Abstract
We present an electron diffraction study of three sillenites, Bi12SiO20, Bi25FeO39, and Bi25InO39 synthesized using the solid-state method. We explore a hypothesis, inspired by optical studies in the literature, that suggests that trivalent sillenites have additional disorder not present in the tetravalent compounds. Electron diffraction patterns of Bi25FeO39 and Bi25InO39 show streaks that confirm deviations from the ideal sillenite structure. Multi-slice simulations of electron-diffraction patterns are presented for different perturbations to the sillenite structure - partial substitution of the M site by Bi3+, random and ordered oxygen-vacancies, and a frozen-phonon model. Although comparison of experimental data to simulations cannot be conclusive, we consider the streaks as evidence of short-range ordered oxygen-vacancies.
Highlights
We present an electron diffraction study of three sillenites, Bi12SiO20, Bi25FeO39, and Bi25InO39 synthesized using the solid-state method
Multi-slice simulations of electron-diffraction patterns are presented for different perturbations to the sillenite structure - partial substitution of the M site by Bi3+, random and ordered oxygen-vacancies, and a frozen-phonon model
The center of each tetrahedron is labelled as the M site and houses metal cations
Summary
Sillenites are technologically-interesting due to their photo-refractive,[1,2,3,4] piezo-electric,[5] electro-optical,[6,7,8,9,10] and elasto-optical properties.[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]. The sillenite structure is exclusive to bismuth oxygen compounds and is described by the meta-stable parent-compound γ -Bi2O3 which houses the Bi3+ ion in the center of the tetrahedron.[33] In the sillenites, Bi12SiO20 and Bi12GeO20, the sillenite structure is stabilized by fully replacing the Bi3+ ion by small tetravalent ions. These two tetravalent sillenites have the simplest structure of all sillenites and their structure will be referred to as the ideal structure. Quantitative comparisons between the simulations and experimental ED patterns are difficult and should not be considered conclusive; the results suggest that short-range ordered oxygen-vacancies are responsible for the streaking in the ED patterns
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