Browsing by Author "Rowda, B"

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    Structures of 6H perovskites Ba3CaSb2O9 and Ba3SrSb2O9 determined by synchrotron x-ray diffraction, neutron powder diffraction and ab initio calculations
    (International Union of Crystallography, 2008-04) Rowda, B; Avdeev, M; Lee, PL; Henry, PF; Ling, CD
    The structures of the 6H perovskites Ba3 B 2+Sb5+ 2O9, B = Ca and Sr, have been solved and refined using synchrotron X-ray and neutron powder diffraction data. Ba3CaSb2O9 and Ba3SrSb2O9 have monoclinic C2/c and triclinic space-group symmetries, respectively, while Ba3MgSb2O9 has ideal hexagonal P63/mmc space-group symmetry. The symmetry-lowering distortions are a consequence of internal chemical pressure' owing to the increasing effective ionic radius of the alkaline-earth cation in the perovskite B site from Mg2+ (0.72 Å) to Ca2+ (1.00 Å) to Sr2+ (1.18 Å). Increasing the effective ionic radius further to Ba2+ (1.35 Å) leads to decomposition at room temperature. The driving force behind the transition from P63/mmc to C2/c is the need to alleviate underbonding of Ba2+ cations in the perovskite A site via octahedral rotations, while the transition from C2/c to is driven by the need to regularize the shape of the Sb2O9 face-sharing octahedral dimers. Ab initio geometry-optimization calculations were used to find a triclinic starting model for Ba3SrSb2O9. © 2008, International Union of Crystallography
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    Structures, phase transitions and microwave dielectric properties of the 6H perovskites Ba3BSb2O9, B=Mg, Ca, Sr, Ba
    (Elsevier, 2009-03) Ling, CD; Rowda, B; Avdeev, M; Pullar, R
    We present a complete temperature-composition phase diagram for Ba3BSb2O9, B = Mg, Ca, Sr, Ba, along with their electrical behavior as a function of B. These compounds have long been recognized as 6H-type perovskites, but (with the exception of B = Mg) their exact Structures and properties were unknown clue to their low symmetries, temperature-dependent phase transitions, and difficulties in synthesizing pure samples. The full range of possible space group symmetries is observed, from ideal hexagonal P6(3)/mmc to monoclinic C2/c to triclinic P1. Direct second-order transitions between these phases are plausible according to group theory, and no evidence was seen for my further intermediate phases. The phase diagram with respect to temperature and the effective ionic radius Of B is remarkably symmetrical for B = Mg, Ca, and Sr. For B = Ba, a first-order phase transition to a locally distorted phase allows a metastable hexagonal phase to persist to lower temperatures than expected before decomposing around 600 K. Electrical measurements revealed that dielectric permittivity corrected for porosity does not change significantly as a function of B and is in a good agreement with the values predicted by the Clausius-Mossotti equation. © 2008, Elsevier Ltd.