Crystallization behavior of Ge-Sb-Se glasses in the compositional range for use as molded lenses

We have investigated compositional dependences of crystallization events of Ge-Sb-Se glasses across the compositional range appropriate for molded lens applications in the long-wavelength infrared spectrum. The onset temperature of crystallization turned out to feature no significant dependence on compositional changes of the constituent atoms, unlike glass transition temperature, Vickers hardness and thermal expansion coefficient, all of which revealed a much more significant correlation against germanium or selenium content. In an effort to unveil glass-structural attributes to the observed onset temperature of crystallization values, both mean coordination number and average bond energy were taken into consideration: The onset temperature of crystallization values appeared to be linearly proportional to average bond energy, whereas no such a correlation was verified in the case of mean coordination number. We delineated these behaviors, with an emphasis paid to the off-stoichiometry of each glass composition, in terms of network connectivity and chemical bond energy of the glass structure. In addition, crystalline phases precipitated during the bulk crystallization were identified, and the effects of the surface crystallization were analyzed in connection with infrared transmittance and hardness.