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

硅笼状晶体是一种共价键合笼状晶体，由共面共面的Si_(20)、Si_(24)和Si_(28)多面体组成，其中占据客体原子。这些化合物作为新的超导体、特殊的宽禁带半导体和一类新的热电材料继续吸引着人们的注意。用高压拉曼光谱研究了主客体晶格的振动性质以及客体-主客体的相互作用，测量了I类硅笼合物Ba8Si；46&gt；、K_8Si_&lt；46&gt；和I_8Si_&lt；44&gt；I_2和III型Ba_&lt；24&gt；Si_&lt；100&gt；的高压拉曼散射高达30 Gpa。在40-110 cm~(-1)~(-1)~gt~(-1)附近，在两种或三种笼子中发现了与Ba、K、I原子有关的低频振动模。在110-450 cm^&lt；-1&gt；附近还观察到了硅骨架拉曼光谱。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与宿主的相互作用密切相关。在高压下发现了非晶硅的非晶化不可逆转变，并通过比较高压下的非晶硅进行了讨论。结合这些结果和硅笼状结构的高压相变，我们研究了它们与客体和笼状结构的关系以及笼状结构的稳定性。

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.