MICROSCOPIC CHARACTERIZATION OF IMPURITIES IN INSULATORS THROUGH EPR

Methods for obtaining information on the true impurity-ligand distances, RI, and covalency for transition-metal impurities in insulators are reviewed in this article. As regards RI both the superposition model (SPM) and the method based on the analysis of the isotropic superhyperfine (shf) constant As, are discussed. It is pointed out that: (1) the degree of validity of the empirical SPM for understanding the zero-field splitting is still not known due to the lack of systems for which metal-ligand distances or their variations under pressure have been determined previously; (2) several reported values of t2 and b2 need revision because they have been derived ignoring the local relaxation. Examples involving Mn2+ in local D4h and D2h symmetries are discussed in detail. The determination of RI from As, based on the results of reliable molecular orbital calculations, gives reasonable results for Mn2+ in octahedral and hexahedral geometries and also for "unstable" impurities like Cr+, Ni+, Ag0 and Cu0. For Mn2+ in fluoroperovskites the agreement with the RI values from EXAFS and optical data is good. The foundations of this method which allow one to detect ΔRI changes down to ~0.02 pm are carefully analysed. The determination of covalency for d9 impurities in insulators is examined critically and the necessity of using the right MO framework for deriving reliable results is stressed. In particular the role played by the often neglected charge transfer excitations, which can dominate the g-shifts of d9 as well as of d7 and s1 impurities, is emphasized. The reported values of the covalency for several d9 systems are consistent with the electronegativity of the impurity and the ligands. Also they indicate that in KCl:Ag2+ the unpaired electron spends more time on the ligands than on Ag2+. Finally, the importance of lattice relaxation for a correct understanding of both the g-shifts of Ni+ centres and the anisotropic shf constant Ap of d5 impurities is pointed out. © 1990.