POSITRON STUDIES OF POLYCRYSTALLINE TIC

The mean positron lifetime tau, positron diffusion length L, and the positron and electron work functions (phi(+) and phi(-)) for polycrystalline TiC have been experimentally determined. The results were tau = 160(2) ps, L(+) = 138(27) nm and phi(-) = 3.96(0.08) eV; phi(+) was shown to be almost certainly positive. These results strongly support the suggestion from recent first-principles electronic structure and positron state calculations that positions are trapped by and annihilate in metal vacancies in this material. XPS measurements indicate that the trapping sites may be predominantly in thin carbon-rich layers between grains, a picture which may also explain the long near-surface diffusion length.