New anion-conducting solid solutions Bi1−xTex(O,F)2+δ (x > 0.5) and glass–ceramic material on their base

The anion-excess fluorite-like solid solutions with general composition Bi1−xTex(O,F)2+δ (x > 0.5) have been synthesized by a solid state reaction of TeO2, BiF3 and Bi2O3 at 873 K with following quenching. The homogeneity areas and polymorphism of the I ↔ IV Bi1−xTex(O,F)2+δ phases were investigated. The crystal structure of the low temperature IV-Bi1−xTex(O,F)2+δ phase has been solved using electron diffraction and X-ray powder diffraction (a = 11.53051(9) Å, S.G. Ia-3, RI = 0.046, RP = 0.041). Glass formation area in the Bi2O3–BiF3–TeO2 (10% TiO2) system was investigated. IVBi1−xTex(O,F)2+δ phase starts to crystallize at short-time (0.5–3 h) annealing of oxyfluoride glasses at temperatures above Tg (600–615 K). The ionic conductivity of the crystalline Bi1−xTex(O,F)2+δ phase and corresponding glass-ceramics was investigated. Activation energy of conductivity Ea = 0.41(2) eV for the IV-Bi1−xTex(O,F)2+δ crystalline samples and Ea = 0.73 eV for the glass-ceramic samples were obtained. Investigation of the oxyfluoride samples with a constant cation ratio demonstrates essential influence of excess fluorine anions on the ionic conductivity.

Graphical abstractThe homogeneity areas and polymorphism of anion-excess fluorite-like solid solutions with general composition Bi1−xTex(O,F)2+δ (x > 0.5) have been investigated. Their crystallographic and structural characterization have been conducted. The influence of chemical composition on ionic conductivity both in crystalline and glass–ceramic forms has been determined.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The homogeneity areas and polymorphism of the Bi1−xTex(O,F)2+δ (x > 0.5) phases have been investigated. ► The crystal structure of the low temperature IV-Bi1−xTex(O,F)2+δ phase has been determined. ► The ionic conductivity of the crystalline Bi1−xTex(O,F)2+δ phase and glass–ceramic samples has been studied.