The structure factors and normal-incidence reflectances of the alkali-
metal-doped fulleride crystals M6C60 (where M = K, Rb, or Cs) were cal
culated, and the peak reflectances are in the 2-20% range for incident
wavelengths of 9-17 angstrom, which indicates that C60 may be a promi
sing transmissive (spacer) material for normal-incidence mirrors that
have relatively high normal-incidence reflectance in the x-ray region.