Raman spectroscopy evidence of inhomogeneous disorder in the bismuth-oxygen framework of Bi25InO39and other sillenites

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.…”

“…These two tetravalent sillenites have the simplest structure of all sillenites and their structure will be referred to as the ideal structure. In Bi 25 FeO 39 and Bi 25 InO 39 the structure is stabilized by trivalent ions; but there are two important differences in the stoichiometry. First, the M site is equally occupied by M 3+ and Bi 3+ ions.…”

“…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.…”

“…A recent Raman spectroscopy study showed that the vibrational modes have considerably larger widths in trivalent compounds than in tetravalent compounds. 25 The wider distribution of resonant frequencies suggests differing force constants across neighbouring unit cells. Since the X-ray spectra of the trivalent sillenites do not show deviations from the ideal structure, whichever structural deviation is present must average to the ideal structure for a large number of unit cells.…”

“…The measurements prove a deviation of the trivalent sillenites from the ideal sillenite structure and corroborate the hypothesis from previous optical studies. 25 Various structural models were explored using multi-slice simulations of the ED patterns. We present simulations involving random substitutions of the M site by Bi 3+ , random and ordered oxygenvacancies, and frozen phonons.…”