Pressure-optical studies of GeS2glasses and crystals: Implications for network topology

Abstract

Germanium disulfide exhibits three solid forms: an amorphous form ($a$-Ge${\mathrm{S}}_2$), a layer-structure crystalline form (2D-Ge${\mathrm{S}}_2$), and a quartzlike crystalline form (3D-Ge${\mathrm{S}}_2$). We have carried out a series of experiments to determine the effect of pressure on the optical-absorption edge and the near-infrared refractive index of all three forms. We find that pressure causes the absorption edge to red-shift and the refractive index to increase, the sensitivity to pressure being greatest for $a$-Ge${\mathrm{S}}_2$, less for 2D-Ge${\mathrm{S}}_2$, and least for 3D-Ge${\mathrm{S}}_2$. The size of the initial effect of pressure on the band gap of $a$-Ge${\mathrm{S}}_2$ (-23 meV/kbar) is among the largest known for any semiconductor. Analysis of our pressure-optical data for all three forms, taken together with a recently established correlation between covalent-network dimensionality and photoelastic response, leads us to conclude that $a$-Ge${\mathrm{S}}_2$ is not a 3D-network glass akin to silica, but instead has lower network dimensionality. This is consistent with a class of molecular-glass models such as the Flory model for 1D-network glasses and the Phillips "partially polymerized cluster" model.