A HIGH-PRESSURE RAMAN-STUDY OF CALCIUM MOLYBDATE

In this work we present a Raman study of CaMoO4 as a function of hydrostatic pressure up to 23 GPa at room temperature. Pressure induced changes in the Raman spectra and in the phonon frequencies are attributed to two phase transitions observed around 8.2 and 15 GPa, which are reversible. The B-g mode at 113 cm(-1), associated with the MoO42--MoO42- vibration along the c-axis, exhibits a softening in the scheelite structure, while in the high pressure phases its slope becomes positive. The nu(1), nu(3), stretching modes of the MoO42- group exhibit drastic changes in their pressure dependencies at both phase transitions. This behavior is connected with the increase in the coordination number of the Mo ion in the high pressure phases. These characteristics are compared with the high pressure behavior of other molybdates and tungstates.