Size effect in nonstoichiometric titanium monoxide and vanadium carbide nanocrystals measured by positron lifetime spectroscopy

A size effect in nonstoichiometric titanium monoxide TiO_y and vanadium carbide VC_y nanocrystals containing high concentrations of structural vacancies was detected by electron-positron annihilation. The effect consists of a considerable increase in the positron lifetime with the specific surface area. An analysis of the intensity of a long lifetime surface component in the positron annihilation spectra and the average positron lifetime for different nonstoichiometric compounds indicated a universal character of the observed size effect.