Low temperature and High-pressure Raman Study of Superconductivity and Cage-structure Stability of Silicon Clathrates

硅笼形物是由Si_<20>、Si_<24>和Si_<28>三种共面多面体组成的共价键合笼状晶体。这些化合物作为新型超导体、特殊的宽禁带半导体和一类新型热电材料不断受到人们的关注。利用高压拉曼光谱研究了客体和主晶格的振动特性以及客体与主晶格的相互作用。测量了ⅰ型硅包合物Ba_8Si_<46>、K_8Si_<46>、I_8Si_<44>I_2和ⅲ型Ba_<24>Si_<100>的高压拉曼散射，达到约30 GPa。Ba， K和I原子在两种或三种笼内的低频振动模式在40-110 cm^<-1>附近被发现。硅骨架拉曼带也在110-450 cm^<-1>附近被观测到。压力诱导相变发生在高压下。它们与客体原子的运动和客体-主相互作用密切相关。在高压下，发现了压力诱导的非晶态不可逆转变，并通过对高压下非晶态硅的比较进行了讨论。考虑到这些结果和Si包合物的高压相变，我们研究了它们依赖于客体和笼形结构的特性，以及笼形结构的稳定性。

Silicon clathrates are covalent bonded cage-like crystals composed of face-sharing Si_<20>, Si_<24>, and Si_<28> polyhedra in which guest atoms are occupied. These compounds continue to attract much attention as new superconductors, unusual wide-bandgap semiconductor, and a new class of thermo-electric materials. Vibrational properties of guests and host lattice, and the guest-host interactions are studied by high-pressure Raman spectroscopy.High-pressure Raman scattering of type-I silicon clathrates, Ba_8Si_<46>, K_8Si_<46>, and I_8Si_<44>I_2, and type-III Ba_<24>Si_<100> were measured up to about 30 GPa. Low-frequency vibrational modes associated with Ba, K, and I atoms isnside two or three kinds of cages were found around 40-110 cm^<-1>. The silicon framework Raman bands were also observed around 110-450 cm^<-1>. Pressure-induced phase transitions occurred at high pressures. They are closely related to the motions of guest atoms and the guest-host interactions. At higher pressures, the pressure-induced amorphized irreversible transitions were found, which were discussed by comparing the amorphous silicon at high pressures.By considering these results and high-pressure phase transitions of Si clathrates, we investigate their characteristics depending on guests and cage-structures, and the stability of cage-structures.