New theory of the gamma-alpha phase transition in Ce: quadrupolar ordering

We present a theoretical model of the “isostructural” γ − α phase transition in Ce which is based on quadrupolar interactions due to coupled charge density fluctuations of 4 f electrons and of conduction electrons. Conduction electrons are treated in tight-binding approximation. The γ − α transition is described as an orientational ordering of quadrupolar electronic densities in a Pa ¯3 structure. The quadrupolar order of the conduction electron densities is complementary to the quadrupolar order of 4 f electron densities. The inclusion of conduction electrons leads to an increase of the lattice contraction at the γ − α transition in comparison to the sole effect of 4 f electrons. We calculate the Bragg scattering law and suggest synchrotron radiation experiments in order to check the Pa ¯3 structure. The theory is capable of accounting for transitions to phases of non-cubic symmetry, but it is not sufficient to describe the magnetic phenomena which we ascribe to the Kondo mechanism. We also present a microscopic derivation of multipolar interactions and discuss the crystal field of γ -Ce.