11B NMR studies of the short range order in wide composition range xNa2S + (1 − x)B2S3 glasses

Abstract

The 11 B NMR spectra of high purity (low oxygen contamination) x Na 2 S+ (1 − x )B 2 S 3 glasses with 0 ≤ x ≤ 0.80 are reported. Glasses were prepared in both the low alkali (0 ≤ x ≤ 0.30) and high alkali (0.60 ≤ x ≤ 0.80) glass-forming ranges. These two glass-forming ranges are similar in composition to the two glass-forming ranges in the analogous x Na 2 O + (1 − x )B 2 O 3 system. As in this latter system, the 11 B NMR spectrum of vitreous B 2 S 3 (v-B 2 S 3 ) consists of a quadrupolar broadened single line and confirms the short range order (SRO) of trigonally coordinated borons. As Na 2 S is added, again in similarity to the oxide system, a single narrow line grows at the resonance frequency and confirms the SRO of the highly symmetric tetrahedral boron group. By contrast with the oxides, however, the fractions of tetrahedral borons that are created by the addition of Na 2 S to B 2 S 3 are a factor of three to four times greater at any given composition. Here, rather than forming two tetrahedral borons for each added S = anion, the rate is likely to be six or eight tetrahedral borons per added S = anion. A structural model incorporating this feature of the sulfide glasses has been developed. After this initial large increase in the fraction of tetrahedral borons in the low alkali glass-forming region, the fraction begins to decrease, and by 75 mol% Na 2 S is approximately zero. This decrease in the tetrahedral boron fraction is associated with the formation of the trigonal, totally depolymerized orthothioborate group, Na 3 BS 3 .