Polymorphism of BaTa 2O 6

Abstract

Phase equilibria studies in the system BaB 2O 4|BaTa 2O 6 disclosed three modifications of the latter compound: a low temperature modification isomorphous with orthorhombic BaNb 2O 6 which forms below about 1150°C, a tetragonal modification having the potassium tungsten bronze structure which forms between about 1150°C and 1300°C, and a hexagonal modification stable from 1300°C to its melting point at about 1850–1900°C. Crystals of the orthorhombic and hexagonal modifications have been grown from this system by the slow-cooling technique. The tetragonal form could only be obtained as a very fine precipitate. Attempts were made to grow the hexagonal modification from the melt using the Czochralski technique; it was found that not only the hexagonal, but the orthorhombic and tetragonal forms as well, could be grown in this fashion. The phase transformation of crystal fragments was investigated. Only transformations from the orthorhombic and tetragonal forms to the hexagonal form were observed to occur in times less than 100 hours in the appropriate temperature ranges.