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SPECIATION EQUILIBRIA, CLUSTERING, AND CHEMICAL-EXCHANGE KINETICS IN NONOXIDE GLASSES AND MELTS - HIGH-TEMPERATURE P-31 NMR-STUDY OF THE SYSTEM PHOSPHORUS-SELENIUM
被引:19
作者:
MAXWELL, R
[1
]
ECKERT, H
[1
]
机构:
[1] UNIV CALIF SANTA BARBARA,DEPT CHEM,SANTA BARBARA,CA 93106
关键词:
D O I:
10.1021/ja00064a039
中图分类号:
O6 [化学];
学科分类号:
0703 ;
摘要:
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.
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页码:4747 / 4753
页数:7
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