Study of Ba₃M^IIM^IVWO₉ (M^II = Ca, Zn; M^IV = Ti, Zr) perovskite oxides: competition between 3C and 6H perovskite structures

Abstract

We describe an investigation of Ba₃M^IIM^IVWO₉ oxides for M^II = Ca, Zn, and other divalent metals and M^IV = Ti, Zr. In general, a 1:2-ordered 6H (hexagonal, P6₃/mmc) perovskite structure is stabilized at high temperatures (1300°C) for all of the Ba₃M^IITiWO₉ oxides investigated. An intermediate phase possessing a partially ordered 1:1 double perovskite (3C) structure with the cation distribution, Ba₂(Zn_2/3Ti_1/3)(W_2/3Ti_1/3)O₆, is obtained at 1200°C for Ba₃ZnTiWO₉. Sr substitution for Ba in the latter stabilizes the cubic 3C structure instead of the 6H structure. A metastable Ba₃CaZrWO₉ that adopts the 3C (cubic, Fm₃m) structure has also been synthesized by a low-temperature metathesis route. Besides yielding several new perovskite oxides that may be useful as dielectric ceramics, the present investigation provides new insights into the complex interplay of crystal chemistry (tolerance factor) and chemical bonding (anion polarization and d⁰-induced distortion of metal-oxygen octahedra) in the stabilization of 6H versus 3C perovskite structures for the Ba₃M^IIM^IVWO₉ series.