Determination of local geometries around tellurium in TeO2-Nb2O5 and TeO2-Al2O3 oxide glasses by XANES and EXAFS: Investigation of electronic properties of evidenced oxygen clusters by ab initio calculations

The origin of the high nonlinear optical response in tellurite glasses is tentatively elucidated from a detailed structural investigation of typical glasses existing in the TeO2-Al2O3 and TeO2-Nb2O5 systems joined to a study of their electronic properties through ab initio calculations. As the proportions of Al2O3 or Nb2O5 increase, the TeO4 trigonal bipyramids are progressively transformed into a TeO3 + 1 asymetric polyhedron, the geometry of which is actually very close to the TeO3 trigonal pyramids already found in some crystal structures, e.g., in ZnTeO3. Ab initio calculations have been performed on TeO4 and TeO3 clusters. The highest occupied molecular orbital is of antibonding type and combines 5s AO of tellurium and 2p AO of oxygens. The energy difference between this orbitals and the next vacant molecular orbital is lower for the TeO4 entity than for the TeO3 cluster. This result is consistent with a higher polarizability of the TeO4 entity and accordingly with the higher nonlinear index coefficient n(2) of the richest tellurium glasses. (C) 1996 Academic Press, Inc.