A comparative study of negative thermal expansion materials Sc2(MoO4)3 and Al2(WO4)3 crystals

Polarized Raman spectra and polycrystalline spectra of isomorphous Sc2(MoO4)3 and Al2(WO4)3 crystals have been measured at room temperature and ambient pressure. Some Raman spectra of Sc2(MoO4)3 were taken at different values of pressures in the range of 0.0–3.0 GPa. Lattice dynamics calculations have been performed on the orthorhombic phase (ambient phase) of these materials using a rigid ion model in order to calculate the phonon frequencies at the Brillouin zone Center (k = 0). The calculated results fit qualitatively well the Raman spectra observed in these materials at room temperature and ambient pressure, and allowed us to establish their Raman signature. A higher coupling among the modes of the Al2(WO4)3 material compared with Sc2(MoO4)3 material was observed. The pressure dependent studies revealed that the 362 and 433 cm−1 modes of Sc2(MoO4)3 and Al2(WO4)3, respectively, exhibit unusually high ∂ω/∂P values. The presence of these unstable modes in both materials is probably related to the mechanism of the phase transition from the monoclinic high-pressure phase to the unknown phase of lower symmetry.