SPECIATION EQUILIBRIA, CLUSTERING, AND CHEMICAL-EXCHANGE KINETICS IN NONOXIDE GLASSES AND MELTS - HIGH-TEMPERATURE P-31 NMR-STUDY OF THE SYSTEM PHOSPHORUS-SELENIUM

Speciation equilibria and dynamic exchange processes occurring in phos phorus-selenium melts above the glass transition temperature are chara cterized by in-situ static P-31 NMR over the temperature range 25-degr ees-C less-than-or-equal-to T less-than-or-equal-to 650-degrees-C. In glasses with low phosphorus contents, the spectra monitor the decompos ition of tetrahedral Se=PSe3/2 units occurring at temperatures below 2 00-degrees-C. In glasses with phosphorus contents greater-than-or-equa l-to 40 atom %, within the temperature region 200-degrees-C less-than- or-equal-to T less-than-or-equal-to 350-degrees-C, a network depolymer ization process occurs, leading to the creation of molecular P4Se3 uni ts. This process can be described by a phenomenological equilibrium co nstant, whose experimental temperature dependence indicates a reaction enthalpy of 30-40 kJ/mol. Above 400-degrees-C the experimental spectr a are affected by chemical exchange between molecular P4Se3 and the mo lten-glass matrix. The temperature-dependent rate constants derived fr om explicit line-shape simulations yield an activation energy of 116 k J/mol for this process. Extrapolation of these temperature-dependent p rocesses to the glass transition temperature indicates that P-Se glass es with phosphorus concentrations below 50 atom % generally show littl e evidence for intermediate-range order, except for some polymerized P 4Se3-precursor states within the concentration range 40-50 atom % P. T he kinetic data further suggest the glass transition in the P-Se syste m is associated with slow chemical exchange processes associated with bond breakage/bond formation, as previously shown for silicate glasses .