POSITRON-ANNIHILATION IN C-60

Positron lifetime experiments have been performed in C60 as a function of (1) pressure up to 14.7 kbar, (2) temperature between 10 and 300 K, and (3) oxygen pressure up to 2 atm. Nearly single lifetimes (356 ps, 98%) have been observed in C60 solids. A large lifetime-pressure variation [(9 - 3 +/- 0.5) X 10(-3) kbar-1] and a monotonic lifetime-temperature variation [(1.5 +/- 0.2) X 10(-4) K-1] are observed while the presence of oxygen does not change the positron lifetime. These results are interpreted in terms that the positron is mainly annihilating with electrons near the interstitial octahedral sites of fcc lattices. Results of the calculated positron density distribution support this interpretation.