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The colorless sample of Ca 2 MgWO 6 has been prepared by conventional solid-state reaction of stoichiometric amounts of CaCO 3 , WO 3 and MgO and characterized by Rietveld refinement of the powder XRD data. This compound crystallizes in a monoclinic lattice (space group: P 2 1 / n , No. 14) with unit cell parameters, a = 5.4255(1) A, b = 5.5510(0) A, c = 7.7224(1) A and β = 89.916(2)°, V = 232.574(3) A 3 . Both the Mg 2+ and W 6+ ions have octahedral coordination with oxygen atoms. The Ca 2+ ions form distorted eight-coordinated polyhedra with oxygen atoms, with Ca O bond lengths ranging from 2.35 to 2.74 A. The high temperature and thermal expansion behaviors of Ca 2 MgWO 6 have been studied by high temperature XRD (HT-XRD) and dilatometer. At 1000 °C, the unit cell parameters for Ca 2 MgWO 6 are: a = 5.5266(2) A, b = 5.5733(2) A, c = 7.8418(4) A and β = 89.98(1)°, V = 241.54(1) A 3 . The unit cell parameters of Ca 2 MgWO 6 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 α a , α b and α c 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. |
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