Two-dimensional angular correlation of positron annihilation radiation experiments were carried out, at temperatures ranging from 15 to 295K, on electron-irradiated monocrystals which contained di-vacancies with the charge states: V22-, V21- or V20. The spectra of non-irradiated crystals were also obtained, and were found to exhibit characteristic anisotropies. By exploiting the latter, the spectra which were associated with the di-vacancies were separated from the partial bulk contribution. The spectra for the various charge states were essentially isotropic and were very close to each other. However, by using a specimen with oriented di-vacancies, V21-, a small but definite anisotropy from the di-vacancies was obtained A small increase in the full-width at half-maximum of the spectra from the di-vacancies was observed with decreasing temperature. The trapped fractions at the di-vacancies increased markedly with decreasing temperature; not only for negative, but also for neutral, di-vacancies. Spectra from vacancy-O complexes were also obtained. The experimentally observed spectra were expected to provide useful information on the electronic and atomic structures of the di-vacancies.

M.Hasegawa, T.Chiba, A.Kawasuso, T.Akahane, M.Suezawa, S.Yamaguchi, K.Sumino: Materials Science Forum, 1995, 196-201, 1481-90