Analysis of phase transition and expansion behaviour of Al ₂(WO₄)₃ by temperature-regulated X-ray diffraction

X-ray diffraction measurements of Al₂(WO₄), were carried out under various temperatures from 123 K to 373 K. Diffraction patterns above 273 K and below 253 K could be successfully explained as orthorhombic Pbcn (No. 60) and monoclinic P2₁/n (No. 14) with superstructure, respectively, which were in agreement with the results of convergent-beam electron diffraction. Thermal expansion of low-temperature monoclinic phase and slight shrinkage of high-temperature orthorhombic phase were observed from the temperature dependence of lattice constants and molar volume, showing agreement with the result of dilatometry. It is revealed that abrupt expansion of Al ₂(WO₄), at the structural phase transition is attributed to an abrupt increase of the c-axis, which is parallel to the 2₁ screw axis and perpendicular to the n-glide plane.