Characterization of silver tellurite

Abstract

Silver tellurite (Ag 2TeO 3) was prepared from an aqueous solution and also by oxidation of Ag 2Te in air at 973 K. Pure, stoichiometric Ag 2TeO 3 was obtained by recrystallization from an ammoniacal solution. The phase crystallized from aqueous media (α) is orthorhombic, while the high-temperature preparation is monoclinic (β). The α-phase is metastable and transforms irreversibly to the β-phase above 573 K. DTA studies showed that Ag 2TeO 3 melts congruently at 857 K. The phase diagram in the composition region of Ag 2TeO 3TeO 2 was determined. It showed a eutectic at 710 K and 70 mole% TeO 2 of the Ag 2OTeO 2 system. The isothermal section at 700 K of the ternary phase diagram of AgTeO system was constructed from the results of this study. The free energy of formation of Ag 2TeO 3 is >−65 kcal/mole. This upper limit of stability indicates that it cannot form in a nuclear reactor. Freezing behavior of Ag 2OTeO 2 melt showed a tendency to glass formation in the composition range of 61–72 mole% TeO 2. Freezing of the melt also appears to produce “tellurite,” the naturally occurring mineral form of TeO 2.