Heterovalent cation-substituted aurivillius phases, Bi₂SrNaNb₂TaO₁₂ and Bi₂Sr₂Nb_3-xM_xO₁₂(M = Zr, Hf, Fe, Zn)

Abstract

We describe the synthesis and structural characterization of new aliovalent cation-substituted n = 3 Aurivillius phases of the formulas, Bi₂SrNaNb₂TaO₁₂ (I), Bi₂Sr₂Nb₂ZrO₁₂ (II), Bi₂Sr₂Nb_2.5Fe_0.5O₁₂ (III) and Bi₂Sr₂Nb_2.67Zn_0.33O₁₂ (IV). Energy dispersive X-ray (EDX) investigation of the chemical compositions showed that while cation-stoichiometric materials are formed for I and II, single-phase materials are obtained only for the compositions given for III and IV suggesting that the compositions tend to be oxygen-stoichiometric. The results show that aliovalent cation-substituted n = 3 Aurivillius phases similar to Bi₂Sr₂Nb₂M^IVO₁₂ (M = Ti, Mn) exist for several metal cations, viz., Ta^V, Zr^IV, Fe^III and Zn^II. Refinement of the crystal structures from the powder X-ray diffraction (XRD) data for I and III revealed that bismuth is essentially confined to the Bi₂O₂ layers and the aliovalent cation shows a preference for the middle perovskite sheet. The present work is significant for two reasons: firstly, the presence of cations such as Sr²⁺, Na⁺ (that do not contain lone pair s² electrons) in the perovskite slabs seems to render the structure centrosymmetric; secondly, a preferential/partial ordering of octahedral site cations in the perovskite slabs obtains in these materials, that seems to be dictated by a second order Jahn-Teller effect associated with d⁰ cations.