Hygroscopicity and bulk thermal expansion in $Y_2W_3O_{12}$

Negative thermal expansion material, $Y_2W_3O_{12}$ has been synthesized by the solid-state method and bulk thermal expansion of the material has been investigated from 300 to 1100 K. The material reversibly forms a trihydrate composition whose X-ray diffraction pattern can be indexed to an orthorhombic unit cell with $a = 10.098(1) \AA$ , $b = 13.315(3) \AA$, $c = 9.691(4) \AA$. The cell volume of the hydrated pattern is 7% smaller than the unhydrated cell volume. According to the dilatometric studies, the material shows a 3–6% increase in the linear strain at about 400 K, which can be attributed to the removal of water. Sintering the material at 1473 K leads to large grain size of $>100 \mu m$, which results in a large hysteresis in the bulk thermal expansion behavior. Hot pressing at 1273 K under a uniaxial pressure of 25 MPa results in a fine-grained $(2-5 \mu m)$ ceramic. Glazing the ceramic prevents moisture pick up and a linear thermal expansion over the entire temperature range 1100–300 K and an average linear thermal expansion co-efficient of $-9.65 \times 10^{-6}/K$ is observed. The effect of water on the thermal expansion behavior of this system is discussed