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

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Abstract
The fourth-order B4 and sixth-order B6 crystal electric field (CEF) parameters are evaluated for NdAl2, 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 B4 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 B6 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 REAl2 series (RE represents a rare earth). The effect of some internal structure on the ions is also considered.