们的搜索结果表明，正交'O-I' TiO 2结构（Pbca，Z=8）是稳定的，该化合物之间的40和44 GPa。我们的搜索发现，在0和70 GPa之间的二氟化钙的静态晶格水平没有新的相位;然而，具有P62 m对称性的相位在这个压力范围内接近稳定，并且我们的计算预测，该相位在高温下是稳定的。P62 m结构在大体积下表现出不稳定的声子模式，这可能是在高温下向超离子态转变的信号。第二组二氟化物是等电子的一些其他AB 2型化合物，如二氧化硅和二氧化钛，我们讨论我们的结果，根据这些相似之处。
英文摘要：
We investigate the high-pressure behaviour of beryllium, magnesium and calcium difluorides using ab initio random structure searching and density functional theory (DFT) calculations, over the pressure range 0-70 GPa. Beryllium fluoride exhibits extensive polymorphism at low pressures, and we find two new phases for this compound - the silica moganite and CaCl2 structures - which are stable over the wide pressure range 12-57 GPa. For magnesium fluoride, our searching results show that the orthorhombic `O-I' TiO2 structure (Pbca, Z=8) is stable for this compound between 40 and 44 GPa. Our searches find no new phases at the static-lattice level for calcium difluoride between 0 and 70 GPa; however, a phase with P62m symmetry is close to stability over this pressure range, and our calculations predict that this phase is stabilised at high temperature. The P62m structure exhibits an unstable phonon mode at large volumes which may signal a transition to a superionic state at high temperatures. The Group-II difluorides are isoelectronic to a number of other AB2-type compounds such as SiO2 and TiO2, and we discuss our results in light of these similarities.
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