π-Bonding approach in the modified charge model

Abstract

Originating from the modified point charge model (MPCM) showed recently to be successful in calculations of crystal field parameters (CFP) in the σ-bonding limit (I), a new π-bonding approach (II) is proposed. Unlike (I) which only defines R eff and q eff the π-bonding approach requires extra adjustable parameters: one ξ for cubic and at least two parameters for lower crystal field symmetries. These adjustable parameters reflect a displacement of the electric charges whenever instead of the σ, the π-bonding occurs. It has been shown for the cubic symmetry case that predicted by (I) signs of A 4〈 r 4〉 and A 6〈 r 6〉 crystal field parameters may be reversed if (II) is applied. Since in many ƒ-electron intermetallics the CFP signs determined experimentally are different from those expected in the framework of (I), the use of (II) seems to be justified. Setting up ξ = ϵ L /2 ϵ M the (II) gives an excellent agreement with the experimental CFP values for many RE-intermetallics and is considered to be a further extension of MPCM over the non-semiconducting ƒ-electron systems.