Synthesis and structural studies of lanthanide substituted bismuth–titanium pyrochlores

The identity of the pyrochlore phase seen during the synthesis of ferroelectric Bi 4− xLn x Ti 3O 12 Aurivillius oxides is shown to be Bi 2/3 Ln 4/3Ti 2O 7. This pyrochlore is only stable for Ln 3+=Sm 3+ or smaller. For larger lanthanides the layered Aurivillius oxide is favoured. The presence of six-fold disorder, associated with the Bi 6 s 2 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 Ln 3+=Eu, Ho and Yb are presented. The pyrochlore phase formed during the synthesis of the ferroelectric oxides Bi 4− xLn x Ti 3O 12 is shown to be Bi 2/3 Ln 4/3Ti 2O 7. This is found to be stable only for Ln 3+cations smaller than Sm 3+. Displacive disorder of the Bi and Ln cations is observed.