The stability of Na-doped Bi2(NbCr)O7−x pyrochlores: The non-existence of “(BiNa)(NbCr)O6”

We show that the pyrochlore described as “(BiNa)(NbCr)O6” is in fact a non-stoichiometric pyrochlore with an approximate composition of (Bi1.33Na0.67)(Nb1.33Cr0.67)O7−x. Refinement of this structure using constant wavelength powder neutron diffraction data reveals the presence of vacancies in the anion sites coupled with displacive disorder of the Bi and Na cations. This is necessary to achieve a satisfactory bonding arrangement for both the Bi and Na cations that occupy the pyrochlore A-type sites. Attempts to prepare other pyrochlores in the series Bi2−xNax(NbCr)O6 were unsuccessful and it appears that the pyrochlore is only stable only over a very narrow composition range. The structure of the pure Bi pyrochlore Bi2(NbCr)O7 is also described.