Complex Polymorphic Behavior and Dielectric Properties of Perovskite-Related Sr(SR1/3Nb2/3)O3
Igor Levin, Julia Y. Chan, J J. Scott, L Farber, Terrell A. Vanderah, James E. Maslar
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. Analysis of well-equilibrated specimens annealed at various temperatures indicated that two thermodynamically stable polymorphs occur above and below T=1250 C. The high-temperature (HT) polymorph exhibits an average cubic structure (Fm 3m, 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 6ac 3, b 2ac 2, c ac 6, b 90 ) with a distant, 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-Sr4 Nb2O9 phase upon annealing in the 1100 C-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-Sr4 Nb2O9 was 30 with near-linear temperature dependence. The peculiar dielectric behavior ofHT-Sr4Nb2O9 was ascribed to the competitive responses of nanodomains having somewhat different structures and compositions
, Chan, J.
, Scott, J.
, Farber, L.
, Vanderah, T.
and Maslar, J.
Complex Polymorphic Behavior and Dielectric Properties of Perovskite-Related Sr(SR<sub>1/3</sub>Nb<sub>2/3</sub>)O<sub>3</sub>, Journal of Solid State Chemistry
(Accessed December 10, 2023)