X-ray and thermal investigations of LaFeTeO6 and LaCrTeO6 compounds

LaFeTeO6 and LaCrTeO6 were prepared by the solid-state reaction of La2O3, M2O3 (M = Fe or Cr) and TeO2 in 1:1:2 molar ratio. The compounds were characterised by X-ray and thermogravimetric techniques. Thermal and X-ray studies indicated that LaFeTeO6 and LaCrTeO6 vapourize incongruently according to the reactions:LaFeTeO6(s) → LaFeO3(s) + TeO2(g) + (1/2)O2(g)LaCrTeO6(s) → LaCrO3(s) + TeO2(g) + (1/2)O2(g)X-ray diffraction data of both the compounds have been indexed on the hexagonal system. Vapour pressures of TeO2(g) over LaFeO3(s) and LaCrO3(s) were independently measured by employing the Knudsen effusion mass loss (KEML) technique. From the partial pressures of TeO2(g) measured, the standard Gibbs free energy of formation of (ΔfG°) LaFeTeO6(s) and LaCrTeO6(s) were obtained and could be represented by the following relations:ΔfG°(LaFeTeO6(s))±15 kJ/mol=−1902.0+0.5953T (1104–1205 K),  ΔfG°(LaCrTeO6(s))±12 kJ/mol=−612.0+0.3488T (1106–1210 K)