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 phosphorus-selenium melts above the glass transition temperature are characterized by in-situ static P-31 NMR over the temperature range 25-degrees-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 decomposition of tetrahedral Se=PSe3/2 units occurring at temperatures below 200-degrees-C. In glasses with phosphorus contents greater-than-or-equal-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 depolymerization process occurs, leading to the creation of molecular P4Se3 units. This process can be described by a phenomenological equilibrium constant, whose experimental temperature dependence indicates a reaction enthalpy of 30-40 kJ/mol. Above 400-degrees-C the experimental spectra are affected by chemical exchange between molecular P4Se3 and the molten-glass matrix. The temperature-dependent rate constants derived from explicit line-shape simulations yield an activation energy of 116 kJ/mol for this process. Extrapolation of these temperature-dependent processes to the glass transition temperature indicates that P-Se glasses with phosphorus concentrations below 50 atom % generally show little evidence for intermediate-range order, except for some polymerized P4Se3-precursor states within the concentration range 40-50 atom % P. The kinetic data further suggest the glass transition in the P-Se system is associated with slow chemical exchange processes associated with bond breakage/bond formation, as previously shown for silicate glasses.