Nano Crystallites and Clustered Species in Modified Sodium Telluride Glasses

The structural properties, crystalline and  clustering  behavior of xNa2O·(1-x)TeO2 (15 £ x £ 65 mol% glasses have been investigated by FTIR spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning   electron microscopy (SEM ). The experimental results have demonstrated that the basic structural units in telluride glasses change from highly strained TeO4 trigonal bipyramids to more regular TeO3 units containing non bridging oxygen atoms (NBO). The concentration of NBO atoms in telluride network increases with  increasing? Na2O content. The present results suggest that (NBO) atoms in telluride glasses  exist in  their pure form in glasses of up to 35 mol% Na2O.  But at higher Na2O concentrations, NBO atoms do not exist in their pure form; that is,  the majority of  NBO atoms  are coordinated with high concentration from Na cations  to form  nano-crystalline clusters. The  results based on  X-ray diffraction (XRD), scanning electron microscopy (SEM) and  differential scanning calomerty (DSC) reveal the presence of two crystalline phases: ? TeO2  and   Na2Te4O9 phases during the crystallization process of the prepared glass in the region < 50 mol% Na2O. But  at higher Na2O concentrations, Na2Te2O5 and NaTeO3 crystalline clustered phase  in the nono size scale are the dominant type. This means that NBO in  TeO3 trigonal pyramids will interact with the excess of Na cation resulting in the three-dimensional network containing nano size clusters  particularly in telluride glasses  containing 50 and 65 mol% Na2O. An increase in Na2O concentration in Na2O-rich telluride  network results in increasing Na coordination instead of breaking more tellurium–oxygen bonds.