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 °C for Ho sample and 200–800 °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 × 10−6 °C−1, −34.50 × 10−6 °C−1, −11.85 × 10−6 °C−1 and −12.09 × 10−6 °C−1, respectively. Thermogravimetry was used to study their hygroscopicity in air.

This figure is the polyhedron model of Ho2W3O12 showing HoO6 octahedra (green) and MoO4 tetrahedra (purple). (For interpretation of the references to colour in this text, the reader is referred to the web version of this article.) In this study Ho2W3O12 was found to adopt monoclinic structure when sample was cooled down slowly in furnace, different from the previous report where Ho2W3O12 was reported to adopt orthorhombic structure when sample was quenched in water. The heat treatment condition plays a big role in determining the structure of Ho2W3O12.Figure optionsDownload as PowerPoint slide