Temperature Dependence of Doppler-Broadened Positron Annihilation Radiation in Hexagonal Close-Packed Metals.

Available from UMI in association with The British Library. An investigation into positron annihilation in hexagonal-close-packed (h.c.p.) metals was carried out using the Doppler Broadening technique. The Doppler broadened lineshape was characterised in terms of the conventional peak-height (S) and wing (W) parameters obtained after a step background subtraction of the annihilation peak. The temperature dependence of these parameters was studied for annihilation photons emerging in the (0001) and (1120) directions of single crystal Zn in the temperature range 100-600 K and single crystal Tl in the range 100-520 K. The monovacancy formation enthalpy, H_sp{ rm 1v}{rm F}, was calculated from the two state trapping model assuming a simple linear temperature dependence of the free annihilation lineshape parameter. H_sp{rm 1v} {rm F} was found, within the precision of the data, to be independent of the crystallographic direction and was evaluated as 0.47 +/- 0.03 eV for Zn and 0.51 +/- 0.15 eV for Tl. The magnitude of the slopes of the prevacancy regions in the (1120) direction of Zn were found to be twice that of the (0001) direction. This anisotropy was not observed in Tl in which the lineshape parameters exhibited a very weak temperature dependence. The lineshape parameters were not affected by the phase transition from h.c.p. to b.c.c. in Tl.