High-pressure X-ray diffraction study of SrMoO4 and pressure-induced structural changes

SrMoO₄ was studied under compression up to 25 GPa by angle-dispersive X-ray diffraction. A phase transition was observed from the scheelite-structured ambient phase (space group I4₁/a) to a monoclinic fergusonite phase (space group I2/a) at 12.2(9) GPa. The unit-cell parameters of the high-pressure phase are a=5.265(9) Å, b=11.191(9) Å, c=5.195 (5) Å, and β=90.9(1)°, Z=4 at 13.1 GPa. There is no significant volume collapse at the phase transition. No additional phase transitions were observed and on release of pressure the initial phase is recovered, implying that the observed structural modifications are reversible. The reported transition appeared to be a ferroelastic second-order transformation producing a structure that is a monoclinic distortion of the low-pressure phase and was previously observed in compounds isostructural to SrMoO₄. A possible mechanism for the transition is proposed and its character is discussed in terms of the present data and the Landau theory. Finally, the room temperature equation of states is reported and the anisotropic compressibility of the studied crystal is discussed in terms of the compression of the Sr-O and Mo-O bonds.