Negative thermal expansion in $Er_2W_3O_{12}$ and $Yb_2W_3O_{12}$ by high temperature X-ray diffraction

Negative thermal expansion in the orthorhombic members of $A_2W_3O_{12}$ (A = Er, Yb, Y and Lu) tungstates is measured by high temperature X-ray diffraction. Two new compounds showing contraction in this iso-structural series, $Yb_2W_3O_{12}$ and Er2W3O12 are identified. Axial thermal expansion coefficients of $Y_2W_3O_{12}$, $Er_2W_3O_{12}$, $Yb_2W_3O_{12}$ and $Lu_2W_3O_{12}$ calculated from high temperature X-ray diffraction (473–1073 K) shows rare earth size effect. The linear thermal expansion coefficient decreases as the ‘A’ cation size increases and the thermal contraction along the b-axis becomes more negative