The angular overlap model of ligand field theory for f elements: An intuitive approach building bridges between theory and experiment

Abstract

Lanthanoid ions are well-known for their characteristic optical and magnetic properties interesting for a wide range of applications. Most of these properties show a subtle dependence on the surrounding ligands. Unlike the transition metal ions with their often encountered octahedral or tetrahedral coordination in compounds, rare earth ions are typically found in coordination geometries with low symmetries and high coordination numbers. The quantitative treatment of the ligand field in the so-called Wybourne scheme then becomes overparametrized and not readily insightful but merely a technical tool. In this review, we want to present the alternative approach by the chemically intuitive angular overlap model (AOM) of ligand field theory that is independent from symmetry and allows to decompose any coordination geometry into its separate metal–ligand bonds with well-known bonding character. It is the goal of this review to demonstrate its strength in the semi-quantitative description of opto-magnetic properties of the rare earth ions on carefully selected examples from practice. Finally, we compare the AOM to conventional Wybourne-parametrized ligand field theory and show how the two schemes of ligand field theory are related to one another. Overall, this review offers a perspective on future approaches to ligand field calculations with f elements with the benefit of full transparency and intuitive understanding rather than technical fitting.