EPR and ENDOR analysis of Fe3+ impurity centers in fluoroelpasolite lattices

Abstract

Fe³⁺ ions in hexagonal and cubic fluoroelpasolite crystals (A^I₂B^IM^IIIF₆) have been investigated in a combined Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) study. A detailed analysis of the ENDOR spectra for the nearest ¹⁹F and ²³Na shells in X (9.5 GHz) and Q band (34 GHz) allowed the complex EPR spectra to be disentangled and to determine the spin Hamiltonian parameters for the various S = 5/2 Fe³⁺ centres. W-band (95 GHz) EPR measurements as a function of temperature were performed to provide unambiguous evidence about the absolute signs of the Zero Field Splitting (ZFS) and SuperHyperFine (SHF) parameters for Fe³⁺ in Cs₂NaAlF₆ as already determined from the ENDOR work. It could be concluded that all principal ¹⁹F hyperfine values were positive, in agreement with earlier assignments in the literature for related systems. A comparative analysis of the ¹⁹F SHF data for Fe³⁺ at a perfectly octahedral site in the cubic crystal, and at two slightly trigonally distorted environments in the hexagonal crystals, indicates that the metal-to-ligand distance changes upon doping. The obtained set of parameters concerning one defect in various analogous environments can furthermore be used to test different methods of theoretical calculations for ZFS and SHF values.