Raman studies of A2MWO6 tungstate double perovskites

Abstract

The Raman spectra of seven A₂MWO₆ tungstate double perovskites are analysed. Ba₂MgWO₆ is a cubic double perovskite with Fmm symmetry and its Raman spectrum contain three modes that can be assigned in a straightforward manner. A fourth mode, the asymmetric stretch of the [WO₆]⁶⁻ octahedron, is too weak to be observed. The symmetry of Ba₂CaWO₆ is lowered to tetragonal I4/m due to octahedral tilting, but the distortion is sufficiently subtle that the extra bands predicted to appear in the Raman spectrum are not observed. The remaining five compounds have additional octahedral tilts that lower the symmetry to monoclinic P2₁/n. The further reduction of symmetry leads to the appearance of additional lattice modes involving translations of the A-site cations and librations of the octahedra. Comparing the Raman spectra of fourteen different A₂MWO₆ tungstate double perovskites shows that the frequency of the symmetric stretch (ν₁) of the [WO₆]⁶⁻ octahedron is relatively low for cubic perovskites with tolerance factors greater than one due to underbonding of the tungsten and/or M cation. The frequency of this mode increases rapidly as the tolerance factor drops below one, before decreasing gradually as the octahedral tilting gets larger. The frequency of the oxygen bending mode (ν₅) is shown to be dependent on the mass of the A-site cation due to coupling of the internal bending mode with external A-site cation translation modes.