Nature self-imposed limits and existence of intrinsically incomparable crystal (ligand) field parameter sets for transition ions at orthorhombic, monoclinic, and triclinic symmetry sites in crystals: pitfalls and blessings

Abstract

Central quantities in spectroscopy and magnetism of transition ions in crystals are crystal field parameters (CFPs). Nature self-imposed limits on orthorhombic and lower symmetry CFPs, and existence of intrinsically incomparable CFP sets are elucidated. Comparisons of such CFP sets result in controversial claims, incorrect structural conclusions or CFP systematics, and misinterpretation of spectroscopic data. Considerable extent of such pitfalls in literature indicates many researchers remain unaware of the intrinsic features of low symmetry CFPs. This calls for coordinated remedial actions. To avoid pitfalls, comparative methodology based on standardization of the rhombicity ratio to express CFP sets in the same region of CF parameter space is proposed. Wider utilization of alternative CFP sets in the multiple correlated fitting technique to improve reliability (blessings) of final fitted CFPs is advocated. This review may be of interest to physicists and chemists working on various materials as well as to science historians and philosophers.