Anisotropic thermal expansion behavior in tetragonal Sr2MgWO6

The colorless sample of Sr2MgWO6 was prepared by conventional solid-state reaction of stoichiometric amounts of SrCO3, WO3 and MgO and characterized by powder X-ray diffraction (XRD) studies. In the temperature range of 300-15 K, detailed structural studies on Sr2MgWO6 were carried out by Rietveld refinements of the observed powder neutron diffraction (ND) data. At ambient temperature Sr2MgWO6 crystallizes in tetragonal (space group I4/m) lattice with unit cell parameters: a = 5.5882(2) and c = 7.9452(7) A, V = 248.11(3) A3, Z = 2. This tetragonal (I4/m) structure is retained down to 15 K. The structural analysis of the sample at different temperatures indicates no appreciable change in the Mg-O and W-O bond lengths. However, a decrease in the tilt angle is observed with increasing temperature. The negative thermal expansion of the c-axis is attributed to increasing transverse displacement amplitude of the bridging oxygen atoms along this axis