Perturbed-angular-correlation measurements of trivalent indium defects in silver bromide

Abstract

The structure and thermodynamics of defects involving ions and charge-compensating silver vacancies in AgBr were studied using perturbed angular correlation (PAC) spectroscopy. We saw two main defect configurations, both attributed to two Ag vacancies bound to the ion at next-nearest-silver-neighbour positions. A reversible transition occurred between the two configurations at approximately 80 K. The predominant complex below 80 K has and . The `zz' axis of the electric field gradient (EFG) tensor for this complex points along a crystal axis. This complex is attributed 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 and . The `zz' axis of the EFG tensor for this complex also lies along a crystal axis. This complex is attributed to a configuration with 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 , but the quadrupole interaction parameters for this complex could not be precisely determined. The transition between the complexes that dominate above and below 80 K is reported and analysed in terms of equilibrium thermodynamics, allowing determination of differences in the formation enthalpy h and entropy s for the two complexes. We obtain , and . Structural studies for other trivalent impurities in AgBr are briefly reviewed, and the PAC results are compared with those results and with earlier PAC results for in AgCl.