Aurivillius Phases in the Bi 4 Ti 3 O 12 /BiFeO 3 System: Thermal Behaviour and Crystal Structure M. Krzhizhanovskaya a, *, S. Filatov a , V. Gusarov b , P. Paufler c , R. Bubnova b , M. Morozov b , and D. C. Meyer c St. Petersburg/Russia, a Department of Crystallography, St. Petersburg State University, and b Institute of the Silicate Chemistry of the Russian Academy of Science c Dresden/Germany, Technische Universität, Institut für Strukturphysik Received February 10th, 2005; revised March 21st, 2005. Dedicated to Professor Rüdiger Kniep on the Occasion of his 60 th Birthday Abstract. 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 ther- mal expansion attributed to an orthorhombic-to-tetragonal trans- formation was revealed. For all the compounds, the lattice parame- ter c vs temperature T dependence has three regions in the range of T = 20 -750 °C interpreted as (1) expansion of the initial or- thorhombic phase, (2) a pronounced structure reconstruction to the tetragonal phase, (3) an expansion of the tetragonal phase. Introduction The Aurivillius family of layered Bi-containing oxides is well known for its ferroelectric properties. Although these phases were first described more than 50 years ago [1] and their ferroelectric properties were found 10 years later by [2], layered perovskites still attract considerable interest in particular due to the recent observation of fatigue free be- haviour and low coercive fields in thin films [3]. According to [4] Bi titanates are of great technological interest because of their applications as non-volatile ferroelectric memories and as high-temperature piezoelectic materials. Usually, crystal structures of these phases are described as a combi- nation or intergrowth of (Bi 2 O 2 ) 2+ layers and (A n-1 B n O 3n+3 ) 2- perovskite-like blocks, where A is a twelve co-ordinated cation, e.g. Na, K, Ca, Sr, Ba, Pb, Bi, etc., and B is an octahedral cation such as Fe, Ti, Nb, Ta, Cr, etc. Here n is the number of octahedral layers in a per- ovskite-like block. The structural studies of these phases concern mainly the compounds with n = 1... 4 or their in- tergrowth analogues. Compounds from the Bi 4 Ti 3 O 12 /BiFeO 3 system represent semiconducting, ferroelectric and ferromagnetic properties * Dr. Maria Krzhizhanovskaya Department of Crystallography St.Petersburg State University University Embankment 7/9 St. Petersburg 199034, Russia Tel.: +7 -812-328-1234 fax: +7-812-328-5479 E-mail: masha krj@mail.ru Z. Anorg. Allg. Chem. 2005, 631, 1603-1608 DOI: 10.1002/zaac.200500130  2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1603 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 Riet- veld 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) A ˚ (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 . Keywords: Perovskite phases; Phase transitions; Thermal expan- sion; X-ray scattering [5, 7-9]. Bi 4 Ti 3 O 12 (n=3) shows high-electrical conduc- tivity like Bi 5 Ti 3 FeO 15 (n=4) [4]. For the latter, measure- ments of the magnetoelectric coefficient were reported in [6]. According to [7], Bi 5 Ti 3 FeO 15 and Bi 6 Ti 3 Fe 2 O 18 exhibit simultaneous electrical and magnetic ordering. The mag- netic measurements of Bi 8 Fe 4 Ti 3 O 24 (n=7) indicate antifer- romagnetic behaviour for the compound [8]. An eight- layered compound Bi 9 Fe 5 Ti 3 O 27 behaves like a superparam- agnet [9] and transforms from that state to an antiferromag- netic state around 400 K. In the Bi 4 Ti 3 O 12 /BiFeO 3 system compounds with the general formula Bi 2 Bi n-1 (Ti,Fe) n O 3n+3 ,n = 3, 3.5, 4, 4.5, 5, 6, 8 were synthesised by solid state reaction and charac- terised by X-ray diffraction (XRD) in [5]; the phases with n = 4, 5 and 8 were found before also in [10]. Diffraction data for the n = 3, 4, 5, 8 compounds were presented in the PDF database. The crystal structures of these compounds with n = 3, 3.5, 4 were determined in the orthorhombic system by [1, 11-14, and 21], respectively. The orthorhom- bic cell parameters, a and b, of these structures are in the range 5.40-5.47 A ˚ . The number of perovskite-like layers varies along the [001] direction and the c value changes cor- respondingly: 32.8 A ˚ (n=3) [11], 37.1 A ˚ (n=3-4) [12], 41.2 A ˚ (n=4) [14]. The structure of Bi 4 Ti 3 O 12 was deter- mined both on the basis of an orthorhombic [22] and a monoclinic [15] space group. Up to now the nature of the ferroelectric-paraelectric (FE-PE) transition is not fully resolved. Recently, a number of high temperature powder neutron diffraction (PND) studies has revealed that the initial polar orthorhombic structure transforms either into a non-polar orthorhombic,