A 125Te and 23Na NMR investigation of the structure and crystallisation of sodium tellurite glasses

Te-125 static nuclear magnetic resonance (NMR) and Na-23 and Te-125 magic angle spinning (MAS) NMR have been used, in conjunction with X-ray diffraction, to examine the structure and crystallisation behaviour of glasses of composition xNa(2)O.(1-x)TeO2 (0.075less than or equal toxless than or equal to0.4). The MAS NMR Na-23 spectra from the glasses are broad and featureless but shift by similar to + 5 ppm with increased x, i.e. as the system becomes more ionic. The static Te-125 NMR spectra show an increase in axial symmetry with increasing x, indicating a shift from predominantly [TeO4] to [TeO3] structural units. The Na-23 and Te-125 spectra from the crystallised samples have been fitted to obtain information on the sites in the metastable crystal phases, which are the first to form on heating and which are therefore more closely related to the glass structure than thermodynamically stable crystal phases. New sodium tellurite phases are reported, including a sodium stabilised, face centred cubic phase related to delta-TeO2; a metastable form of Na2Te4O9 containing 3 sodium and 4 tellurium sites; and a metastable form of Na2Te2O5 containing 2 sodium sites. There is evidence of oxidation of Te-IV to Te-VI occurring in glasses with high values of x and, at x = 0.40 and 0.50 (outside the glass forming range), some sodium metatellurate (Na2TeO4) is formed at the same time as sodium metatellurite (Na2TeO3). The Te-125 shift is very sensitive to environment within the sodium tellurite system, covering more than 320 ppm, with anisotropies varying from 640 to 1540ppm. The lack of features in the Te-125 spectra of the glass phases, combined with the large shift range and high but variable anisotropy, means than it is not possible to obtain a unique fit to any presumed species present. Furthermore, the chemical shift anisotropy parameters for three of the four Te sites in the Na2Te4O9 phase are found to lie outside the range used for previous simulations of glass spectra. (C) 2004 Elsevier Inc. All rights reserved.