Raman and x-ray diffraction studies of Ba doped germanium clathrate Ba8Ge43 at high pressures

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High-pressure Raman and x-ray diffraxtion (XRD) measurements of a defect clathrate Ba8Ge43□3 have been carried out at room temperature up to 40 GPa. Three vibrational modes associated with guest Ba atoms were observed in the low-frequency region, and the structureless spectrum due to Ge vacancies was found in the framework vibrational region. The Raman spectra shows 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 Ba8Ge43 around 30–40 GPa. The isostructural phase transition often present in intercalated silicon clathrates and associated with a large volume reduction was not observed for Ba8Ge43 up to 40 GPa. The pressure dependence of the lattice constant (a) normalized by a0 at 1 bar (a/a0) shows the continuous decrease with pressure until amorphization. From the good coincidence of this curve between Ba8Ge43 and Ba8Si46 at pressures above 15 GPa, we propose that the isostructural phase transition found for Ba8Si46 at 15 GPa may be originated from a defect-induced transformation to Ba8Si43□3 with the help of their theoretical equation of state by the first-principles calculations.
2007-05-15