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

The 11B NMR spectra of xRb2S+(1−x)B2S3 glasses in the range 0⩽x⩽0.75 and of xCs2S+(1−x)B2S3 glasses in the range 0⩽x⩽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 11B NMR spectra of the di-thioborate polycrystals (c-Rb2S:2B2S3 and c-Cs2S:2B2S3) whose structures appear to be comprised of two BS4 tetrahedrals and two BS3 trigonals (N4∼0.5) like that in the alkali di-borate phases for both Rb and Cs. Unlike the 11B 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.