Crystal structure and thermal expansion behavior of Ca2MgWO6

• Published in: Materials chemistry and physics 2006-08, Vol.98 (2-3), p.486-493
• Main Authors: Patwe, S.J., Achary, S.N., Mathews, M.D., Tyagi, A.K.
• Format: Article
• Language: English
• ISSN: 0254-0584
• DOI: 10.1016/j.matchemphys.2005.09.071

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: P2(1)/n, No. 14) with unit cell parameters, a = 5.4255(1)A(ngstrom), b = 5.5510(0)A(ngstrom), c = 7.7224(l) A(ngstrom) and beta = 89.916(2) deg , V= 232.574(3)A(ngstrom)3. Both the Mg2+ and W6+ ions have octahedral coordination with oxygen atoms. The Ca2+ ions form distorted eight-coordinated polyhedra with oxygen atoms, with Ca-O bond lengths ranging from 2.35 to 2.74A(ngstrom). The high temperature and thermal expansion behaviors of Ca2MgWO6 have been studied by high temperature XRD (HT-XRD) and dilatometer. At 1000 deg C, the unit cell parameters for Ca2MgWO6 are: a = 5.5266(2)A(ngstrom), b = 5.5733(2)A(ngstrom), c = 7.8418(4)A(ngstrom) and beta = 89.98(1) deg , V= 241.54(1)A(ngstrom)3. The unit cell parameters of Ca2MgWO6 lattice gradually increase with the increase in temperature. Also, the monoclinic angle gradually approaches to 90 deg 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 deg C, the typical coefficients of thermal expansions coefficients alpha(a), alpha(b )and alpha(c) are: 18.9(1) x 10(-6), 4.0(1) x 10(-6) and 15.8(2) x 10(-6) deg C(-1), respectively. A small discontinuity around 700 deg 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.