Journal of Molecular Structure, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF 111 (1983) 71-77 Elsevier Science Publishers B.V., Amsterdam -hinted in The Netherlvlds 71 HYPERFINE STRUCTURE IN Ga NQR OF GaS AND GaSe T-J. EASTOW, I.D. CM7pBZLL and S.N. STUART CSIRO Division of Chemical Ph-jsics, P-0. BOX 160, Claytor., Victoria, Australia 3168. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA AESTRACT Fine structure is reported in the 69'71 Ga NQR spectra of Gas and GaSe. The 69Ga (I = ‘,j and 'lGa (I = 3') spectra are qualitatively different for each compound. This is attributed to an indirect spin-spin interaction 'between nearest-neighbour Ga atom pairs. A model agreement with experiment is presented. giving reasonable quantitative INTRODUCTION Complex "Ga NQR li.ne shapes have recently been reported for GaS and GaSe (ref. 1). These are layer compounds possessing a common layer unit consisting of an X-Ga-Ga-S sequence where X is S or Se (see Fig. I)_ The crystal structure of both compounds is based on a regular stacking of these units. T'he three commonly observed stacking polytypes are labelled 5. 71 and E, viz. 9 (2x; r6 /mmc) 3 - one distinguishable Ga site per unit cell E (ZH; P6n2) - two distinguishable Ga sites per unit cell y (3R; R3m) - two distinguishable Ga sites par unit cell. Fig. 1. The four-fold S-PI-!.I-X layer unit of GaS and GaSe. The Ga ions are tie small shaded circles. GaS appears to exist exclusively in the i?-fora? accordkg to all available X-ray structure determinations (ref. 2), except under cvtreme conditions (refs. OOZZ-2860/83/$03.00 0 1983 Elsevier Science Publishers B.V.