Experimental and theoretical study of electron momentum density of A(n)C(60) (n=3,4) and comparison to pristine C-60

Compton profile measurements on KnC60 (n=3 and 4), CsRb2C60, Rb4C60, and C- 60 powders have been carried out using inelastically scattered photons. We compare the experimental Compton profile difference (CPD) of KnC60 (n = 3,4 ) and that of C-60 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 o rbitals for each compound and to compare them, as a function of alkali inte rcalation. For all the samples we have obtained an overall agreement betwee n 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, simi lar measurements performed on heavy ions intercalated compounds (Rb4C60 and Rb2CsC60) show clearly that CPD depends on the number of ions and not on t heir nature. We put forward two possible hypotheses in order to explain the mismatch between theoretical and experimental CPD's of K4C60: (1) The orie ntations of C-60 molecules in these compounds vary with n and they are not taken in account by calculations. (2) Jahn-Teller effect for the negatively charged C-60 molecule in the solid. [S0163-1829(99)12447-0].