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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%).

Alkali-based barium borosilicate glasses having similar composition to the Trombay nuclear research reactor waste base glass containing iron were prepared and irradiated by gamma rays. The radiation-induced defect centers were investigated using electron paramagnetic resonance (EPR) technique. The results showed the formation of silicon hole centers and electron trap centers apart from boron-based oxy hole centers in the glass due to the irradiation. The EPR Hamiltonian parameters for these radicals were evaluated by simulation technique using Bruker SIMFONIA computer program. Glasses with varying iron content were irradiated with the same dose to evaluate the effect of iron content on the defect center concentration. A spin counting technique was employed to evaluate the number of defect centers produced in each glass. It was observed that the defect center concentration reduced as a function of increasing iron content. This was attributed to the charge trapping process of ferric ions in the amorphous system. *manojm@barc.gov.in

Here we demonstrate the selective bulk scale synthesis of delaminated graphene sheets by a proper choice of magnetic field modulating an electric-arc. An ultra-high purity glassy graphite anode was sublimated in an argon atmosphere. Carbon nanotubes (CNTs), as well as graphene sheets were found inside the deposit formed on the cathode. Both the high purity CNTs as well as graphene sheets, with minimal structural defects, were synthesized separately by varying the strength and orientation of the external magnetic field generated by arrays of permanent magnets. The as-synthesized carbonaceous samples were characterized with the help of transmission electron microscopy, selected area electron diffraction (SAED), Raman spectroscopy (RS) and thermogravimetry for optimizing the highest selective production of delaminated graphenes. This optimization was done by varying the strength and orientation of the external magnetic field.The as-synthesized graphene sheets exhibited relatively high degree of graphitization and low structural defect density as confirmed by RS. They were found to exhibit higher oxidation temperature (767°C) than that of the carbon nanocrystalline (690°C) particles as inferred from the thermogravimatric analysis. Moreover, they were found to form 'scroll-like CNTs' at their edges on account of their surface energy minimization. This was confirmed by the SAED analysis. With this new technique, we could successfully synthesize delaminated graphenes at a rate of few g/h.

The stannides RERhSn (RE = Ho -Yb) and ScTSn (T = Pd, Pt) were prepared by reaction of the elements in sealed tantalum tubes in a high-frequency furnace, by arc-melting, or by a tin-flux technique in quartz tubes. The rhodium based stannides crystallize with the ZrNiAl type structure, space group P62m. The four structures were refined from single crystal X-ray data: a = 754.5(3), c = 377.1(1) pm, wR2 = 0.0357, 233 F 2 values for HoRhSn, a = 753.3(1), c = 372.16(8) pm, wR2 = 0.0721, 233 F 2 values for ErRhSn, a = 753.7(3), c = 369.0(2) pm, wR2 = 0.0671,233 F 2 values for TmRhSn, and a = 753.17(5), c = 366.53(4) pm, wR2 = 0.0566, 180 F 2 values for YbRhSn with 14 parameters for each refinement. ScPdSn and ScPtSn adopt the HfRhSn type, a superstructure of ZrNiAl, space group P62c: a = 747.5(1), c = 710.2(1) pm, for ScPdSn, and a = 738.37(9), c = 729.47(9) pm, wR2 = 0.0452,369 F 2 values, 18 variables for ScPtSn. Structural motifs in these stannides are transition metal centered trigonal prisms formed by the rare earth and tin atoms. While these prisms are regular in the rhodium based stannides, significant distortions occur in ScPdSn and ScPtSn. The formation of the superstructure can be ascribed to packing reasons. The shortest interatomic distances occur between the transition metal (T) and tin atoms. These atoms form three-dimensional [FSn] networks in which the rare earth atoms fill distorted hexagonal channels. The series RERhSn displays a somewhat unique behavior. The a lattice parameter is more or less independent of the rare earth element, while the c lattice parameter shows the expected lanthanoid contraction. ll9Sn Mössbauer spectroscopic data of the rhodium stannides show signals at isomer shifts varying from 1.77 to 1.82 mm/s subject to quadrupole splitting between 0.75 to 0.82 mm/s.

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