Chemical Bonding, 10Dq Parameter and Superexchange in the Model Compound KNiF₃

Abstract

The cubic field splitting parameter, 10Dq, plays a central role in the ligand field theory on insulating transition metal compounds. Experimental data prove that 10Dq depends on changes of the metal‐ligand distance, R. Despite this fact has consequences on optical and magnetic properties of such compounds, its actual origin is still controversial. Seeking to clarify that crucial issue, this work is focused on KNiF3, a reference system among insulating transition metal compounds. By means of first principles calculations we show that, the R‐dependence of 10Dq does not arise neither from the crystal field contribution nor from the covalent admixture of 3d(Ni) with valence 2p(F) orbitals. Indeed, we prove that it is mainly due to the covalency with 2s(F) orbitals, highly sensitive to R variations. The calculations show that the 3d‐2pσ and 3d‐2pπ admixtures raise practically equal the energy of antibonding eg and t2g orbitals of NiF64‐ units in KNiF3 thus leading to a null contribution to 10Dq. This conclusion is not significantly altered when considering the excited state 3T2(t2g5eg3). The different influence of chemical bonding on the superexchange constant, J, and 10Dq is also discussed in a second step. The meaning of 10Dq is also briefly analyzed.