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 d ouble molybdate KTb(MoO4)(2) were carried out in a diamond-anvil cell up to similar to 45 GPa at room temperature, using both an in-house X- ray source and synchrotron radiation. The results confirm the phase tr ansition near 3 GPa reported from high-pressure Raman studies and opti cal observation. This phase transition is from an orthorhombic (phase I) to a monoclinic phase (phase II) and is reversible. The volume chan ge at the transition pressure is insignificant, which implies that the re is no change in coordination around Mo in phase (II); this finding is in good agreement with earlier Raman observations. The monoclinic p hase becomes highly disordered above 35 GPa. However, no X-ray evidenc e for complete amorphization is observed to the highest pressure reach ed, 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 initi al derivative of the bulk modulus (K-0') as 4, we obtain the bulk modu lus (K-0) of 35(+/-2) GPa for phase (I). Using the finite strain analy sis, we obtain the zero-pressure bulk modulus (K-0) for phase II as 87 GPa. (C) 1997 Elsevier Science Ltd. All rights reserved.