State mixing and the cubic crystal field approximation for rare earth ions: the case of the Er3+ ion in axial crystal fields

Abstract

The validity of the cubic crystal field (CCF) approximation for the interpretation of the magnetic resonance properties of the Er3+ ion in crystal fields with tetragonal and trigonal symmetry is examined. The ground state paramagnetic resonance principal g values are explicitly calculated in terms of the cubic crystal field eigenstates as a function of axial crystal field strength. It is shown that, depending on the ground state crystal field eigenstate, the widely accepted CCF approximation of simply taking the average of the trace of the g tensor and equating it to the g value found in cubic symmetry can lead to a misinterpretation of the ground state Stark level and the lattice coordination of the ion. The implications for experimentally reported results are discussed.

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