High-Pressure Study of $Gd_2(MoO_4)_3$ by Raman Scattering and Ab Initio Calculations

High-pressure induced phase transitions in a single crystal of gadolinium molybdate, $Gd_2(MoO_4)_3$ were studied by the Raman spectroscopy and ab initio calculations. The amorphization of the sample takes place at about 6 GPa in a mixture of alcohol as a pressure transmitting medium and begins as soon as 3 GPa in argon. In both media, the amorphization is irreversible in the 0-9 GPa investigated pressure range. The joint ab initio and Raman results allowed us to conclude that rotations of $MoO_4$ tetrahedra are the primary structural changes involved in the first phase transition (at about 2 GPa) explaining the softening of the low frequency modes at about 50 $cm^{-1}$. In addition, a progressive distortion of tetrahedra followed by a coordination change (IV-VI) of Mo atoms is observed through the five structural transitions including amorphization. This mechanism based on the steric hindrance of polyhedra is believed to be the most relevant for explaining the amorphization of $Gd_2(MoO_4)_3$.