A high pressure X-ray diffraction study of KTb(MoO₄)₂: pressure-induced phase transition and equation of state

In situ high pressure X-ray diffraction studies on potassium terbium double molybdate KTb(MoO4)2 were carried out in a diamond-anvil cell up to ~45 GPa at room temperature, using both an in-house X-ray source and synchrotron radiation. The results confirm the phase transition near 3 GPa reported from high-pressure Raman studies and optical observation. This phase transition is from an orthorhombic (phase I) to a monoclinic phase (phase II) and is reversible. The volume change at the transition pressure is insignificant, which implies that there is no change in coordination around Mo in phase (II); this finding is in good agreement with earlier Raman observations. The monoclinic phase becomes highly disordered above 35 GPa. However, no X-ray evidence for complete amorphization is observed to the highest pressure reached, namely 45 GPa in this study. Fitting the P−V data obtained in this study to the Birch-Murnaghan equation of state and assuming the initial derivative of the bulk modulus (K0′) as 4, we obtain the bulk modulus (K0) of 35( +- 2) GPa for phase (I). Using the finite strain analysis, we obtain the zero-pressure bulk modulus (K0) for phase II as 87 GPa