ELECTRIC-FIELD GRADIENT IN MG AND ZN DETECTED BY BETA-EMITTERS, LI-8 AND B-12

The quadrupole coupling constants of Li-8 and B-12 in hcp Mg and Zn ar e determined by use of a newly developed nuclear quadrupole resonance technique (NNQR) as \eqQ(Li-8 in Mg)/h\ = 3.0 +/- 0.3 kHz, \eqQ(Li-8 i n Zn)/h\ = 33.5 +/- 2 kHz, and \eqQ(B-12 in Mg)/h\ = 47.0 +/- 0.1 kHz. Correspondingly, the electric field gradients at room temperature are deduced: \q(Li-8 in Mg)\ = (3.81 +/- 0.39) x 10(18), \q(Li-8 in Zn)\ = (4.25 +/- 0.27) x 10(19), and \q(B-12 in Mg)\ = (1.47 +/- 0.03) x 10 (20), all in V/m2. The experiments are compared with the results of fi rst-principles super-cell band structure calculations which can treat local lattice relaxations around the impurity nuclei. The calculations show that the most favorable location of these light interstitials in hcp Mg is not the octahedral-like sites which have the biggest inters titial volume, but the basal trigonal sites with a local lattice expan sion of as big as 30%. Calculated electric field gradients at the impu rity nuclei reproduce the experimental values fairly well.