The NQR and NMR studies of icosahedral borides

Boron NMR and NQR studies have been performed on three icosahedral borides: - and -rhombohedral boron and boron carbide (B₁₂C₃). Two ¹¹B NMR peaks, separated by significant chemical shifts in the range from 130 ppm to 280 ppm, were clearly observed for all of the icosahedral borides that were not enriched in the ¹⁰B isotope. A single peak, however, was found for the ¹⁰B enriched boron carbide powder (90.6 at.% enrichment.) Moreover, the peak separation in the ¹¹B NMR spectrum for the unenriched -boron was reduced when the sample was crushed into a fine powder. In addition to NMR responses, four strong NQR responses were observed for boron carbides from different manufacturers. Two resonance signals, observed at 513 kHz and 2769 kHz, correspond to one of the icosahedral boron sites and the boron in the CBC chain, respectively. The other two NQR responses, having frequencies of 361 and 380 kHz, are either ¹⁰B responses for the chain site or ¹¹B responses for the other boron sites in the icosahedra. The NQR responses are not only consistent with the previous NMR studies performed independently by Silver and Bray (1959 J. Chem. Phys. 31 247) and by Hynes and Alexander (1971 J. Chem. Phys. 54 5296, 1972 J. Chem. Phys. Erratum 56), but also provide much higher accuracy for the values of the quadrupolar parameters.