Study of insulator to metal transition in Rb4C60 by coherent and incoherent X-ray scattering under pressure

The equation of state (EOS) of Rb4C60 is determined by high-pressure X-ray diffraction measurements. We observe an abrupt jump in the compressibility of Rb4C60 between 0.5 and 0.8 GPa which is attributed to a structural phase transition preserving the tetragonal symmetry. Pressure effects on electronic density are studied by measuring Compton profiles below and above the transition. The ab initio calculation of the electronic structure is used to obtain theoretical equation of state, which agrees well with the experimental results. The calculations are used to quantitatively identify different contributions to the Compton profiles. In particular, we point out an unexpected contraction of the C60 molecular volume under pressure which brings about significant effects on the electronic density of Rb4C60 compound.