Revisiting pressure-induced phase transition in silicon clathrates using Ge substitution

$\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{39}\mathrm{G}{\mathrm{e}}_{7}$ and $\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{29}\mathrm{G}{\mathrm{e}}_{17}$ have been studied at high pressure using x-ray diffraction and x-ray absorption spectroscopy (XAS) at the Ge $K$ edge. In $\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{39}\mathrm{G}{\mathrm{e}}_{7}$, a transition is observed similar to the one in $\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{46}$, apparently isostructural. However, the XAS data analysis shows that the transformation is related to the off-centering of the Ba atoms. A theoretical model based on a Landau potential suggests that this transition is second order, with a symmetry-breaking mechanism related to the Ba displacement probably initiated by the vacancy creation or local distortion predicted theoretically. This analysis gives a coherent picture of the phase transition mechanism. In the case of $\mathrm{B}{\mathrm{a}}_{8}\mathrm{S}{\mathrm{i}}_{29}\mathrm{G}{\mathrm{e}}_{17}$, such phase transition is not observed as the Ba atoms appear already off-center at ambient pressure.