Characterization of ternary compounds in the BaO : Fe2O3 : TiO2 system: Ba6Fe45Ti17O106 and BaFe11Ti3O23

Single crystals of Ba6Fe45Ti17O106 and BaFe11Ti3O23 were obtained as major and minor coproducts, respectively, by slow-cooling an off-stoichiometric B aO:Fe2O3:TiO2 melt. The former compound exhibits variable stoichiometry, Ba 6Fe48-xTi14+xO106, with the Fe:Ti ratio dependent upon the partial pressure of oxygen. The value of x corresponds to the equivalents of reduction that occur to maintain electroneutrality as the Ti-content increases. When prep ared in air, this phase occurs at x = 3 with the stoichiometry Ba6Fe45Ti17O 106, while in 100% oxygen the x-value approaches zero with the resulting st oichiometry Ba6Fe48Ti14O106 (all Fe3+ and Ti4+). The structures of Ba6Fe45T i17O106 and BaFe11Ti3O23 were solved using single-crystal X-ray diffraction methods. Ba6Fe45Ti17O106 was prepared in polycrystalline form, and further structural details, including accurate Fe/Ti occupancy factors, were deter mined by a combined refinement using neutron and synchrotron powder diffrac tion data. (Ba6Fe45Ti17O106: Space group C2/m (No. 12); 19.390(1) Angstrom, b = 20.260(1) Angstrom, c = 10.076(1) Angstrom, beta= 105.27(1)degrees; V = 3818.5(3) Angstrom(3); Z = 2; rho(calc) = 5.08 g/cm(3). Ba6Fe11 Ti3O23: S pace group C2/c (No. 15); a = 19.561(1) Angstrom, b= 8.6614(7) A, c = 10.12 0(1) Angstrom, beta = 105.62(1)degrees; V = 1651.1(3) Angstrom(3); Z = 4; r ho(calc)=5.08g/cm(3).) Both compounds adopt eight-layer close-packed struct ures built from alternating ccp and hcp [O,(Ba, O)] layers stacked along th e a-direction with a (ch)(4) repeat sequence. Both structures feature octah edral sites occupied by a mixture of Fe and Ti as well as tetrahedral sites occupied by Fe3+; the structural formulas are (Ba6 Fe6)-Ba-XII-Fe-IV (VI)( Fe39Ti17)O-106 and Ba-XII Fe-IV(2) (VI)(Fe9Ti3)O-23. Both compounds are par tially reduced; the former contains 3 moles of Fe2+ (or Ti3+) per formula u nit, and the latter contains 1 mole. The formation of Fe2+ is considered mo re likely than Ti3+, but could not be experimentally confirmed. BaFe11Ti3O2 3 is apparently metastable in air when cooled from above the solidus and co uld not be prepared as a polycrystalline sample. Indexed experimental Xray powder diffraction data for Ba6Fe45Ti17O106 are given. Polycrystalline samp les of this compound were used to measure its magnetic and electrical prope rties. The magnetic behavior of Ba6Fe45Ti17O106 above room temperature up t o 1073K was found to obey the Curie-Weiss law, which indicated a small effe ctive magnetic moment (34 mu(B) per mole Ba6Fe45Ti17O106) and a large negat ive temperature intercept (- 806 K). Electrical resistivity measurements be tween room temperature and 120 K revealed nonmetallic behavior with an acti vation energy on the order of 0.17eV. At 347 MWz under ambient conditions, Ba6Fe45Ti17O106 exhibited a relative p ermittivity of 24 and a dielectric loss tangent of 0.10. (C) 1999 Academic Press.