High-pressure Raman investigation on CdMoO₄ and pressure-induced phase transformations

Abstract

Cadmium molybdate (CdMoO₄) has been investigated by high-pressure Raman spectroscopy to 40 GPa in the diamond cell. Changes in the Raman spectral features reveal two pressure-induced phase transitions in the pressure range investigated. Detailed analysis of the Raman data and comparison with the spectra of CdWO₄ confirm that the first transition near 12 GPa is from the scheelite phase to the wolframite phase. The second phase change occurring near 25 GPa from the wolframite phase is marked by a large downward shift (120 cm^-1) of the υ₁ mode frequency of Mo-O polyhedron. By comparison with the υ₁ mode frequency of the WO₆, it is concluded that the Mo-O coordination becomes truly octahedral in the new high-pressure phase (phase III) of CdMoO₄. The behavior is compared with that of CdWO₄ which undergoes an analogous pressure-induced phase transition near 20 GPa. Our recent high-pressure x-ray diffraction study on CdMoO₄ fully supports the above conclusions.