The fibers produced by co-evaporation of C-60 and potassium, and subse
quent treatment in toluene, have been investigated by electron microsc
opy and optical spectroscopy. The electron diffraction pattern can be
indexed in terms of the known orthorhombic (KC60)(n) structure. The fr
equencies of the vibrational modes indicate charge transfer between K
and C-60, Similar to bulk samples; no anisotropy of optical properties
has been observed. The detailed analysis of the electron diffraction
and micrographs reveal that, contrary to expectations, the direction o
f polymerization does not coincide with the direction of the long axis
of the fibers. Furthermore, the structure is twinned, explaining the
absence of optical anisotropy.