High-pressure Raman scattering and x-ray diffraction of phase transitions in MoO3

The high-pressure behavior of molybdenum trioxides (MoO3) has been investigated by angle-dispersive synchrotron x-ray powder diffraction and Raman spectroscopy techniques in a diamond anvil cell up to 43 and 30 GPa, respectively. In the pressure range of up to 43 GPa, structural phase transitions from the orthorhombic alpha-MoO3 phase (Pbnm) to the monoclinic MoO3-II phase (P2(1)/m), and then to the monoclinic MoO3-III phase (P2(1)/c), occurred at pressures of about 12 and 25 GPa at room temperature, respectively. Our observation of the transition from the orthorhombic alpha-MoO3 to the monoclinic MoO3-II phase is in disagreement with earlier studies in which the phase transition could not be obtained when only pressure is applied. The changes in the Mo-O distances and O-Mo-O and Mo-O-Mo angles may explain the changes in Raman spectrum. The pressure dependence of the volume of two monoclinic high-pressure phases is described by a third-order Birch-Murnaghan equation of state, which yields a bulk modulus value of B-0=143.41(3) GPa with B-0(')=12, and B-0=261.9(3) GPa with B-0(')=3.5.