Negative thermal expansion in rare earth molybdates

Negative thermal expansion in rare earth molybdates of $A_2Mo_3O_{12}$ 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 $La_2Mo_3O_{12}$ is also reported to demonstrate the effect of crystal structure