Crystal structure and thermal expansion behavior of Ca2MgWO6

The colorless sample of Ca2MgWO6 has been prepared by conventional solid-state reaction of stoichiometric amounts of CaCO3, WO3 and MgO and characterized by Rietveld refinement of the powder XRD data. This compound crystallizes in a monoclinic lattice (space group: P21/n, No. 14) with unit cell parameters, a = 5.4255(1) Å, b = 5.5510(0) Å, c = 7.7224(1) Å and β = 89.916(2)°, V = 232.574(3) Å3. Both the Mg2+ and W6+ ions have octahedral coordination with oxygen atoms. The Ca2+ ions form distorted eight-coordinated polyhedra with oxygen atoms, with CaO bond lengths ranging from 2.35 to 2.74 Å. The high temperature and thermal expansion behaviors of Ca2MgWO6 have been studied by high temperature XRD (HT-XRD) and dilatometer. At 1000 °C, the unit cell parameters for Ca2MgWO6 are: a = 5.5266(2) Å, b = 5.5733(2) Å, c = 7.8418(4) Å and β = 89.98(1)°, V = 241.54(1) Å3. The unit cell parameters of Ca2MgWO6 lattice gradually increase with the increase in temperature. Also, the monoclinic angle gradually approaches to 90° with increasing temperature. The typical expansion along b axis is found to be significantly lower than the other two axes. In the temperature range of 25–1000 °C, the typical coefficients of thermal expansions coefficients αa, αb and αc are: 18.9(1) × 10−6, 4.0(1) × 10−6 and 15.8(2) × 10−6 °C−1, respectively. A small discontinuity around 700 °C is observed in the variation of unit cell parameters as well as in the variation of percent linear thermal expansion with temperature. This is attributed to a possible existence of a continuous isomorphous phase transition without any drastic change in the lattice symmetry.