Synthesis and structural studies of lanthanide substituted bismuth–titanium pyrochlores

The identity of the pyrochlore phase seen during the synthesis of ferroelectric Bi4−xLnxTi3O12 Aurivillius oxides is shown to be Bi2/3Ln4/3Ti2O7. This pyrochlore is only stable for Ln3+=Sm3+ or smaller. For larger lanthanides the layered Aurivillius oxide is favoured. The presence of six-fold disorder, associated with the Bi 6s2 lone pair electrons, is believed to stabilise the unexpected stoichiometry of this oxide. Precise structures, obtained by Rietveld refinement from synchrotron X-ray diffraction data, of three examples Ln3+=Eu, Ho and Yb are presented.