Cation ordering, domain growth, and zinc loss in the microwave dielectric oxide Ba₃ZnTa₂O₉-δ

The perovskite Ba₃ZnTa₂O₉ (BZT) is the parent of a family of high dielectric constant, low-loss oxides currently applied in mobile telecommunications base stations. The preparation of BZT microwave dielectric resonators with optimal dielectric properties requires careful attention to thermal treatment to control the formation of domains in which the Ta and Zn cations order on the octahedral sites of the perovskite structure. In this paper we use powder X-ray and neutron diffraction together with X-ray fluorescence and electron microscopy to demonstrate the strong correlation between the size of the cation-ordered domains, the extent of the ordering within them, and their composition, particularly the ZnO content. Phase separation into two trigonal BZT phases, which differ in the extent of cation order and the size of the ordered domains, is a common feature of all the synthetic procedures. A zinc-deficient impurity phase of nominal stoichiometry Ba₈ZnTa₆O₂₄, involving face- as well as corner-sharing octahedra, is identified in strongly zinc-deficient samples. A mechanism in which a fully ordered BZT phase grows at the expense of a slightly Zn-deficient and partially ordered phase is proposed.