Systematization of crystal field parameters for trivalent rare-earth (RE3+) ions at orthorhombic sites in selected laser materials—standardization approach Original Research Article

The crucial intrinsic features of orthorhombic crystal field (CF) parameter (CFP) sets, i.e. existence of alternative disparate sets, which belong to different regions in the CFP space, and incorrectness of comparison of such CFP sets, are elucidated. Several cases of incorrect comparisons of fitted CFP sets obtained by various authors have been revealed by survey of spectroscopic studies of rare-earth ions at orthorhombic sites in technologically important crystals. This finding indicates that these features are not fully realized and calls for clarification of implications of these features for interpretation of spectroscopic data. For this purpose, we outline standardization approach for dealing with intrinsically incompatible CFP sets. It is shown that direct comparisons of disparate yet correlated CFP sets lead to incorrect conclusions concerning the relative magnitudes of CFPs and structural implications. The axis systems in which the theoretical CFPs are expressed or which are assigned to the fitted CFPs are also discussed. The standardization approach is employed for systematization and comparative analysis of the orthorhombic non-standard CFP sets for selected systems, i.e. Er3+:SrLaAlO4, Ce3+:Ce2Ge2In, Eu3+:BaFCl, Er3+:LiKGdF5, Eu3+:SBN, Pr3+:K2PrCl5 and Pr3+:K2YF5. The present results may facilitate future spectroscopic determination of CFPs and increase reliability of fitted and theoretical CFPs for fN ions at orthorhombic sites.