Jahn-Teller Effect on Rare Earth Dioxides

A number of experimental facts found in PrO2 and UO2, such as anomalies in the magnetic excitations spectrum, the reduction of the ordered magnetic moment for T < TN and the distortion of the oxygen cage are interpreted as consequences of electron-phonon coupling and cooperative effects. A simple microscopic model including crystal field, magnetic exchange interaction and cooperative dynamical Jahn-Teller coupling is here proposed. A coupling to phonons of trigonal symmetry is found appropriate to describe the experimental results. A single-centre model can explain the reduction of the ordered magnetic moment and also the main features of the differential cross section; however, in PrO2, a cooperative interaction is necessary to interpret the observed distortion of the oxygen cage and the observed splitting of the ground state into two doublets in the paramagnetic phase.