Structural evolutions of mixed-chalcogen Ge-Sb-S-Se glasses for use as infrared lenses

Abstract

Ternary Ge-Sb-Se glass which is one the prototypical chalcogenide glasses features a quite broad glass forming region mainly covering Se-sufficient compositions. However, as far as moldability issue is concerned, only a limited area belonging to Se-deficient compositions has been verified to be more adequate for use as molded lens transparent up to the long-wavelength infrared range, i.e., 8–12 μm. Commercially available Ge27.5Sb12.5Se60 (at%) glass well exemplifies practicality of the Se-deficient Ge-Sb-Se glasses, which are categorized commonly as a low refractive index dispersion glass. We discover that partial substitution of selenium with sulfur in this glass augments its thermal and mechanical properties, and at the same time its refractive index becomes more dispersive over wavelength. Here, EXAFS technique employed in this study to elucidate local structural evolutions of each constituent in quaternary Ge27.5Sb12.5SxSe60-x glasses confirms that 1) the empirical 8−N rule is preserved for each atom; 2) both homopolar Ge–Sb and Ch-Ch (Ch = S or Sb) bonds are hardly found; 3) Ge atom prefers S to Se as its nearest neighbor, whereas such a preference is absent for Sb; 4) homopolar Ge–Ge bonds abound more than estimation derived from preservation of chemical order, and are formed at the expense of Ge–Se bonds rather than Ge–S bonds. These findings suggest that the present sulfur-selenide glasses are supposed to possess an identical short-range topological order, i.e., their mean coordination number is unaltered, but their chemical order is broken especially in the vicinity of Ge atom. The violated chemical order related with Ge atoms should be taken into consideration when interpreting their composition-dependent properties and performing compositional optimization with regard to molded lens applications.