X-RAY ROOM-TEMPERATURE STRUCTURE FROM SINGLE-CRYSTAL DATA, POWDER DIFFRACTION MEASUREMENTS AND OPTICAL STUDIES OF THE AURIVILLIUS PHASE BI5(TI3FE)O15

The room temperature structure of the ferroelastic and presumably ferroelectric Aurivillius type compound Bi5(Ti3Fe)O15 (M(r) = 1484.4, D(x) = 8.065 Mg/m3, Z = 4) was refined by X-rays on a weakly twinned crystal prepared in a Bi2O3/B2O3 flux with TiO2/Fe2O3 in stoichiometrical proportions. The lattice constants were found to be orthorhombic with a = 5.4318(6), b = 41.149(4) and c = 5.4691(12)angstrom. Refinement in space group Fmm2 converged at R(R(w)) = 4.6 (4.2%) with 538 averaged reflections. As the crystal showed <5% of twin domains, a supplementary powder refinement was performed to confirm space group Fmm2. The refinement gave confidential values R(R(w)) = 3.1 (4.3%) and R(Bragg) = 7.4%. No superstructure reflections were found. The structure is built up of layers, composed of two slightly and two strongly distorted perovskite units. These layers are separated by oxygen layers. Birefringence, DELTAn, was restudied at room temperature on a wedge shaped crystal and found to be 0.080 +/- 4. When studied as a function of temperature DELTAn revealed two phase transitions of first and second order, at 560-degrees-C and 750-degrees-C, respectively. Based on the present structure determination of the Fmm2 room temperature phase, on group subgroup considerations and polarized light microscopy studies, the following phase sequence is suggested: Fmm2 <--(I)-560-degrees-C--> Fmmm <--(II)-750-degrees-C--> I4/mmm. Etching of the crystal surface with concentrated hydrochloric acid produces typical etch figure alignments which point in the direction of the crystal axes a and c and form an angle of 45-degrees with the traces of the (101) domain walls.