Complex Polymorphic Behavior and Dielectric Properties of Perovskite-Related Sr (Sr1/3Nb2/3)O3

The complex structural behavior of the perovskite-related compound Sr4Nb2O9 [=Sr (Sr1/3Nb2/3)O3] has been investigated using electron, X-ray, and neutron powder diffraction. nalysis of well-equilibrated specimens annealed at various temperatures indicated that two Athermodynamically stable polymorphs occur above and below T=1250°C. The hightemperature (HT) polymorph exhibits an average cubic structure (Fm m, a~2ac, where "c" refers to the cubic ~4 ? perovskite unit cell) with 1:1 (NaCl-type) partial ordering of Sr2+ and Nb5+ on the B sites. The low-temperature (LT) phase is monoclinic (P21/n, a~6acsquare root=radical beta°) with a distinct, yet unknown, B-cation arrangement. Examination of the HT cubic polymorph using electron diffraction revealed diffuse intensity contours that were consistent with additional local ordering/cluster formation on the sites of the mixed (Sr/Nb) fcc cubic sublattice of the 1:1 ordered B-cation array. Concentration of intensity observed at certain points within the diffuse intensity contours was attributed to superstructure formation. Additionally, refinements of the HT structure using X-ray and neutron powder diffraction data indicated local displacements of both oxygen and A-site Sr atoms which resembled those associated with octahedral tilting. HT-Sr4Nb2O9 specimens subsequently annealed at 900°C exhibited remarkably complex chemical and structural behavior: The contours of diffuse intensity were replaced by arrays of sharp superlattice reflections corresponding to at least four distinct metastable superstructures, all derived from the 1:1 B-cation arrangement. Compositional analysis indicated that these phases exhibit Sr/Nb ratios slightly different from 2/1; that is, they are not true polymorphs, but rather a series of structurally distinct phases with compositions near Sr4Nb2O9. The metastable superstructures were attributed to ordering of Sr and Nb on the mixed B sites in the 1:1 ordered array, possibly combined with either A-site or oxygen vacancy ordering, depending on the Sr/Nb ratio in the particular phase. The metastable phases transform to the stable LT-Sr4Nb2O9 phase upon annealing in the 1100–1200°C temperature range. The dielectric properties of HT- and LT-Sr4Nb2O9 were measured by capacitive methods at 1 MHz. HT-Sr4Nb2O9 exhibited an ambient permittivity of 40 with a non-monotonic temperature dependence, while that for LT-Sr4Nb2O9 was 30 with near-linear temperature dependence. The peculiar dielectric behavior of HT-Sr4Nb2O9 was ascribed to the competitive responses of nanodomains having somewhat different structures and compositions.