Jaephil Cho

Raman spectroscopy studies of xNa2S + (1 - x)B2S3 glasses and polycrystals

The Raman spectra of binary xNa2S+(1−x)B2S3 glasses and polycrystals have been measured for the first time and are used to develop a structural model of the sodium thioborate glasses. The Raman spectra confirm our previous infrared (IR) experimental conclusions that the structure of vitreous (ν-B2S3) is comprised of B3(0) groups and six-membered rings. It was also found that as sodium sulfide is added to the glass in the low alkali (x<0.35) glass forming region, the B4 groups are formed at the expense of the B3(0) groups first and then from the six-membered ring groups. The Raman spectra are also consistent with the presence of a pyramidal structural arrangement of B4 groups with trigonally coordinated sulfur atoms. This structure could explain the existence of the super-stoichiometric amounts of B4 groups found using nuclear magnetic resonance (NMR). Glasses in the high alkali region (0.50<x<0.80) progressively change from being comprised of metathioborate rings to being comprised of B3(3) groups. The Raman spectra also confirms the IR spectra which saw no evidence of B3(2) groups in these sodium thioborate glasses.

Infrared spectra of lithium thioborate glasses and polycrystals

Abstract Glasses and polycrystals in the x Li 2 S + (1 − x )B 2 S 3 system have been prepared over the composition range 0 ≤ x Li 2 S ≤ 0.80. The glass-forming range was observed to be ≈0.48 ≤ x ≤ 0.80 for liquids splat-quenched from ≈ 850°C into films ≈ 0.25 mm in thickness. The infrared spectra of the glasses were interpreted as a progressive compositional shift from the glassesith x ≈ 0.5 being based upon a mixture of trigonal (probably six-membered rings) and a high fraction tetrahedral boron groups to glasses with x ≈ 0.80 being based upon isolated trigonal orthothioborate anions. As with nearly all alkali borate and thioborate glasses in the high alkali region, x > 0.5, a decreasing fraction of tetrahedral borons is observed. The fraction of tetrahedral borons for glasses with x ≈ 0.55 is very large, but diminishes to zero for glasses with x = 0.75 and 0.80. Polycrystals were prepared at x = 0.33(≈ c-Li 2 B 4 S 7 ), 0.5 (≈ c-LiBS 2 ) and 0.75 (c-Li 3 BS 3 ). For the first two phases, all borons were found to be tetrahedrally coordinated, whereas for the last phase all borons were found to be in trigonal coordination.

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

Abstract 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.

Density measurements of xK2S + (1 - x)B2S3 (0 ⩽ x ⩽ 0.75) glasses: correlation with short range order

Abstract Densities of potassium thioborate glasses, xK2S + (1 - x)B2S3, are reported over the range, 0 ⩽ x ⩽ 0.75. The variation of the density with x is quite strong and is characterized by a maximum at x ⋍ 0.3 . The density increases from 1.7 g/ml for pure B2S3 to 1.92 g/ml at x = 0.35 and decreases thereafter. The increase in density in the 0 ⩽ x ⩽ 0.35 range is attributed to the increasing fraction of the tetrahedral boron group, K+BS4−, which has heavier mass but only slightly larger volume. The density decrease in the 0.40 ⩽ x ⩽ 0.75 range is associated with the decreasing fraction of these groups with the concomitant increase in the fraction of trigonal boron groups with increasing numbers of non-bridging (terminal) sulfurs. Using estimates of the fractions of short-range order groups in the alkali thioborates, the molar volumes of these groups were determined by a best fit of the density data. The calculated densities were found to agree with the experimental values. The molar volumes of the structural units present in the potassium thioborate glasses were found to be larger than those in the corresponding sodium thioborate glasses. The infrared spectrum analysis of the polycrystalline pyrothioborate phase, 2K2S:B2S3, shows strong evidence that this phase does not form in this system.