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.