Pressure-induced amorphization of Tb₂ (MoO₄)₃: a high pressure Raman and X-ray diffraction study

Abstract

High pressure Raman and infrared spectroscopic studies on Tb₂ (MoO₄)₃ (TMO) have been carried out at ambient temperature in the diamond cell to 10 GPa hydrostatic pressure. These experiments have revealed pressure-induced phase transitions in TMO near 2 GPa and 6.2 GPa. The first transition is from Pba2 (β') phase to another undetermined crystalline phase designated phase II, and the second transition is to an amorphous state. On releasing pressure there is a partial reversion to the crystalline state. The amorphization has been confirmed by a high pressure X-ray diffraction study on the material. The Raman data indicate that the amorphization is due to disordering of the MoO₄ tetrahedral units. Further, it is inferred from the nature of the Raman bands in the amorphized material that the Mo-O bond lengths and bond angles have a range of values instead of a few set values. The results of the present study as well as previous high pressure-high temperature quenching experiments strongly support that pressure-induced amorphization in TMO is a consequence of the kinetically-impeded β-to-α-phase transition in TMO. The rare earth trimolybdates crystallizing in the β' structure are all expected to undergo similar pressure-induced amorphization.