Sign change of the c.e.f. parameters in light rare earth compounds in relation with the delocalization of the 4f shell

The crystal field level scheme of RX3 (R = Ce, Pr, Nd ; X = In, Pb) intermetallic compounds has been determined by fitting susceptibility curves and specific heat Schottky anomalies ; an inelastic neutron diffraction experiment has been performed for NdIn3. The fourth order C. E. F. parameter A04 < r4 > of the cérium compounds is positive ; it decreases for the praseodymium compounds and becomes negative for the neodymium compounds. The sixth order C. E. F. parameter A0 6 < r6 > exhibits the same trend to negative values from praseodymium to neodymium compounds. This sign change of the C. E. F. parameters appears to be related to the small energy difference between the conduction band and the 4f levels at the beginning of the first rare earth series.