Experimental and theoretical study of electron momentum density of AnC60 (n=3,4) and comparison to pristine C60

Compton profile measurements on K_nC₆₀ (n=3 and 4), CsRb₂C₆₀, Rb₄C₆₀, and C₆₀ powders have been carried out using inelastically scattered photons. We compare the experimental Compton profile difference (CPD) of K_nC₆₀ (n=3,4) and that of C₆₀ with the corresponding calculated results, obtained from ab initio self-consistent field calculations of the energy band structure. This permits us to isolate the contribution of the distortion of the C₆₀ orbitals for each compound and to compare them, as a function of alkali intercalation. For all the samples we have obtained an overall agreement between theory and experiment. Nevertheless, in the case of K₄C₆₀, the comparison between the calculated and measured CPD's widths is not satisfactory. The calculated width is larger than the experimental results. Furthermore, similar measurements performed on heavy ions intercalated compounds (Rb₄C₆₀ and Rb₂CsC₆₀) show clearly that CPD depends on the number of ions and not on their nature. We put forward two possible hypotheses in order to explain the mismatch between theoretical and experimental CPD's of K₄C₆₀: (1) The orientations of C₆₀ molecules in these compounds vary with n and they are not taken in account by calculations. (2) Jahn-Teller effect for the negatively charged C₆₀ molecule in the solid.