Crystal Structure and Properties of Ba11FeTi27O66.5

Abstract

AbstractThe crystal structure of Ba11FeTi27O66.5 was determined using single‐crystal and powder X‐ray diffraction methods. This phase crystallizes in the monoclinic space group C2/m (No. 12) (a = 23.324(1) Å, b = 11.388(1) Å, c = 9.8499(3) Å, β = 90.104(3)°; Z = 2; ρcalcd. = 4.98 g/cm3), and exhibits a 10‐layer structure built from close‐packed [O,(Ba,O)] layers with a stacking sequence (cchhc)2. Octahedral sites are occupied by a mixture of Fe3+ and Ti4+, with some preferential ordering suggested by analysis of bond valence sums. The structure features vertex‐, edge‐, and face‐sharing of the [Ti(Fe)O6] octahedra. Indexed X‐ray powder diffraction data for a polycrystalline specimen are given. Ba11FeTi27O66.5 and the 8‐layer phase Ba4Fe2Ti10O27 are built from the same types of polyhedral layers, some of which feature vacant sites between two Ba ions, which substitute for three oxygens in a row. The single‐crystal results suggest that the basic structural formula of the phase is A11B28O66+x, with the value of x (and hence the Fe/Ti ratio) determined by partial occupancy of one of these vacant sites. Variation of this occupancy factor with synthesis temperature may account for apparent slight differences in the stoichiometry of this phase in polycrystalline and single‐crystal form. However, solid solution formation was not observed for polycrystalline specimens. A comparison of the crystal structure obtained for Ba11FeTi27O66.5 with that previously proposed for “Zr4+‐stabilized Ba2Ti5O12” indicates that the phase “Ba2Ti5O12” is actually a ternary compound which forms upon addition (either deliberately or inadvertently) of a trivalent ion such as Fe3+ or Al3+. The specimens Ba11Al2Ti26O66, Ba11Al2Ti24Sn2O66, and Ba11Al2Ti24Zr2O66 were also prepared and were found to form the A11B28O66+x‐type phase. Ba11FeTi27O66.5 exhibits paramagnetic behavior that deviates somewhat from the Curie−Weiss Law below 75 K. Application of this formalism to the 1/χ vs. T data above 75 K yields an effective moment consistent with the presence of high‐spin Fe3+ (S = 5/2), and a negative Weiss constant (about −25 K) indicating weak cooperative magnetic interactions that are overall antiferromagnetic. The relative permittivity and dielectric loss tangent of a sintered polycrystalline disk were measured at 5.33 GHz, yielding values (corrected for theoretical density) of 55 and 7.7(±0.3) × 10−4, respectively. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)