The EPR spectra of Gd3+ and Eu2+ in glassy systems

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Abstract
A critical analysis is given of the EPR spectrum exhibited by the rare‐earth S‐state ions, Gd3+ and Eu2+, in glassy and disordered polycrystalline materials. The analysis of this spectrum and of its previous interpretations is based on (a) a set of criteria derived from a wide range of experimental EPR and optical data, and (b) a first principles computer simulation method which explicitly incorporates broad distributions in the crystal field interaction parameters. It is found that all four previous interpretations of the glassy spectrum are unsatisfactory, each failing to satisfy two or more of the criteria imposed by the full range of data. The correct general solution to the spectrum is unequivocally established and shown to be a convolution of (a) a broad and essentially unimodal distribution of second‐order crystal field parameters, b02, with a maximum in the approximate range 0.051≲b02 ≲0.056 cm−1, and (b) a broad distribution of asymmetry parameters, λ’=b22/b02, with appreciable probability over the whole range 0.0≤ λ’≤1.0. The prominent features in the X‐band spectrum at g∼6.0 and 2.8 are found to be the result of specific EPR transitions that are stationary with respect to b02, λ’, and the orientation angles of the applied field H over a wide range. The quantitive results indicate that the site symmetries of the RE ions are essentially very low and disordered, and are best characterized by a single low‐symmetry ‘‘glassy type’’ site.