Static B 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 0x0.75 and of xCs₂S+(1?x)B₂S₃ glasses in the range 0x0.60 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 (N₄0.5) 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.