LOCAL-STRUCTURE OF ASXTE100-X GLASSES STUDIED BY DIFFERENTIAL X-RAY ANOMALOUS SCATTERING AND X-RAY-ABSORPTION SPECTROSCOPY

The local structure of AsxTe100-x glassy semiconductors has been studied using x-ray anomalous scattering and x-ray-absorption spectroscopy as a function of the composition. Two important findings are (a) tellurium (Te) atoms are twofold coordinated in arsenic (As) rich alloys (x > 40), while the Te coordination suddenly increases to 2.4 at the stoichiometric As40Te60 composition, which indicates a large fraction (about 50%) of threefold Te sites in this glass. This percentage of occupancy of the three-fold Te sites remains almost constant with increasing further the Te concentration; (b) a significant level of chemical disorder, marked by the existence of homopolar As-As and Te-Te pairs in the first coordination shell, is observed for all the investigated glasses. At the As40Te60 composition, nearly about the same amount of As-As and Te-Te pairs do exist in the first coordination shell. Thus the stoichiometric As40Te60 composition is not a chemical threshold as it is the case for the Se- or S-based glasses. Based on these results, a number of peculiar and unique features in the physical properties of these glasses, such as the glass transition temperature (T(g)), the microhardness, the glass forming ability, and the electrical conductivity, can be understood.