A Stoichiometric Model for Sillenites

Abstract

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+ 5x )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.

Electron diffraction study of the sillenites Bi12SiO20, Bi25FeO39 and Bi25InO39: Evidence of short-range ordering of oxygen-vacancies in the trivalent sillenites

“…11,12 The last decade has witnessed an increase in the study of these compounds for photocatalysis and novel applications. [13][14][15][16][17] The physical and electronic structure of sillenites remains important and continues to motivate synthesis, [18][19][20][21][22] optical, [23][24][25] first principles, [26][27][28] dielectric, 29 magnetism, 30 and high-pressure studies. 31,32 Figure 1 shows a representative sillenite structure.…”

“…And second, the tetrahedra with a Bi 3+ ion have an oxygen vacancy which makes room for the 6s 2 lone pair of bismuth. 19 Divalent, pentavalent, and even hexavalent compounds can also be synthesized, and the partial occupancy of the M site varies for different valences. More detail about the structure can be found in references 19, 34.…”

“…The stoichiometry and structures described above correspond to the models proposed by . 19 This comprehensive model of sillenites is dependent on the conclusion by Radaev and Simonov that the parent γ -Bi 2 O 3 compound houses Bi 3+ in the center of the tetrahedron, 33,35,36 and that it is unlikely that Bi 5+ partially occupies the M site of divalent and trivalent sillenites. 19 Some recent publications instead claim a Bi 25 FeO 40 chemical formula for the iron sillenite, 17,22 and some justify the stoichiometry by citing work from Craig and Stephenson.…”

“…19 This comprehensive model of sillenites is dependent on the conclusion by Radaev and Simonov that the parent γ -Bi 2 O 3 compound houses Bi 3+ in the center of the tetrahedron, 33,35,36 and that it is unlikely that Bi 5+ partially occupies the M site of divalent and trivalent sillenites. 19 Some recent publications instead claim a Bi 25 FeO 40 chemical formula for the iron sillenite, 17,22 and some justify the stoichiometry by citing work from Craig and Stephenson. 37 Unfortunately, some of the publications claiming the Bi 25 FeO 40 chemical formula 17,22,38 do not comment on the more recent, heavily cited, and comprehensive model of sillenites that uses Bi 3+ partial occupation of the M site.…”

“…37 Unfortunately, some of the publications claiming the Bi 25 FeO 40 chemical formula 17,22,38 do not comment on the more recent, heavily cited, and comprehensive model of sillenites that uses Bi 3+ partial occupation of the M site. 19 The controversy is further complicated by the fact that some of the studies quoting the Bi 25 FeO 40 formula use the hydrothermal growth method. 22 In our study we used the solid-state growth method; therefore, we use the Bi 25 FeO 39 chemical formula due to the lack of experimental evidence for the stabilization of Bi 5+ in systems grown by the solid-state method.…”