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Publication Citation: Subsolidus Phase Equilibria and Crystal Chemistry in the System BaO-TiO2-Ta2O5

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Author(s): Terrell A. Vanderah; Robert S. Roth; T Siegrist; W Febo; J. M. Loezos; Winnie K. Wong-Ng;
Title: Subsolidus Phase Equilibria and Crystal Chemistry in the System BaO-TiO2-Ta2O5
Published: January 01, 2003
Abstract: Subsolidus phase relations have been determined for the BaO: TiO2Ta2O5 system. Approximately 100 specimens were prepared in air at temperatures near but below the solidus (1275 C to 1500 C). For the binary BaO: Ta2O5 subsystem, the formation of all three polymorphs of BaTa2O6 (orthorhombic, tetagonal, and hexagonal) was confirmed. The present study found no stable phases forming between Ba5Ta4O15 and BaO. In the ternary system, along the line BaTiO3-Ba5Ta4O15, the occurrence of the previously reported 8L and 10L hexagonal perovskite derivatives was confirmed, albeit with somewhat wider homogeneity ranges; BaTiO3 dissolves Ta5+ up to the limiting composition BaTi0.64Ta0.29O3 which contains 7% B-cation vacancies and exhibits a dilated cubic unit cell (a=4.0505(2) ). The formation of Ba3Ti4Ta4O21, a member of the hexagonal A3M8O21-type ternary oxides, was confirmed as well as its solid solution, for which a different mechanism of formation is suggested. Several new compounds have been found, including four members of the orthorhombic (space group Cmcm) rutile-slab homologous series, BaTinTa4O11+2n, with n-values 3,5,7, 9; the unit cells for these compounds are given. Three ternary phases with close-packed [Ba,O] layer structures related to that of 6L Ba4Ti13O30 were found: 13L Ba18Ti53Ta2O129, 7Lba10Ti27Ta2O69, and 8Lba6Ti14Ta2O39. The crystal structures of the 13L and 7L phases were determined by single-crystal X-ray diffraction (13L Ba18Ti53Ta2O129: cchcchcchcchc, C2/m, a=9.859(2) , b=17.067(5) , c=30.618(8) , Β=96, 11(2) , Z=2; 7L Ba10Ti27Ta2O69: cchchc, C2/m, a=9.855(3) , b=17.081(7) , c=16719(7) , B=101.18(3) ; z=2(m and are described and compared with those of the 6L and 8L members of this family. Phases with tetragonal tungsten bronze related structures occur over large compositonal ranges in the BaO:TiO2:Ta2O5 system, both within the ternary (Ba6-xTi2-2xTa8+2xO30, x=0->).723), and along the BaO-Ta2O5 binary (from 43 to 26 mol% BaO), which in turn dissolves up to {difference} 12 mol% TiO2 to form extensive single phase fields with different TTB superstructures. X-ray powder diffraction data are given for three binary BaO:Ta2O5 compounds exhibiting the TTB structure and two superstructures. The large compositional ranges of the TTB-type phases are accompanied by only small changes in specific volume. The TTB-type regions within the ternary and those emanating from the BaO-Ta2O5 binary do not co-exist in equilibrium, which likely reflects fundamental differences in the structural mechanisms that accommodate the variable stoichiometries. The similarities and differences between the BaO:TiO2: Ta2O5 and BaO:TiO2Nb2O5 systems are described.
Citation: Solid State Sciences
Volume: 5 No. 1
Keywords: BA18Ti53Ta2O129;Ba3Ti4Ta4O21;Ba6Ti14Ta2O39;BaO:TiO2;barium titanium tantalates;BaTi0.64Ta0.29O3;BaTinTa4O11+2n;Ta2O5;tetragonal tungsten bronze
Research Areas: Ceramics, Materials Science