Phonon induced instabilities in intermediate valence compounds

The coupling of 4f electrons and longitudinal optical phonons and its implications on different types of structural phase transitions in intermediate valence compounds is discussed within the framework of the periodic Anderson model. Two types of interactions are considered. First, the usual density type of coupling of 4f electrons and phonons leads to a weakly temperature dependent renormalization of the positionE ₀ of the 4f level with respect to the 5d band. Secondly, phonon induced 4f–5d interband transitions lead to a renormalization of the hybridization energyV of 4f and conduction electrons. For appropriate parameter values the latter effect gives rise to discontinuous changes of the occupation of the 4f state.The tendency towards a ferromagnetic or antiferromagnetic instability is essentially suppressed by the presence of phonons.The dependence of the susceptibility on temperature and onE ₀ is calculated and a jump is found whenever a discontinuous change of the occupation of the 4f state occurs.Work performed within the research program of the Sonderforschungsbereich 125, Aachen-Jülich-Köln