Static 11B NMR studies of the short range order in alkali metal modified B2S3 glasses

The B-11 NMR spectra of xRb(2)S + (1 - x)B2S3 glasses in the range 0 less than or equal to x less than or equal to 0.75 and of xCs(2)S + (1 - x)B2S3 glasses in the range 0 less than or equal to x less than or equal to 0.60 are reported. The addition of Rb2S to B2S3 creates on average approximately two and one-half tetrahedral borons for each added sulfur ion, whereas It is found that the addition of Cs2S creates approximately 2 tetrahedral borons for each added sulfur ion. This behavior while more similar to that seen in the alkali borate glasses, contrasts that seen in the Na and K thioborate glasses, where six to eight and three, respectively, tetrahedral borons are formed for every sulfide anion added to the glass. These findings are supported by the IR and B-11 NMR spectra of the di-thioborate polycrystals (c-Rb2S:2B(2)S(3) and c-Cs2S:2B(2)S(3)) whose structures appear to be comprised of two BS4 tetrahedrals and two BS3 trigonals (N-4 similar to 0.5) like that in the alkali di-borate phases for both Rb and Cs. Unlike the B-11 NMR resonances of the sodium thioborate glasses where a single sharp line is observed for the tetrahedral boron site and a single quadrupolar broadened line is observed for all the trigonal sites, a third resonance line is observed at high alkali fractions for the rubidium and cesium thioborate glasses. This new structural feature may arise from asymmetric MBS2 (meta-thioborate groups) or tetrahedral boron groups possessing a non-bridging sulfur. (C) 2000 Elsevier Science B.V. All rights reserved.