Structural Study ofxNa2S + (1 −x)B2S3Glasses and Polycrystals by Multiple-Quantum MAS NMR of11B and23Na

Abstract

Glasses and polycrystals in the series xNa2S + (1 − x)B2S3 have been prepared and studied by magic angle spinning (MAS) NMR and by two-dimensional multiple-quantum (MQ) MAS NMR of 11B and 23Na. These techniques, when applied at various magnetic fields and combined with computer simulations of the spectra, provide new insights into the structure of the polycrystalline samples. Isotropic chemical shifts, quadrupolar parameters, and relative concentrations of the various boron sites are obtained by NMR and correlated with the known structures of boron trisulfide (x = 0), sodium metathioborate (x = 0.5) and sodium orthothioborate (x = 0.75). A structural model of polycrystalline sodium dithioborate (x = 0.33) is proposed. The MQMAS NMR method significantly enhanced the resolution in 11B spectra of xNa2S + (1 − x)B2S3 glasses and proved instrumental in finding and identifying various structural units present within these materials as the modification of the B2S3 network progressed with increasing Na2S content. The dominant 11B resonances observed in the glassy samples represent the same basic structural units that were observed in the polycrystalline compounds. In addition, several new resonances featuring trigonally and tetrahedrally coordinated boron atoms in various transitional structures between dithioborate and metathioborate, or between metathioborate and orthothioborate, were found. 23Na NMR proved less informative, especially in the glassy samples where the motion of the sodium ions between various sites precluded the observation of well-resolved spectra.