Temperature and pressure dependence of Raman-active phonons of CaMoO4:: an anharmonicity study

The Raman spectra of tetragonal CaMoO4 (scheelite structure, C-4h(6) space group) have been measured in the temperature range 12-1300K. At high temperatures, the frequency and linewidth of all Raman-active phonons vary almost linearly with temperature, indicating that the three-phonon decay processes are dominant over the four-phonon ones. All Raman phonons display normal negative (partial derivativeomega/partial derivativeT) slopes throughout the temperature range, except for the lowest-frequency B-g phonon at 111.5 cm(-1) which is almost temperature independent in the region 12-400 K, but then for T > 400 K it shows a slight (normal) softening with temperature. The reduced (partial derivativeomega/partial derivativeT)/omega slopes of the internal modes (vibrations of atoms within the tetrahedral MoO42- units) are an order of magnitude smaller than the respective slopes of the external ones (pure lattice modes). There are no discontinuities or sharp changes of slope in the omega (T) plots, implying that CaMoO4 remains stable over the entire temperature range. Combining the temperature-dependent Raman data of this work and the previously reported pressure-dependent Raman data on this crystal (Christofilos D, Kourouklis G A and Ves S 1995 J. Phys. Chem. Solids 56 1125), as well as the thermal expansion coefficient (T) and compressibility kappa (T) data, it has been possible to separate and evaluate quantitatively the volume (expansion) Deltaomega(vol) and the pure temperature (anharmonic) Deltaomega(anh) contributions to the total Deltaomega(tot) shift of phonons with temperature. It has been found that for most phonons at high temperatures, the volume effect is greater than the pure temperature one, thus indicating that most of the bonds in CaMoO4 are predominantly ionic in character.