CHEMICAL-EQUILIBRIA IN GLASS-FORMING MELTS - HIGH-TEMPERATURE P-31 AND SE-77 NMR OF THE PHOSPHORUS SELENIUM SYSTEM

Molten-state equilibria governing the structure of phosphorus-selenium glasses are studied by high-temperature P-31 and Se-77 NMR over the t emperature range 25-degrees-C less-than-or-equal-to T less-than-or-equ al-to 550-degrees-C. The spectra reveal that generally ca. 100 K above the glass transition temperature motional narrowing occurs concomitan t with characteristic chemical exchange processes. In the fast exchang e limit, the P-31 and Se-77 chemical shifts reflect the average specia tions of phosphorus and selenium atoms, respectively. In melts with lo w phosphorus contents (less-than-or-equal-to 20 atom % P) the P-31 NMR spectra are dominated by the chemical equilibrium PSe3/2 + Se --> Se= PSe3/2. Detailed composition-dependent analysis of the P-31 chemical s hifts in the fast-exchange limits reveals that this equilibrium is tem perature dependent, with a reaction enthalpy around -5 +/- 1 kJ/mol an d a reaction entropy of -16 +/- 1 J/(mol K). In melts with phosphorus contents greater-than-or-equal-to 40 atom %, a thermal depolymerizatio n process results in the formation of molecular P4Se3 and PSe3/2 group s at the expense of P-P bonded structures. The spectra are analyzed in terms of temperature-dependent equilibrium constants, characterized b y a reaction enthalpy of ca. 51 +/- 4 kJ/mol and a reaction entropy of 50 +/- 7 J/(mol K), respectively. In glass containing 48 atom % P, th e phosphorus speciation is affected by hysteresis, due to the crystall ization and incongruent melting of P4Se4.