A monovacancy-divacancy model interpretation of positron annihilation measurements in aluminium

Abstract

A monovacancy-model analysis of new temperature-dependent (in the range 293-903K) data of the peak-count rate, long-slit angular correlation of annihilation radiation (1D ACAR) for Al single crystals yields an apparent value for the monovacancy formation enthalpy (H_1v^F) of (0.76+or-0.04) eV. However, lifetime experiments of lower temperatures (293-645K) have yielded H_1v^F=(0.66+or-0.02) eV. A constrained monovacancy-divacancy analysis of the 1D ACAR data, incorporating theoretical predictions of the relative peak-count rates arising from monovacancy- and divacancy-trapped positrons, can resolve this apparent discrepancy between the lifetime results and momentum measurements (Doppler broadening or angular correlation) of the vacancy formation enthalpy in Al. This analysis yields a divacancy binding enthalpy of H_2v^b=(0.30+or-0.13) eV, and indicates that at the melting point about 60% of the positrons annihilate from divacancy-trapped states in Al, consistent with the interpretation of recent 2D ACAR results. Introduction of the ratio of the positron trapping rate into divacancies to that into monovacancies, mu _2v/ mu _1v, as deduced from the jellium-model calculations of McMullen et al. (1981) allows one to estimate the divacancy binding entropy S_2v^b as well. The results are compared with those of previous investigations of vacancy formation and self-diffusion in Al.