Materials Chemistry and Physics 75 (2002) 105–109 Doping effects of BiFeO 3 in layered perovskite SrBi 2 Nb 2 O 9 Haoshuang Gu a,b , J.M. Xue a , Xingsen Gao a , John Wang a,∗ a Faculty of Science, Department of Materials Science, National University of Singapore, Singapore 119260, Singapore b Faculty of Physics & Electronic Technology, Hubei University, Wuhan 430062, PR China Abstract Nanocrystallites of ferroelectric xBiFeO 3 –(1 - x)SrBi 2 Nb 2 O 9 (SBFN) with x ranging from 0 to 0.2 were synthesized by mechanical activation of mixed oxides at room temperature. The resulting SBFN exhibit nanocrystalline particles, which consist of layered perovskite crystallites in the range 14–24 nm, depending on the level of BiFeO 3 doping in SBN. Sintered SBFN exhibited a single-phase layered perovskite structure with no detectable secondary phase being present during crystallization. The grain morphology changes from a lamellar structure in SBN to a granular structure at x = 0.2. Lattice dimensions of SBN are decreased slightly by doping with BiFeO 3 , which also leads to a lower sintering temperature, while the Curie point was shifted upwards with increasing BiFeO 3 doping. The peak impedance of SBFN decreases with frequency and increases with increase level of BiFeO 3 doping in SrBi 2 Nb 2 O 9 . A dielectric enhancement was observed in 0.2BiFeO 3 –0.8SrBi 2 Nb 2 O 9 . © 2002 Elsevier Science B.V. All rights reserved. Keywords: xBiFeO 3 –(1 - x)SrBi 2 Nb 2 O 9 ; Mechanical activation; Nanocrystallites; Impedance; Dielectric properties 1. Introduction Bismuth layered perovskite of SrBi 2 Nb 2 O 9 (SBN) be- longs to the family of Aurivillius type structures, which is regular intergrowth of (A m-1 B m O 3m+1 ) 2- perovskite-like layers and bismuth oxygen (Bi 2 O 2 ) 2+ slabs [1]. SBN is ferroelectric at room temperature with the orthorhombic to tetragonal phase transition occurring at 430 ◦ C. It exhibits excellent fatigue resistance and is capable of withstanding 10 12 erase/rewrite operations for nonvolatile random access memory (NvRAM) applications, [2–4] making it an excel- lent candidate for substituting PZT and PZT-based materials [5–8]. The conventional solid-state reaction for synthesiz- ing SBN is limited by the formation of nonferroelectric pyrochore phases resulting from the loss of bismuth compo- nent at elevated temperatures, together with the undesirable particle characteristics derived from the particle coarsening at the calcinations temperature [9]. To overcome the dis- advantages of conventional ceramic processing, alternative routes have been attempted, for example, Asai et al. [10] synthesized SBN by a novel aqueous solution route in order to lower the sintering temperature. Forbess et al. [12], and Wu and Cao [11] observed that substitution of Nb 5+ with vanadium, and Sr 2+ with lanthanum or calcium can lower the sintering temperature and at the same time a degree of enhancement in dielectric properties is also observed. ∗ Corresponding author. Tel.: +65-8742958; fax: +65-7763604. E-mail address: maswangj@nus.edu.sg (J. Wang). Mechanical activation was applied to synthesize amor- phous, nanocrystalline and intermetallic compounds [13–15]. More recently, it has also been employed for syn- thesis of functional ceramic materials with perovskite and layered perovskite type structures [16–18]. Pardo et al. [18] synthesized an amorphous SBN after mechanical activation for 168 h. A significant dielectric enhancement has been observed in SrBi 2 Nb 2 O 9 when doped with BiFeO 3 by sin- tering the mechanically activated oxide composition [19]. In this paper, xBiFeO 3 –(1 - x)SrBi 2 Nb 2 O 9 (where x = 0, 0.1 and 0.2) were synthesized by sintering the mechanically activated oxide compositions, followed by an investigation into the microstructure, dielectric and impedance properties of sintered SBFN. 2. Experiments The experimental procedure used in this work has been detailed in Ref. [13]. The starting materials used were SrO, Bi 2 O 3 , and Nb 2 O 5 , all with purity of >99.5% (Aldrich) and Fe 2 O 3 with purity of >99% (Fluka). These oxides were mixed together at an appropriate weight ratio as designed for each of the various xBiFeO 3 –(1 - x)SrBi 2 Nb 2 O 9 composi- tions with 3.0 wt.% excess Bi 2 O 3 , which was to compensate for the likely loss of Bi 2 O 3 at elevated temperatures. The oxide mixtures were ball-milled together for 24 h in ethanol and were subsequently dried and sieved. They were then sub- jected to mechanical activation. The mechanically activated 0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0254-0584(02)00040-8