Redetermination of the structure of ALa2WO7 (A=Ba, Sr) with fluorite-like metal ordering

The crystal structures of ALa2WO7 (A=Ba, and Sr) at room temperature were re-determined by the Rietveld method using the combined X-ray and neutron powder diffraction data. The compounds are confirmed to be isomorphic, crystallizing in the space group P1121/b. In ALa2WO7 the ordering of metal atoms is fluorite-like, but it differs from that of the fluorite-defect compounds of the formula Ln3MO7 (Ln=lanthanide or Y, M=pentavalent metal). The structure of ALa2WO7 consists of isolated WO6 octahedra, whereas in the normal Ln3MO7 the MO6 octahedra share corners forming one-dimensional chains. Although ALa2WO7 has a centric space group, La ions are not situate at the centre of symmetry, which explains the 5D0→7F2 transition being dominant in emission spectrum of Eu-doped materials.

Schematic drawing of the crystal structures of ALa2WO7 (A=Ba, Sr) (left) and Ln3MO7 (right) showing the different arrangement of metal octahedra. Note that the example of Ln3MO7 is the structure of La3TaO7 with the space group Cmcm. Figure optionsDownload as PowerPoint slideHighlights
► The structural ambiguity of BaLa2WO7 has been resolved.
► The details of the crystal structure of SrLa2WO7 are reported for the first time.
► ALa2WO7 (A=Ba, Sr) closely resemble β-La3RuO7 and one of the La3IrO7 polymorph.