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

The high-pressure behavior of molybdenum trioxides $\left({\text{MoO}}_3\right)$ 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 $\alpha {\text{-MoO}}_3$ phase $\left(Pbnm\right)$ to the monoclinic ${\text{MoO}}_3\text{-II}$ phase $\left(P{2}_1/m\right)$ , and then to the monoclinic ${\text{MoO}}_3\text{-III}$ phase $\left(P{2}_1/c\right)$ , occurred at pressures of about 12 and 25 GPa at room temperature, respectively. Our observation of the transition from the orthorhombic $\alpha {\text{-MoO}}_3$ to the monoclinic ${\text{MoO}}_3\text{-II}$ 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 $B_0=143.41\left(3\right)\text{GPa}$ with $B_0^{\prime }=12$ , and $B_0=261.9\left(3\right)\text{GPa}$ with $B_0^{\prime }=3.5$ .