Order–disorder in In3+ perovskites: The example of A(In2/3B″1/3)O3 (A=Ba, Sr; B″=W, U)

We describe the preparation and structural characterization of four In-containing perovskites from neutron powder diffraction (NPD) and X-ray powder diffraction (XRPD) data. Sr3In2B″O9 and Ba(In2/3B″1/3)O3 (B″=W, U) were synthesized by standard ceramic procedures. The crystal structure of the W-containing perovskites and Ba(In2/3U1/3)O3 have been revisited based on our high-resolution NPD and XRPD data, while for the new U-containing perovskite Sr3In2UO9 the structural refinement was carried out from high-resolution XRPD data. At room temperature, the crystal structure for the two Sr phases is monoclinic, space group P21/n, where the In atoms occupy two different sites Sr2[In]2d[In1/3B″2/3]2cO6, with a=5.7548(2) Å, b=5.7706(2) Å, c=8.1432(3) Å, β=90.01(1)° for B″=W and a=5.861(1) Å, b=5.908(1) Å, c=8.315(2) Å, β=89.98(1)° for B″=U. The two phases with A=Ba should be described in a simple cubic perovskite unit cell (S.G. Pm3¯m) with In and B″ distributed at random at the octahedral sites, with a=4.16111(1) Å and 4.24941(1) Å for W and U compounds, respectively.

The structure of the new uranium-based double perovskite Sr3In2UO9 is described and the true symmetry of the other title compounds are revisited. The presence of long-range ordering in the Sr samples, by contrast with the Ba perovskites, is related with the smaller unit cell and B–B distances in the Sr oxides, promoting the electrostatic repulsions between highly charged W6+ and U6+ cations as driving force for the long-range B-site ordering.Figure optionsDownload as PowerPoint slide