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 (H1vF) of (0.76+or-0.04) eV. However, lifetime experiments of lower temperatures (293-645K) have yielded H1vF=(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 H2vb=(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 S2vb as well. The results are compared with those of previous investigations of vacancy formation and self-diffusion in Al.