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) Å, V = 248.11(3) Å3, 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 MgO and WO 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.