Three-layer Aurivillius phases containing magnetic transition metal cations: Bi2−xSr2+x(Nb,Ta)2+xM1−xO12, M=Ru4+, Ir4+, Mn4+, x≈0.5

We report the synthesis of Aurivillius-type phases incorporating magnetic M4+ cations (M=Mn, Ru, Ir), based on the substitution of M4+ for Ti4+ in Bi2Sr2(Nb,Ta)2TiO12. The key to incorporating these magnetic transition metal cations appears to be the partial substitution of Sr2+ for Bi3+ in the α-PbO-type layer of the Aurivillius phase, leading to a concomitant decrease in the M4+ content; i.e., the composition of the prepared compounds was Bi2−xSr2+x(Nb,Ta)2+xM1−xO12, x≈0.5. These compounds only exist over a narrow range of x, between an apparent minimum (x≈0.4) Sr2+ content in the α-PbO-type [Bi2O2] layer required for Aurivillius phases to form with magnetic M4+ cations, and an apparent maximum (x≈0.6) Sr2+ substitution in this [Bi2O2] layer. Rietveld-refinement of synchrotron X-ray powder diffraction data making use of anomalous dispersion at the Nb and Ru K edges show that the overwhelming majority of the incorporated M cations occupy the central of the three MO6 octahedral layers in the perovskite-type block. Magnetic susceptibility measurements are presented and discussed in the context of the potential for multiferroic (magnetoelectric) properties in these materials.