A comparative study of negative thermal expansion materials Sc 2(MoO 4) 3 and Al 2(WO 4) 3 crystals

Abstract

Polarized Raman spectra and polycrystalline spectra of isomorphous Sc 2(MoO 4) 3 and Al 2(WO 4) 3 crystals have been measured at room temperature and ambient pressure. Some Raman spectra of Sc 2(MoO 4) 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 Al 2(WO 4) 3 material compared with Sc 2(MoO 4) 3 material was observed. The pressure dependent studies revealed that the 362 and 433 cm −1 modes of Sc 2(MoO 4) 3 and Al 2(WO 4) 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.