PERTURBED-ANGULAR-CORRELATION MEASUREMENTS OF TRIVALENT INDIUM DEFECTS IN SILVER BROMIDE

The structure and thermodynamics of defects involving In3+ ions and ch arge-compensating silver vacancies in AgBr were studied using perturbe d angular correlation (PAC) spectroscopy. We saw two main defect confi gurations, both attributed to two Ag vacancies bound to the In3+ ion a t next-nearest-silver-neighbour positions. A reversible transition occ urred between the two configurations at approximately 80 K. The predom inant complex below 80 K has nu(Q1) = 40 +/- 1 MHz and eta(1) approxim ate to 0. The 'zz' axis of the electric field gradient (EFG) tensor fo r this complex points along a (100) crystal axis. This complex is attr ibuted to a collinear configuration with two silver vacancies at next- nearest-neighbour sites on opposite sides of the indium ion. The other complex, predominant between 80 K and the onset of diffusion-induced damping near 200 K, has nu(Q2) = 20 +/- 1 MHz and eta(2) = 0.35 +/- 0. 15. The 'zz' axis of the EFG tensor for this complex also lies along a (100) crystal axis. This complex is attributed to a configuration wit h two silver vacancies at next-nearest sites, forming a right triangle with the indium probe at the vertex. A third complex occurs near the transition, characterized by nu(Q3) approximate to 30 MHz but the quad rupole interaction parameters for this complex could not be precisely determined. The transition between the complexes that dominate above a nd below 80 K is reported and analysed in terms of equilibrium thermod ynamics, allowing determination of differences in the formation enthal py h and entropy s for the two complexes. We obtain Delta h = h(2) = h (1) = 40 +/- 15 meV, and Delta s = s(2) - s(1) = (6 +/- 2)k(B). Struct ural studies for other trivalent impurities in AgBr are briefly review ed, and the PAC results are compared with those results and with earli er PAC results for In3+ in AgCl.