Negative thermal expansion in rare earth molybdates

Negative thermal expansion in rare earth molybdates of A₂Mo₃O₁₂ family (A=Y, Er, Yb and Lu) is measured by high temperature X-ray diffraction and dilatometry. Rare earth molybdates which are isostructural with the corresponding rare earth tungstates, also exhibit this phenomena attributed to transverse acoustic vibrations. The rare earth molybdates of A₂M₃O₁₂ family with an orthorhombic structure (A=Y, Er, Yb and Lu) are highly hygroscopic and exhibit negative thermal expansion after the complete removal of water molecules. Axial thermal expansion co-efficient calculated from high temperature X-ray diffraction (RT-1073K) shows rare earth size effect. As the ‘A’ cation decreases in size, the thermal expansion co-efficient along ‘b’ axis and the linear thermal expansion co-efficient become less negative. The thermal expansion behaviour of the tetragonal La₂Mo₃O₁₂ is also reported to demonstrate the effect of crystal structure.