Crystal structure and properties of Ba5Fe4Ti10O31

Using single-crystal and powder X-ray diffraction methods, the crystal structure of Ba5Fe4Ti10O31 was determined and its magnetic and dielectric properties were measured. Ba5Fe4Ti10O31 crystallizes in space group P6(3)/mcm (No. 193) (a=9.9886(2), c=42.226(2) Angstrom: Z=6: rho(calc)=5.15 g cm(-3)) and exhibits an 18-layer close-packed structure built from vacancy-free [O,(Ba,O)] layers. The structure features octahedral sites occupied by a mixture of Fe3+ and Ti4+, with some preferential ordering, and tetrahedral sites occupied by Fe3+ one of which shares faces and is half occupied. Some Ba ions in :he structure display (9+3) coordination with three unusually long Ba-O bond distances. Computation of bond valence sums about the cation sites using the observed bond distances reveals significant deviations from the valence sum rule, indicating that the structure of Ba5Fe4Ti10O31 contains residual strain not relieved by distortion. Indexed experimental X-ray powder diffraction data for Ba5Fe4Ti10O31 are given and are in excellent agreement with those calculated from the results of the single-crystal structure determination. The compound exhibits approximately paramagnetic behavior that deviates somewhat from the Curie Law. Application of this formalism to the 1/chi vs. T data above 250 K yields an effective moment consistent with the presence of high-spin Fe3+ (S=5/2), and a negative Weiss constant (approximate to-190 K) indicating cooperative magnetic interactions that are overall antiferromagnetic. The relative permittivity and dielectric loss tangent of a sintered polycrystalline disk of Ba5Fe4Ti10O31 were measured between 7.1 and 7.7 GHz, yielding values (corrected for theoretical density) of 32 and 3.3(+/-0.3)x10(-3), respectively, that were essentially independent of frequency. (C) 1998 Elsevier Science S.A.