The coordination state of the boron atom in boron compounds may be correlated with the properties of the nuclear magnetic resonance (NMR) signals arising from the boron nuclei. Characteristic resonance line shapes occur when the NMR transitions are perturbed by interactions between the B11 nuclear electrical quadrupole moment and the electric field gradient at the boron site. In polycrystalline borates, boron in trigonal and tetrahedral coordination may be resolved by analysis of first and second-order quadrupolar effects on the NMR transitions. The B11 quadrupole coupling constants have been measured for the simple trigonal BO3 and tetrahedral BO4 groups. The analysis has been extended to include polyborate structures having both trigonal and tetrahedral boron atoms. Results of the NMR study of simple, known bonding configurations are discussed and correlated with measurements of alkali-borate glasses and of miscellaneous borate and mineral compounds.

PHYSICAL REVIEW LETTERS DECEMBER 15, 1958 excitons are in quasi-thermal equilibrium with the free carriers, i.e., if the most likely mode of breakup of the exciton is thermal dissociation. For a given n andwhere % is the binding energy of the exciton, N x> N v , N c are the effective densities of states of the exciton, valence, and conduction bands, respectively, near their respective extrema. Taking T=4°K and assuming that { } 8/2 « 1 and that E x & 10&T«4 x 10"" 3 ev, we obtain n x max « 10~i 2 np under these conditions.

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