Quantum charge density fluctuations and the gamma-alpha phase transition in Ce

We have calculated the quantum quadrupolar interaction due to charge densit y fluctuations of localized 4f-electrons in gamma-Ce by taking into account the angular dependence, the degeneracy of the localized 4f-orbitals and th e spin-orbit coupling. The calculated crystal field of 4f electronic states is in good agreement with neutron diffraction measurements. We show that o rientational ordering of quantum quadrupoles drives a Fm (3) over bar m --> Pa (3) over bar phase transition at similar to 86 K which we assign with t he gamma-alpha transformation. In the Pa (3) over bar phase the centers of mass of the Ce atoms still form a face centered cubic lattice. The theory a ccounts for the first order character of the transition and for the cubic l attice contraction which accompanies the transition. The transition tempera ture increases linearly with pressure. Our approach does not involve Kondo spin fluctuations as the significant process for the phase transition.