Energy levels of terbium(III) in the elpasolite Cs2NaTbBr6. II. A correlation crystal field analysis

A set of more than 100 electronic energy levels in Cs₂NaTbBr₆, extending from the ground state to ⁵H₄, is used to test different models of the correlation crystal field (CCF). These are based on Judd's orthogonal g_iQ ^(k) two-electron operators, and more specifically on contributions due to spin-correlation, or ligand polarization. Similar data from Cs₂NaTbCl₆ and Cs₂NaTbF₆ has also been analysed. Only fourth-rank operators make clear improvements to the quality of the fit in deviant multiplets. Empirically the g₇ ⁽⁴⁾ and g₉ ⁽⁴⁾ operators are found to be the most effective. Although fourth-rank operators achieve modest success in correcting the calculated spread of the multiplets, no single operator has a significant impact on the shortcomings of the one-body crystal field. This result is discussed in terms of the limitations of an effective-operator Hamiltonian.