Structure of triple perovskite BaSr2MgTa2O9 revisited

An experimental investigation of the structure and crystal-chemistry of the triple perovskite BaSr2MgTa2O9 is reported. Polycrystalline BaSr2MgTa2O9 was synthesized via solid-state reaction and its structure quantitatively probed at 10 and 300 ​K using neutron diffraction. Monoclinic (A2/m, a0b¯–b–) and trigonal structural models (P3¯c1, a–a–a–) were fit to diffraction data. The former was found to correctly reproduce experimental intensities, while the latter failed to do so. This difference is attributed to the ability of the monoclinic model to accurately reflect octahedral tilting. BaSr2MgTa2O9 features corner-sharing MgO6 and TaO6 octahedra with barium and strontium cations occupying cubooctahedral holes. Barium and strontium are disordered over the A site, whereas B site cations magnesium and tantalum are ordered. Chemical substitution of strontium for barium in Ba3MgTa2O9 induces tilting of MgO6 and TaO6 octahedra in two directions and geometric distortions of MgO6 octahedra. [Display omitted] •BaSr2MgTa2O9 is a triple perovskite with monoclinic crystal symmetry.•A site cations Ba2+ and Sr2+ are disordered. B site cations Mg2+ and Ta5+ are ordered.•Tilting of MgO6 and TaO6 octahedra occurs in two directions.