High-Pressure Raman Study of Ba Doped Silicon Clathrate

Vibrational properties of Ba doped Si clathrate are investigated at high pressures up to 20 GPa by Raman spectroscopy. Vibrations related with Ba encaged in the Si cages are observed below $100{\mathrm{c}\mathrm{m}}^{-1}$ by low-frequency Raman measurements of ${\mathrm{B}\mathrm{a}}_8{\mathrm{S}\mathrm{i}}_{46}$ and ${\mathrm{B}\mathrm{a}}_{6.6}{\mathrm{S}\mathrm{i}}_{46}$ clathrates. The high-pressure Raman spectra obtained for both compression and decompression processes reveal a new phase transition at 7 GPa, and the reversibility of the phase transition at 15 GPa. We investigate the mechanisms of these phase transitions, the interaction between the guest and host frame, and the volume dependence of the vibrational modes.

Figure 1

Raman spectra of ${\mathrm{B}\mathrm{a}}_8{\mathrm{S}\mathrm{i}}_{46}$ and ${\mathrm{B}\mathrm{a}}_{6.6}{\mathrm{S}\mathrm{i}}_{46}$. In this last compound the Ba is partially removed from the ${\mathrm{S}\mathrm{i}}_{20}$ small cages. The spectra were measured at 0  GPa and room temperature. For comparison, Raman frequencies predicted for hypothetical ${\mathrm{S}\mathrm{i}}_{46}$ by Kahn and Lu [20], Menon et al. [21], and Moriguchi et al. [22] and measured for other Si clathrates by Fang et al. [23]. Frequencies by Kahn and Lu have been scaled from the original results by a factor of 0.85 [20].

Figure 2

Pressure dependence of Raman spectra of ${\mathrm{B}\mathrm{a}}_8{\mathrm{S}\mathrm{i}}_{46}$, measured on the compression and decompression processes. The maximum pressure was 19.8 GPa in the present experiments. Solid circles indicate new peaks concerned with the phase transition at 7 GPa.

Figure 3

Raman frequencies of ${\mathrm{B}\mathrm{a}}_8{\mathrm{S}\mathrm{i}}_{46}$ plotted as a function of pressure. These data were obtained on the compression process.