Valence fluctuations,crystal structure stability and the effects of alloying

The systematics of the effects of alloying on the valence of cerium are discussed. In addition to factors such as the size and valence of the substitute atom and resulting “lattice pressure” and motion of the f level, the possible role of crystal structure stability is examined. In particular, a detailed analysis of both on cerium site and off cerium site alloying of CePd₃ suggests that crystal structure stability considerations are of over-riding importance for this system. For the Cu₃Au structure, the existence of a maximum number N_max of electrons per formula unit, which is determined from studies of Zr(Rh,Pd)₃, allows one correctly to predict the concentrations at which trivalence and saturated valence occur in six related pseudobinary alloy systems. However, these predictions are based on a model which requires:

• 1.(a) that in the mixed valence regime the valence of cerium adjusts so that the total number of electrons per cell is constant at N_max, and
• 2.(b) that cerium be tetravalent in CeRh₃.

The latter requirement is in strong disagreement with studies using microscopic probes such as L_III, absorption, X-ray photoelectron spectroscopy (XPS) and bremsstrahlung isochromat spectroscopy (BIS). Nonetheless, the predictive ability of the model is striking and cannot be dismissed as merely fortuitous.