Ligand dependence of the correlation crystal field

When the crystal-field components of 26 J multiplets in the elpasolites Cs₂NaTbX₆ (X=F,Cl,Br) are modeled by a one-electron crystal-field Hamiltonian, major variations are found in the accuracy with which the splitting of each multiplet is reproduced. The effective crystal field within each multiplet is quantified by a degeneracy-weighted standard deviation. Differences between experimental and calculated splittings of up to 80% of the theoretical value are observed. They provide the most extensive example so far reported of electron correlation induced by the crystal field (CCF). The percentage deviation of each multiplet from the one-electron model decreases in the order Br>Cl>F. This indicates the importance of covalency and/or ligand polarizability contributions to the CCF.