Aurivillius Phases in the Bi 4 Ti 3 O 12 /BiFeO 3 System: Thermal Behaviour and Crystal Structure

Four compounds of the Bi 4 Ti 3 O 12 /BiFeO 3 system with the formula Bi 2 Bi n−1 (Ti,Fe) n O 3n+3 , n = 3, 4, 4.5 and 6 were studied using high‐temperature X‐ray powder diffraction and differential thermoanalysis methods. The crystal structure of the n = 6 phase was refined by the Rietveld method. An unusual behaviour of thermal expansion attributed to an orthorhombic‐to‐tetragonal transformation was revealed. For all the compounds, the lattice parameter c vs temperature T dependence has three regions in the range of T = 20 –750 °C interpreted as (1) expansion of the initial orthorhombic phase, (2) a pronounced structure reconstruction to the tetragonal phase, (3) an expansion of the tetragonal phase. The crystal structure of Bi 7 Ti 3 Fe 3 O 21 based on 6‐layer‐perovskite blocks is proposed from X‐ray powder diffraction data. The Rietveld refinement of the structure in the orthorhombic space group F2mm with lattice parameters a = 5.4699(3), b = 5.4924(3), c = 57.551(3) Å (R p = 9.4, R wp = 11.9, R exp = 4.7, R B = 4.4 %) shows that a distorted 6‐layer model fits the data of Bi 7 Ti 3 Fe 3 O 21 .