A Stoichiometric Model for Sillenites

A critical review of crystallographic studies on sillenite compounds has allowed us to develop a general structural formula for the stoichiometric sillenites: Bi 12 (Bi 4/5 - nx M n + 5 x )O 19.2+ nx . We defined the stoichiometric sillenites as those with an oxygen sublattice that is fully occupied by either oxygen ions or lone electron pairs of Bi 3+ ions. Nonstoichiometric groups of sillenites were identified as being either substoichiometric, with a deficient oxygen sublattice, or superstoichiometric, with an excess of oxygen. The compositions of the sillenites reported in the literature agree well with the proposed stoichiometric model. Two sillenites, Al and Zn, that were found to deviate from this model were experimentally investigated using X-ray diffraction, scanning electron microscopy, and wavelength-dispersive spectroscopy. The composition was found to be different from that reported previously and to correspond to the proposed stoichiometric model.