B-11 NMR-STUDIES OF THE SHORT-RANGE ORDER IN WIDE COMPOSITION RANGE XNA2S-X)B2S3 GLASSES((1)

The B-11 NMR spectra of high purity (low oxygen contamination) xNa(2)S + (1 - x)B2S3 glasses with O less than or equal to x less than or equ al to 0.80 are reported. Glasses were prepared in both the low alkali (0 less than or equal to x less than or equal to 0.30) and high alkali (0.60 less than or equal to x less than or equal to 0.80) ming ranges . These two glass-forming ranges are similar in composition to the two glass-forming ranges in the analogous xNa(2)O + (1 -x)B2O3 system. As in this latter system, the B-11 NMR spectrum of vitreous B2S3 (v-B2S3 ) consists of a quadrupolar broadened single line and confirms the sho rt range order (SRO) of trigonally coordinated borons. As Na2S is adde d, again in similarity to the oxide system, a single narrow line grows at the resonance frequency and confirms the SRO of the highly symmetr ic tetrahedral boron group. By contrast with the oxides, however, the fractions of tetrahedral borons that are created by the addition of Na 2S to B2S3 are a factor of three to four times greater at any given co mposition. Here, rather than forming two tetrahedral borons for each a dded S-= anion, the rate is likely to be six or eight tetrahedral boro ns per added S-= anion. A structural model incorporating this feature of the sulfide glasses has been developed. After this initial large in crease in the fraction of tetrahedral borons in the low alkali glass-f orming region, the fraction begins to decrease, and by 75 mol% Na2S is approximately zero. This decrease in the tetrahedral boron fraction i s associated with the formation of the trigonal, totally depolymerized orthothioborate group, Na3BS3.