In situ high temperature powder neutron diffraction study of undoped and Ca-doped La28−xW4+xO54+3x/2 (x = 0.85)

Abstract

In situ neutron diffraction experiments of 2% Ca-doped and nominally undoped lanthanum tungstate (La_28−xW_4+xO_54+3x/2, with x = 0.85) have been carried out under controlled pD₂O and pO₂ at elevated temperatures. All the diffraction patterns could be refined using an average cubic fluorite-related structure, in accordance with recent reports. The material exhibits disorder of the oxygen and the cation sublattices. Splitting of the oxygen sites around tungsten from the 32f to 96k Wyckoff position in the Fmm space group improves the model and can better represent the oxygen disorder. No phase transition was detected from room temperature up to 800 °C under any of the studied conditions. Expansion of the unit cell constants in the presence of water at intermediate and low temperatures was correlated with the formation of protonic defects. The thermal expansion coefficient for lanthanum tungstate is rather linear under all studied conditions (∼11 × 10⁻⁶ K⁻¹). The in situ diffraction studies are correlated with dilatometry investigations and conductivity measurements.