Role of crystal structure on the thermal expansion of Ln 2W 3O 12 (Ln = La, Nd, Dy, Y, Er and Yb)

Abstract

The negative thermal expansion material Y 2W 3O 12 belongs to Ln 2W 3O 12 family of compositions. The thermal expansion behavior of Ln 2W 3O 12 (Ln = La, Nd, Dy, Y, Er and Yb) members synthesized by the solid-state reaction have been studied and correlated to their crystal structure. The lighter rare earth tungstates (Ln = La, Nd and Dy) crystallize in monoclinic structure ( C 2 / c ) whereas the heavy rare earth tungstates (Ln = Y, Er and Yb) form the trihydrate orthorhombic Ln 2W 3O 12⋅3H 2O at room temperature and above 400 K transforms to unhydrated orthorhombic structure ( P n c a ). The hot pressed (1273 K and 25 MPa) ceramic pellets have been studied for thermal expansion property by dilatometry and high temperature X-ray diffraction. The heavy rare earth tungstates show a large initial expansion up to 400 K, followed by a thermal contraction. The light rare earth tungstates, on the other hand, show thermal expansion. The difference in the thermal expansion behavior in Ln 2W 3O 12 series is attributed to the difference in the structural features. The heavy rare earth tungstates have corner sharing of LnO 6 octahedra with WO 4 tetrahedra, where the now well established mechanism of transverse vibrations operate. The light rare earth tungstates have edge sharing of LnO 8 polyhedra where in such a mechanism is absent.