Guest-Host Interaction and High-pressure Phase Transitions in Semiconductor Clathrates Studied by Raman Spectromopy

1.对缺陷包合物Ba_8Ge_（43）□_3在室温高达40 GPa的条件下进行了高压拉曼和XRD测量。拉曼光谱显示在8 GPa时压力诱导相变，这是由于三键Ge原子的结构畸变和主客电子相互作用的变化所致。Raman光谱和XRD谱图均显示了Ba_8Ge_（43）在30-40 GPa附近非晶化的证据。当Ba_8Ge_（43）达到40 GPa时，未观察到与体积减小相关的等结构相变。在1 bar （; a/s_0）下，晶格常数(a)经a_0归一化后的压强随压强的变化而不断减小，直至非晶化。从Ba_8Ge_（43）和Ba_sSi_（46）在压力大于15 GPa时的曲线的良好重合来看，我们提出Ba_8Si_（46）在压力大于15 GPa时的等结构相变可能是由缺陷引起的向Ba_sSi_（43）□_3的转变引起的。在室温下对ⅲ型锗包合物Ba_(24)Ge_（100）进行了高达26 GPa的高压拉曼测量。与硅包合物Ba_(24)Si_（100）相比，我们观察到客体Ba原子在25-50 cm^（-1）处的低频振动模式，以及宿主Ge原子在50-250 cm^（-1）处的振动模式。在3.2 GPa处发现了高压相变，这可能是由于结构扭曲和主客相互作用增强所致。Ba_(24) Ge_（100）在22gpa以上的压力下不可逆地变成非晶态。该压力小于ⅰ型Ba_8Ge_（43）包合物的压力，说明ⅲ型包合物在高压下结构不如ⅰ型Ge包合物稳定。本研究结果对半导体包合物、高压相变和主客相互作用的一般系统的理解和应用提供了有价值的启示。少

1.High-pressure Raman and XRD measurements of a defect clathrate Ba_8Ge_(43) □_3 have been carried out at room temperature up to 40 GPa. Raman spectra show a pressure-induced phase transition at 8 GPa, which is due to the structural distortion through the three-bonded Ge atoms and to the change in the guest-host electronic interaction.Both Raman spectra and XRD patterns present the evidence for the amorphization of Ba_8Ge_(43) around 30-40 GPa. The isostructural phase transition associated with the large volume reduction was not observed for Ba_8Ge_(43) up to 40 GPa. The pressure dependence of the lattice constant (a) normalized by a_0 at 1 bar ( ; a/s_0) shows the continuous decrease with pressure until amorphization. From the good coincidence of these curves between Ba_8Ge_(43) and Ba_sSi_(46) at pressures above 15 GPa, we propose that the isostructural phase transition found for Ba_8Si_(46) at 15 GPa may be originated from a defect-induced transformation to Ba_sSi_(43) □_3 with the … More help of their theoretical EOS by the first-principles calculations.2.High-pressure Raman measurements of type-III germanium clathrate Ba_(24)Ge_(100) has been made up to 26 GPa at room temperature. We observed low-frequency vibrational (rattling) modes associated with guest Ba atoms at 25-50 cm^(-1), and host Ge framework vibrations around 50-250 cm^(-1), which are compared with those of silicon clathrate Ba_(24)Si_(100). High-pressure phase transition was found at 3.2 GPa, which seems to be due to the structural distortion combined with the enhanced guest-host interactions.Ba_(24) Ge_(100) becomes irreversibly amorphous at pressures above 22 GPa. This pressure is less than that of type-I Ba_8Ge_(43) clathrate, indicating that type-III structure is less stable than type-I Ge clathrate under high pressures.The present results throw valuable light on the understanding and the application of semiconductor clathrates, the high pressure phase tuansitions, and the general system of guest-host interactions. Less