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

硅笼形化合物是由Si_<20>、Si_<24>、Si_<28>多面体共面构成的笼状晶体，其中占据了客体原子。这些化合物作为新型的超导体、特殊的宽带隙半导体和一类新型的热电材料继续引起人们的广泛关注。用高压拉曼光谱研究了客体晶格和主体晶格的振动性质以及客体-主体相互作用，测量了I型硅包合物<46>Ba_8Si_2、<46>K_8Si_2和I_8Si_I_2<44>以及III型<24>Ba_Si_2<100>在30 GPa压力下的高压拉曼散射。在40-110 cm ↑ [2]附近发现了Ba、K和I原子在两种或三种笼中的低频振动模<-1>。硅骨架拉曼谱带也在110-450 cm-2附近观察到<-1>。压力诱导相变发生在高压下。它们与客体原子的运动和主客体相互作用密切相关。在高压下，发现了压力诱导的非晶化不可逆相变，并与高压下的非晶硅相比较，讨论了压力诱导的非晶化不可逆相变，结合硅笼合物的高压相变，研究了笼合物的客体结构和笼结构对笼合物相变特性的影响，以及笼结构的稳定性。

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.