Chemical bond distribution in chalcogenide glasses. A 31P MAS and spin-echo NMR study of glasses in the phosphorus-germanium-selenium system

Abstract

Glasses in the phosphorus-germanium-selenium system have been prepared at compositions within the entire glass-forming region and their properties have been measured by differential scanning calorimetry and 31P nuclear magnetic resonance (NMR) spectroscopy, using spin-echo and magic-angle spinning (MAS) techniques. The glass transition temperature, T g, is a monotonic function of the average coordination number, 〈 r〉, with a distinct change in slope, d T g/d〈 r〉, near 〈 r〉 = 2.4, the theoretically expected percolation threshold. The NMR results are consistent with a structure in which phosphorus and germanium atoms compete for bonding to the selenium atoms but are otherwise randomly distributed. After taking into account the bonding requirements of the germanium atoms, which form tetrahedral GeSe 4 2 units, the presence of germanium in these glasses is found to have a secondary effect on the phosphorus environment. Based on the compositional dependence of the 31P- 31P dipolar second moments, it is suggested that the formation of P-bonded phosphorus atoms is more favored than in binary PSe glasses. 31P MAS-NMR data show that the presence of germanium tends to reduce the concentration of four-coordinated Se= PSe 3 2 units. This effect is attributed to the decrease in available selenium and to the fact that the coordinatively saturated GeSe 4 2 units are unable to provide intermolecular stabilization for the four-coordinated species.