Glass formation and short-range order structures in the BaS + La2S3 + GeS2 system

Infrared (IR) optical materials have enabled a broad range of optical sensing and measurement applications in the mid-wave and long-wave IR. Many IR transmitting glasses are based on covalently-bonded selenides and tellurides, such as As2Se3 and GeTe2, which typically have relatively low glass transition temperatures (Tg) on the order of 200 to 350 °C. Many applications have working temperatures above the Tg of these materials, which compels the development of new IR materials. This work studies the underlying short-range order (SRO) structure and glass formability of a new family of ionically-bonded sulfide glasses, xBaS + yLa2S3 + (1 − x − y)GeS2, to develop high Tg optical materials with a broad IR transmission range. These sulfide glasses were produced by melting sulfide materials inside evacuated and sealed carbon-coated silica ampoules at 1150 °C for 12 h and quenching to room temperature to form glass. Glass samples were then characterized by IR and Raman spectroscopies and differential thermal analysis (DTA). It was found that by increasing the modifier concentration, the predominantly Ge4 SRO units, the superscript defines the number of bridging sulfur (BS) ions in the tetrahedral network found in GeS2 glasses, are ultimately converted to Ge0 units at >40 mol% network modifier content through the generation of non-bridging sulfur (NBS) ions. These molecular ionic units still form a glassy network, with some of the highest reported Tg values to date for a pure sulfide glass. This suggests that this composition has strong ionic bonds between negatively-charged tetrahedral SRO units and the positively-charged modifier cations. While the glass network is depolymerized in the high modifier content glasses though the formation of a high concentration of molecular ionic Ge0 SRO groups, they are, nevertheless, homogeneous glassy materials that exhibit the largest Tg and ΔT (difference between crystallization temperature, Tc, and Tg) values of glasses in this system, making them the optimal glasses for high Tg IR optical components, including, potentially, refractory IR optical fibers.