SPECIATION EQUILIBRIA AND CHEMICAL-BOND DISTRIBUTION IN CHALCOGENIDE CLASSES - A P-31 SPIN-ECHO AND MAS-NMR STUDY OF THE SYSTEM PHOSPHORUS-SELENIUM-TELLURIUM

Glasses in the system phosphorus-selenium-tellurium have been prepared over a wide compositional range and characterized by differential sca nning calorimetry and P-31 solid-state NMR techniques. The NMR data re veal clearly that Se and Te are structurally inequivalent in these gla sses. P-31 spin echo NMR studies reveal that in glasses' with fixed P/ Se ratio the presence of additional tellurium does not influence the d istribution of the P atoms between P-bonded and non-P-bonded structura l units appreciably. Magic angle spinning NMR spectra show no evidence for the formation of any P-Te bonds but distinguish clearly between t hree- 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 resulti ng 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 NMR data can be described phenomenologically in terms of an association equilib rium Se=PSe3/2 + Te <-> Te...Se=PSe3/2, with an average equilibrium co nstant K-2 of 25 (mole fraction)(-1). Spin echo MMR studies reveal tha t the dipolar P-31-P-31 interactions are significantly weaker for the four-coordinated P atoms than for the three-coordinated P atoms. The s elenium-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 NMR data offer a dear-cut structural rationale fo r these observations.