Comprehensive analysis of crystal field parameter datasets for transition ions at low symmetry sites and extracting structural information—Application to Pr 4+ in BaPrO 3

Abstract

Our recent literature survey has revealed several crystal field parameter (CFP) datasets for rare-earth ions at orthorhombic, monoclinic, and triclinic symmetry sites in various hosts. Often CFP datasets are not directly comparable, even for the same ion-host system, whereas the low symmetry effects observed may be either actual or apparent. Careful considerations are required to distinguish the nature of the low symmetry effects and extract useful structural information inherent in the low symmetry CFPs. For this purpose, we propose a comprehensive approach comprising three methods. First method consists in finding the principal values of the second-rank CFPs and the orientation of their principal axis system w.r.t. the original or crystallographic axis system. Second method consists in extending the cubic/axial pseudosymmetry axes method to lower symmetry cases. Third method consists in considering for quantitative comparison of CFP datasets of the closeness factors C p and the norms ratios R p = N A/ N B for the respective H CF terms: p = k = 2, 4, and 6, and the global ( p = gl) ones. Usefulness of the various physically equivalent CFP datasets generated by the first and second method as initial sets for the multiple correlated fitting technique is also discussed. This study benefits from cross-fertilization between the spin Hamiltonian theory used in the EMR area and the CF theory, since these methods have been used before in the EMR area. As an application of this approach, for the first time in the CF theory area, we reanalyze the triclinic-like CFPs and the low symmetry effects involved therein as well as the cubic CFPs for Pr 4+ in BaPrO 3.