The T_g's and heat capacity functions have been measured for a series of Na₂S + B₂S₃ glasses for the first time. Unlike the alkali borates, T_g decreases rapidly as Na₂S is added to B₂S₃. This effect, even in the presence of a rapidly increasing fraction of tetrahedrally coordinated borons, has been associated with the “over crosslinking” effect of the sulfide ion. Unlike the borate glasses where each added oxygen produces two tetrahedral borons, the conversion rate for the thioborates is between four and six. This behavior is suggested to result in the formation of local tightly-bonded molecular-like structures that exhibit less long-range network bonding than the alkali borate glasses. As a result, T_g decreases with added alkali in alkali thioborates rather than increases as in the alkali borate glasses.

The change in heat capacity at T_g, ΔC_p(T_g) has been carefully measured and is found to also decrease dramatically as alkali sulfide is added to the glass. Again this effect is opposite to the trends observed for the alkali borate glasses. The decreasing ΔC_p(T_g) occurs even in the presence of a decreasing T_g. We have tentatively associated the diminishing ΔC_p(T_g) values to the decreasing density of the configurational states above T_g. This is attributed to the high coordination number and site specificity caused by the added alkali sulfide. The glassy state heat capacities were analyzed and found to reach ˜90% of the classical limiting DuLong-Petit value just below T_g for all glasses. This was used to suggest that the diminishing ΔC_p(T_g) values are associated with a unique behavior in the system to become a liquid with very little change in the density of configurational states.