NUCLEAR-QUADRUPOLE INTERACTION OF GA-69 IN AGGAS2 AND AGGASE2

By FT-NMR we measured the first-order shifts for tile resonances of Ga -69 in a single crystal of AgGaS2 and in a AgGaSe2 powder specimen. We determined the nuclear quadrupole coupling constant of (0.40 +/- 0.01 ) MHz for AgGaS2 and (0.55 +/- 0.03) MHz for AgGaSe2. The theory of Li nes is applied to explain the corresponding electric field gradients. After that the rather small values are due to a strong compensation of two contributions. the lattice contribution and the orbital contribut ion. This compensation cannot take place in CuGaS2 with its much highe r coupling constant of 1.8 MHz, because of its small orbital contribut ion due to a very small local deviation from perfect tetrahedral symme try.