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.

Graphical abstractSchematic 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 full-size imageDownload 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.