Pressure-induced phase transitions in select molybdates and tungstates

Abstract

Mao-Bell and symmetric diamond anvil cells were used to study the effect of high-pressure on a series of tungstate and molybdate compounds. The experimental techniques of powder x-ray diffraction and Raman spectroscopy were employed to gain an understanding of the observed phase transitions. Bi[sub]2MoO[sub]6, exhibited a phase transition at ~6.8 GPa that was observed by a stiffening of its bulk modulus from 51 GPa, for the low-pressure phase, to 141.5 GPa for the high-pressure phase. Bi[sub]2W[sub]2O[sub]9 exhibited a phase transition at ~11 GPa that was observed by a change in the x-ray diffraction pattern. Bi[sub]2WO[sub]6 exhibited a phase transition between ~4.8 GPa and 8.3 GPa that was observed by a stiffening of its bulk modulus from 52.2 GPa, for the low-pressure phase, to 121.8 GPa for the high-pressure phase. La[sub]2Mo[sub]4O[sub]15 exhibited two phase transitions one at 2 GPa and another at 17 GPa. The transition at 2 GPa was observed as a change in the x-ray diffraction pattern and has yet to be identified. The transition at 17 GPa was to a glassy, pressure-induced amorphization, state. Sm[sub]2Mo[sub]4O[sub]15 exhibited a phase transition at 5.4 GPa. This transition was observed as a change in the x-ray diffraction pattern and has yet to be identified. MgNb[sub]2O[sub]6 exhibited a phase transition at 10.3 GPa. This transition was observed in both x-ray diffraction and Raman spectroscopy. KNbW[sub]2O[sub]9 exhibited no signs of a phase change up to a pressure of 35 GPa.