Study of the tellurite-rich composition range in the Bi₂O₃-TeO₂ system

The TeO₂ and Bi₂O₃ based glasses and glass ceramics are widely used for various technical needs. However, information about the phase diagram of the Bi₂O₃-TeO₂ system is limited, and the existing data are inconsistent. According to Demina L.A. with co-authors the Bi₂Te₄O₁1 compound has a congruent melting at 662°C and forms two eutectics with neighbors. In another case, according to the Schmidt P. with co-authors, it melts incongruently at 645°C without indication of Liquidus temperature. It was the motivation for the Bi₂Te₄O₁1 melting behavior investigation and the binary Bi₂O₃-TeO₂ system phase diagram correction in the TeO₂ rich area of compositions. As initial materials the glass and solid state sintered samples were used for these purposes. The differential thermal and X-ray analyses were used for glassy and crystallized products identification. The exothermic effect with maximum at 420°C and two endothermal effects with minimum at 635°C and 720 Degree C are clearly observed on the DTA curve of the 80 TeO₂-20 Bi₂O₃ (mol.percent) glass composition corresponding to the Bi₂Te₄O₁1 compound. The product of Bi₂Te₄O₁1 glass powder crystallization at 420°C is the Bi₂Te₄O₁1 compound with melting point of 635 ± 5°C. The second endothermic effect on the DTA curve in the range of temperature 680-765°C with minimum at 720°C, is associated with dissolution of TeO₂ in the melt, formed as result of the Bi₂Te₄O₁1 incongruent melting. The existence of eutectic E₁(87 mol.percent TeO₂) between Bi₂Te₄O₁1 and TeO₂ with a melting point of 580 ±5°C has been confirmed. Incongruent melting promotes the peritectic P₁ (81 mol.percent TeO₂) formation between Bi₂Te₄O₁1 and eutectic E₁(87 mol.percent TeO₂) with a melting point of 635±5°C. Three endothermic effects at 560 °C, 635 °C and 720°C have been observed on the DTA curve of Bi₂Te₄O₁1 compound, obtained by solid state synthesis. Last two endoeffects are the same as on DTA curve of glass sample, and associated with incongruent melting of Bi₂Te₄O₁1. An endothermic effect at 560 °C is connected with phase transition from monoclinic modification of Bi₂Te₄O₁1 to the cubic