High-pressure synthesis and crystal structure of the strontium tungstate Sr₃W₂O₉

The strontium tungstate compound Sr₃W₂O₉ was prepared by a high-pressure synthesis technique. The crystal structure was determined by single-crystal X-ray diffraction and transmission electron microscopy. The structure was found to be a hettotype structure of the high-pressure phase of Ba₃W₂O₉, which has corner-sharing octahedra with a trigonal symmetry. Sr₃W₂O₉ has a monoclinic unit cell of C2/c symmetry. One characteristic of the structure is the breaking of the threefold rotation symmetry existing in the high-pressure phase of Ba₃W₂O₉. The substitution of Sr at the Ba site results in a significant shortening of the interlayer distances of the [AO₃] layers (A = Ba, Sr) and causes a distortion in the crystal structure. In Sr₃W₂O₉, there is an off-centre displacement of W⁶⁺ ions in the WO₆ octahedra. Such a displacement is also observed in the high-pressure phase of Ba₃W₂O₉.