Pressure-induced structural transitions in MOO3·xH2O (x = 1/2, 2) molybdenum trioxide hydrates : a raman study

The high-pressure behaviors of MOO3·1/2H2O and MOO3·2H2O have been investigated by Raman spectroscopy in a diamond anvil cell up to 31.3 and 30.3 GPa, respectively. In the pressure range up to around 30 GPa, both MOO3·1/2H2O and MOO3·2H2O undergo two reversible structural phase transitions. We observed a subtle structural transition due to O−H···O hydrogen bond in MOO3·1/2H2O at 3.3 GPa. We found a soft mode phase transition in MOO3·2H2O at 6.6 GPa. At higher pressures, a frequency discontinuity shift and appearance of new peaks occurred in both MOO3·1/2H2O and MOO3·2H2O, indicating that the second phase transition is a first-order transition. The frequency redshift of the O−H stretching bands of MOO3·1/2H2O and MOO3·2H2O are believed to be related to the enhancement of the O−H···O weak hydrogen bonds under high pressures.