STRUCTURAL PRINCIPLES IN NONOXIDE CHALCOGENIDE GLASSES - SITE SPECIATION IN THE SYSTEM P-AS-SE AS INVESTIGATED BY HIGH-SPEED P-31 MAS-NMR

The local structure of phosphorus-arsenic-selenium glasses of composition (P(1-x)As(x))(1-y)Se(y) was investigated by high-speed magic-angle spinning (MAS) P-31 NMR. At sufficiently high spinning speeds, PSe3/2 and Se = PSe3/2 units yield well-separated resonance lines, whose integrals provide the detailed phosphorus 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, indirectly, 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: RSe3/2 +[Se(n)]1/n --> Se = RSe3/2 (R = P, As), assuming that arsenic substitutes exclusively for three-coordinate P atoms. In the range 0 less-than-or-equal-to x less-than-or-equal-to 0.5, this process is characterized by a universal equilibrium constant of 0.80 (mole fraction)-1 reflecting the thermodynamic equivalence of AsSe3/2 and PSe3/2 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.