Effects of screening by conduction electrons on the crystal electric field in REAl₂intermetallic compounds

The fourth-order B 4 and sixth-order B 6 crystal electric field (CEF) parameters are evaluated for NdAl 2 , taking into account the contribution of the whole lattice of ions, by the use of reciprocal space according to the Duthie-Heine technique. Screening of the CEF by free conduction electrons gives a parameter B 4 of opposite sign to the experimental value. This becomes correct if the screening is removed, the point-ion limit taken and the Al charge made zero. A simple explanation of this striking and unexpected result is offered by screening the rare-earth spheres with electrons with d character, while keeping the Al ones screened by free electrons. The situation for B 6 is less satisfactory, the calculated value being one order of magnitude smaller than the experimental one. These conclusions can be extended to the rest of the REAl 2 series (RE represents a rare earth). The effect of some internal structure on the ions is also considered.