Heterovalent cation-substituted Aurivillius phases, Bi2SrNaNb2TaO12 and Bi2Sr2Nb3−xMxO12 (M = Zr, Hf, Fe, Zn)

We describe the synthesis and structural characterization of new aliovalent cation-substituted n = 3 Aurivillius phases of the formulas, Bi2SrNaNb2TaO12 (I), Bi2Sr2Nb2ZrO12 (II), Bi2Sr2Nb2.5Fe0.5O12 (III) and Bi2Sr2Nb2.67Zn0.33O12 (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 Bi2Sr2Nb2MIVO12 (M = Ti, Mn) exist for several metal cations, viz., TaV, ZrIV, FeIII and ZnII. 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.