Structure and phase transition of Sn-substituted Zr(1−x)SnxW2O8

Abstract

A conventional solid state reaction between ZrO₂, SnO₂ and WO₃ was used to prepare the negative thermal expansion material Zr_(1−x)Sn_xW₂O₈. The strong negative thermal expansion over a broad temperature range, which is well known for the pure zirconium tungstate compound, is also demonstrated in this substituted material. However, the order–disorder phase transition of the cubic materials was shown to shift towards lower temperatures, dependent on the degree of Sn⁴⁺-substitution, by dilatometry and temperature variable X-ray diffraction. This is attributed to the lower bond strength of the Sn–O bond in comparison to the Zr–O bond. The unit cell parameters of the material are significantly smaller due to the insertion of smaller Sn⁴⁺-cations on the Zr⁴⁺-position in the structure. For one composition (x = 0.3), the structure of Zr_(1−x)Sn_xW₂O₈ was studied by neutron diffraction at two temperatures, 293 K and 473 K, corresponding to respectively the low temperature α-, and high temperature β-polymorph of Zr_(1−x)Sn_xW₂O₈. The refined structures were found to be similar to that of ZrW₂O₈ at the same temperatures. Variable temperature X-ray diffraction of the same sample was used to establish the phase transition temperature, by refining the fractional occupancy of the possible tungstate orientations with temperature.