High-pressure phase transformations, pressure-induced amorphization, and polyamorphic transition of the clathrateRb6.15Si46

Abstract

The type-I clathrate ${\text{Rb}}_{6.15}{\text{Si}}_{46}$ with partly empty cage sites has been studied up to 36 GPa using Raman spectroscopy, synchrotron x-ray diffraction in diamond-anvil cells, and ab initio total-energy and lattice-dynamics calculations. A first phase transition is observed at $13\ifmmode\pm\else\textpm\fi{}1\text{ }\text{GPa}$ and a ``volume collapse'' transition within the clathrate structure is then observed at $24\ifmmode\pm\else\textpm\fi{}1\text{ }\text{GPa}$. Pressure-induced amorphization into a high-density amorphous (HDA) state occurs above $P=33\ifmmode\pm\else\textpm\fi{}1\text{ }\text{GPa}$. The HDA form transforms into a low-density amorphous polymorph during decompression. During the compression study using angle dispersive synchrotron x-ray diffraction techniques we measured bulk modulus parameters for rocksalt-structured TaO, included adventitiously in the clathrate sample [${K}_{0}=293(3)\text{ }\text{GPa}$ and ${K}_{0}^{\ensuremath{'}}=5.4(3)$].