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

1.在室温至40 GPa的高压下，对缺陷笼形物Ba_8Ge_（43）□_3进行了高压拉曼和XRD测量。拉曼光谱表明，在8 GPa压力下，由于Ge原子的三键结构畸变和主客电子相互作用的改变，Ba_8Ge_（43）在30-40 GPa压力下发生了非晶化。Ba_8Ge_（43）在40 GPa压力下没有观察到与大体积收缩有关的等结构相变。在1巴（; a/s_0）下用a_0归一化的晶格常数（a）的压力依赖性示出了随着压力的连续减小直到非晶化。从Ba_8Ge_（43）和Ba_sSi_（46）在15 GPa以上的曲线的良好重合性，我们认为Ba_8Si_（46）在15 GPa下的等结构相变可能是由缺陷诱导的Ba_sSi_（43）□_3相变引起的，其中 关于我们 2.在室温下，对第三型锗包合物Ba_（24）Ge_（100）进行了高达26 GPa的高压拉曼测量。我们观察到了与客体Ba原子在25-50 cm^（-1）处有关的低频振动模式和主体Ge骨架在50-250 cm^（-1）处的振动模式，并与硅包合物Ba_（24）Si_（100）的振动模式进行了比较。在3.2GPa时，Ba_（24）Ge_（100）发生高压相变，这可能是由于结构畸变和主客体相互作用增强的结果，当压力超过22 GPa时，Ba_（24）Ge_（100）发生不可逆的非晶化。这一压力小于I型Ba_8Ge_（43）包合物的压力，表明在高压下III型结构的稳定性不如I型Ge包合物，这一结果对理解和应用半导体包合物、高压相变和一般的主客体相互作用体系都有重要意义。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