Density Measurements of Glasses in the Series x Na 2 S + (1 − x )B 2 S 3

Measurements of the densities of glasses in the series x Na 2 S + (1 − x )B 2 S 3 are reported for the first time. As has been found in the corresponding oxyborate series x Na 2 O + (1 − x )B 2 O 3 , the addition of Na 2 S to B 2 S 3 causes the density to increase, from 1.80 g/mL for pure B 2 S 3 to 1.99 g/mL at the limit of the low‐alkali sulfide glass‐forming range of x = 0.25. These data provide evidence for the formation of tetrahedral boron units (BS 4 − ) as alkali is added. Volumes for the trigonal BS 3 and tetrahedral BS 4 − groups of 57.55 and 51.79 Å 3 , respectively, were determined. As has also been found in the oxyborates, the tetrahedral boron group occupies a smaller volume than the trigonal boron group and causes the increase in density with added Na 2 S.