Anisotropy of the positron-annihilation Doppler-broadening profiles with the crystallographic direction of Zn

Doppler-broadened photopeaks resulting from positron annihilation in high-purity, low-dislocation-density Zn single crystals have been characterized with single-number line-shape parameters for annihilation photons emerging in the [0001] and $\left[11\mathrm{}\overline{2}0\right]$ crystallographic directions. Anisotropy of the photopeaks with crystallographic direction is observed through these parameters for sample temperatures between 183 and 568 K. The anisotropy is associated with the asymmetric nature of the positron and electron wave functions. A linear temperature dependence of the characteristic bulk annihilation parameters is deduced from data for both crystallographic directions from 183 to 568 K. The magnitude of the slope of the bulk annihilation parameters for the basal plane was found to be approximately twice that of the parameters for the [0001] direction. When the direction and temperature dependence of the bulk annihilation parameters were considered, the anisotropy of the line-shape parameters did not affect, within the precision of our data, the value of the monovacancy-formation enthalpy deduced from the two-state trapping model.