Influence of alkaline earth oxides on thermal stability of oxyfluoride glass

A new type of glass from the Na₂O–MeO–Al₂O₃–SiO₂–LaF₃ system where MeO = MgO, CaO, BaO and SrO has been studied. The aim of the investigation was to determine, by means of thermal techniques (DTA and DSC), the influence of alkaline earth ions additions on its thermal stability and the ability of LaF₃ phase to crystallization. The effect of LaF₃ crystallization was analyzed in connection with glass composition expressed by the Al₂O₃/(MeO + Na₂O + 3La₂F₆) ratio varying from 0.4 to 0.8 for the alkaline earth admixtures. The compositions of the glasses have been designed so as to make it possible to define the effect of the charge of the ion modifiers (Na⁺, Me²⁺, La³⁺) on the alumina position in the framework of the glass. Two series of glasses were obtained with a different F⁻ content. The formation of LaF₃ depends directly on the strength of the network and can be control by the Al₂O₃/modifiers ratio as well as the content of fluorine ions. Generally, it can be stated that transparent glass-ceramic with nanocrystallization of LaF₃ can be obtained for Al₂O₃/(Na₂O + MeO + 3La₂F₆) ≤0.6 in the examined glasses. The more the ionicity of the alkaline earth ions the greater the tendency for the crystallization of Me₂LaF₇ and MeF₂. In the glass structure the substitution of oxygen ions by F⁻ ions facilitated the crystallization of LaF₃. Simultaneously, it influenced the thermal stability of the aluminosilicate network and induced the crystallization of appropriate silicates during the heat treatment.