Thermal expansion properties of A2(MO4)3(A = Ho and Tm; M = W and Mo)

Compounds Ho2W3O12, Ho2Mo3O12, Tm2W3O12 and Tm2Mo3O12 have been prepared by conventional solid-state reaction. Their crystal structures and thermal expansion properties were investigated by room temperature and high temperature X-ray diffractions. It is found that, in certain temperature ranges (200-700 deg C for Ho sample and 200-800 deg C for the others), Ho2Mo3O12, Tm2W3O12 and Tm2Mo3O12 all adopt orthorhombic structure and show negative thermal expansion whereas Ho2W3O12 adopts monoclinic structure and shows positive thermal expansion. The volume thermal expansion coefficients obtained for Ho2W3O12, Ho2Mo3O12, Tm2W3O12 and Tm2Mo3O12 from high temperature XRD are 29.46 X 10-6 deg C-1, -34.50 X 10-6 deg C-1, -11.85 X 10-6 deg C-1 and -12.09 X 10-6 deg C-1, respectively. Thermogravimetry was used to study their hygroscopicity in air.