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A sodium barium borosilicate glass matrix with a higher solubility of sulfate has been developed recently at Bhabha Atomic Research Centre for vitrification of sulfate bearing high-level nuclear waste. We report here the studies carried out to understand the influence of sulfate ion on the three-dimensional borosilicate network. Experiments were carried out with sodium barium borosilicate base glass samples loaded with varying amounts of SO 4 2À (0-5 mol%). Phase separation studies on the samples revealed that as much as 3 mol% of SO 4 2À can be loaded within the base glass without any phase separation, however, beyond this limit BaSO 4 (barite) crystallizes within the matrix. Thermal analyses of the samples indicated a shift in glass transition temperature from 5341 (0 mol% SO 4 2À ) to 4951C (3 mol% SO 4 2À ) and it remained more or less unaltered afterwards even with high SO 4 2À loading. A similar observation of structure stabilization was obtained from 29 Si MAS-NMR studies also, which showed that with 2 mol% of SO 4 2À loading, the Q 2 :Q 3 ratio changed from 59:41 (for samples with 0 mol% SO 4 2À loading) to 62:38 and it remained almost the same afterwards even with higher SO 4 2À loading. 11 B MAS NMR patterns of the glass samples, however, remained unchanged with SO 4 2À loading ([BO 4 ]:[BO 3 ] 5 38:62). Based on 29 Si and 11 B MAS NMR studies, the authors propose two different ways of interaction of SO 4 2À ions with the borosilicate network: (i) the network modifying action of SO 4 2À ions with -Si-O-Si-linkages, at low SO 4 2À ion concentration (o2 mol%) and (ii) the preferential interaction of SO 4 2À with the Ba 21 ions at high SO 4 2À concentration (42 mol%).

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