Abstract
The valence state of cerium in CeCo5, CeNi5, CeCu 5 and the pseudo-binary Ce(Ni1-xCux)5 compounds has been studied by means of measurements of lattice parameters, magnetic properties and X-ray absorption. Whereas the lattice parameters of CeCu5 correspond to a Ce3+ valence state, X-ray absorption experiments show that, in CeCo5 and CeNi5, Ce has the same valence state, close to 4 +, as in the insulator CeO2. Stoichiometry deviations in the RCo 5 compounds (R = rare earth) can be correlated with the difference in the anomalies of the α lattice parameter of Ce compounds in the RCo 5 and RNi5 series. In Ce(Ni0.1Cu0.9) 5, Ce is in the 3+ state and the thermal variation of the susceptibility is characteristic of crystal field effects in the Ce3+ ion. In contrast the susceptibility of CeNi5 is one order of magnitude smaller and almost isotropic; as in YNi5, it arises mainly from the Ni 3d electrons which are close to the onset of ferromagnetism. In the Ce(Ni1-xCux) 5 compounds, the Ce valence state decreases linearly with x up to x = 0.3 and more rapidly for higher copper concentrations. The evolution of the Ce valence state is discussed in terms of the band structure of the different compounds, especially the relative position of the Fermi level and the 4f 1 state.