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POSITRON STATE AND LIFETIME IN C 6 0/C 7 0o FULLERITES H.-E. SCHAEFER1 , M. FORSTER1 , R. WURSCHUM", 2 , AND F. BANHART 3 'Universitit Stuttgart, Institut fur Theoretische und Angewandte Physik, Pfaffenwald2 ring

57, 7000 Stuttgart 80, Germany, presently at: Massachusetts Institute of Technology, Dep. Materials Science and Engin., Cambridge, MA 02139, USA, 'Max-Planck-Institut fur Metallforschung, Institut fu-r Physik, Heisenbergstr. 1, 7000 Stuttgart 80, Germany ABSTRACT The present paper reports on positron lifetime measurements on Ceo/C 7 0 fullerite powder as a function of temperature and quasi-hydrostatic pressure in order to give an estimate of the positron annihilation site in the fullerite lattice. A single-component positron lifetime of 402 ps is observed which significantly decreases under quasi-hydrostatic pressure. From this and the soft intermolecular properties of the fullerites one can conclude that the positron is annihilated rather in the intermolecular space than inside the fullerene molecules. However, positron trapping at lattice defects, which are observed by high-resolution electron microscopy, cannot be ruled out.

INTRODUCTION Since the discovery of the carbon fullerene C60o [1] and the recent achievements in preparing bulk quantities of C60 [2] wide interdisciplinary interest was focused on the fullerite solids (see, e.g., ref. 3 and present proceedings). For investigating the electronic properties of the fullerite solids or the processes of intercalation and compound formation [4] on an atomic scale, nuclear probes, e.g., positrons, may be used as specific and sensitive probes. The present work aimed at a comparative positron lifetime study of the three allotropic carbon forms (i) C0o/15 mole% 070 fullerite powder, (ii) highly oriented pyrolytic graphite crystals (HOPG), and (iii) natural diamond, as well as on positron lifetime studies in fullerite under quasihydrostatic pressure in order to obtain information about the positron annihilation site in the fullerite lattice. Furthermore, the fullerite material used for the positron lifetime studies was examined by electron microscopy for a crystallographic characterization and the study of lattice defects.

EXPERIMENTAL PROCEDURE AND RESULTS The powder specimens with the typical compositions C60/15 mole% C70 (crystallite size 2 um) prepared by standard techniques [1] were obtained from W. Kri.tschmer (MI'I Heidelberg, Germany; specimen #1) and D.R. Huffman (Univ. Arizona, Tucson, USA; specimen #2). Solvent traces were removed by heating in vacuum at 300 0C for 1 h. Scanning electron microscopy (Fig. 1, specimen #1) shows mostly cube-shaped crystallites which may reflect the cubic crystal structure of the major fraction of the fullerite material. High-resolution transmission electron microscopy performed by using a JEOL 4000 FX microscope reveals predominantly fcc-type diffraction pattern of the fullerites in agreement with earlier x-ray diffraction results [5]. A minor fraction of hcpcrystallites coexists which is supposed to be due to the