No abstract

The region of glass formation in the new glass-forming system phosphorus-sulfurtellurium has been established, and the local structure investigated using 31P wide-line and magic-angle spinning (MAS) NMR as well as vibrational spectroscopies. The results are consistent with the absence of phosphorus-tellurium bonds, and a structure dominated by S=PSs/2 units. Characteristic chemical shift effects and changes in the vibrational frequencies of the P=S bond suggest that these units associate with tellurium in the glasses. In addition, the presence of tellurium tends to suppress the formation of molecular P4Sn species, which form at higher phosphorus concentrations in the binary system. All of these results indicate active participation of tellurium in the network consistent with compositional trends of the glass transition temperatures.

Glasses in the system phosphorus-selenium-tellurium have been prepared over a wide compositional range and characterized by differential scanning calorimetry and 31P solid-state N M R techniques. The N M R data reveal clearly that Se and Te are structurally inequivalent in these glasses. 31P spin echo N M R studies reveal that in glasses with fixed P/Se ratio the presence of additional tellurium does not influence the distribution of the P atoms between P-bonded and non-P-bonded structural units appreciably. Magic angle spinning N M R spectra show no evidence for the formation of any P-Te bonds but distinguish clearly between three-and four-coordinate P atoms. The results indicate that Te plays two distinct roles in these glasses: at lower levels of incorporation, it strongly stabilizes tetrahedral Se=PSe3/2 groups, thereby resulting in significantly increased concentrations of these units. At higher Te contents, partial segregation occurs, presumably due to solubility limitations. In spite of this complexity, the entire set of N M R data can be described phenomenologically in terms of an association equilibrium Se=PSe3/2 + Te -Te...Se=PSe,/,, with an average equilibrium constant K2 of 25 (mole fraction)-'. Spin echo N M R studies reveal that the dipolar 31P-31P interactions are significantly weaker for the four-coordinated P atoms than for the three-coordinated P atoms. The selenium-tellurium interactions proposed for these glasses would result in a net increase in the average coordination number, accounting for significant compositional effects on the glass transition temperatures in this system. The N M R data offer a clear-cut structural rationale for these observations.

Solid-state 31p MAS-NMR provides powerful insights into the local order of phosphorus selenide based glasses. Specifically, it allows a quantitative characterization of the melt equilibrium PSe3/2 + Se -> Se=PSe3/2 describing the stability of four-coordinate P atoms. Using this technique, the effect of isovalent substitution by As, and Te, and of other glass constituents Ge and TI on this equilibrium and on the quantitative phosphorus speciation is studied in detail.