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

1.在室温高达40 Gpa的条件下，测量了缺陷包层结构Ba_8Ge_(43)-3的高压拉曼光谱和X射线衍射谱。拉曼光谱表明，在8 GPa处发生了压致相变，这是由于Ge原子三键的结构扭曲和客体电子相互作用的改变所致。拉曼光谱和X射线衍射谱都证明了Ba8Ge(43)在30-40 GPa.Ba8Ge(43)在40 Gpa以下未观察到与大体积压缩相关的等结构相变。在1bar(；a/S_0)处由a_0归一化的晶格常数(A)与压力的关系显示出随压力的不断减小直到非晶化。根据Ba8Ge(43)和BaSSi(46)在15 Gpa以上压力下的这些曲线的良好符合，我们认为在15 GPa时发现的Ba8Si46的等结构相变可能起源于缺陷诱导的…向BaSi43-3的转变通过第一性原理的计算，为他们的理论状态方程提供了更多的帮助。2.在室温下，对三型锗笼合物Ba_(24)Ge_(100)的高压拉曼光谱进行了高达26 Gpa的测量。我们在25-50 cm^(-1)处观察到了与客体Ba原子相关的低频振动模，在50-250 cm^(-1)处观察到了宿主Ge骨架振动，并与硅笼合物Ba24Si100的振动进行了比较。在3.2 GPa时发现了高压相变，这可能是由于结构变形和增强的客体相互作用共同作用的结果。在高于22 GPa时，Ba24Ge(100)变为不可逆非晶态。这一压力比I型Ba8Ge(43)包合物的压力小，表明III型结构在高压下的稳定性不如I型Ge包合物。这一结果对理解和应用半导体包合物、高压相变和一般的客体相互作用体系具有重要意义。较少

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