Compton scattering has been shown to be a powerful tool for studying the ground state electronic density in real materials. Using synchrotron radiation, we have studied pressure effects on ${\mathrm{Rb}}_4{\mathrm{C}}_{60}$ by measuring the Compton profiles below and above the insulator to metal transition at 0.8 GPa. The experimental results are compared with the corresponding calculated results, obtained from new ab initio energy band structure calculations. These results allow us to quantitatively evaluate contributions to the Compton profiles resulting from the contraction of the unit cell as well as from the contraction of the ${\mathrm{C}}_{60}$ molecule itself. In this paper, we point out an unexpected contraction of the volume of the ${\mathrm{C}}_{60}$ molecule, leading to a major effect on the electronic density of the ${\mathrm{Rb}}_4{\mathrm{C}}_{60}$ compound.

• Received 21 December 2004

DOI:https://doi.org/10.1103/PhysRevB.72.085412