Structural principles in non-oxide chalcogenide glasses. Site speciation in the system PAsSe as investigated by high-speed 31P MAS-NMR

Abstract

The locals structure of phosphorousarsenicselenium glasses of composition ( P 1− x As x ) 1− y Se y was investigated by high-speed magic-angle spinning (MAS) 31P NMR. At sufficiently high spinning speeds, PSe 3 2 and Se= PSe 3 2 units yield well-separated resonance lines, whose integrals provide that detailed phosphorous speciation in these glasses as a function of composition. The microstructure of PSe glasses is retained upon successive substitution of P by As up to substitution levels of x = 0.5. This substitution enhances the tendency of P to be four-coordinate, and indicates, that the site speciations of P and As differ. The results are quantitatively explained in terms of a liquid equilibrium reaction between different short-range-order environments according to: RSe 3 2 + [ Se n] 1 n √ Se= RSe 3 2 ( R = P, As) , assuming that arsenic substitutes exclusively for three-coordinate P atoms. In the range 0 ⩽ x ⩽ 0.5, this process is characterized by a universal equilibrium constant of 0.80 (mole fraction) −1 reflecting the thermodynamic equivalence of AsSe 3 2 and PSe 3 2 groups. Thus, the inability of As to form four-coordinate species is compensated by a higher degree of conversion of the P atoms, such that the resulting glass contains an approximately constant fraction of four-coordinated species.