Standardization of crystal field parameters for rare-earth (RE3+) ions at monoclinic sites in selected laser crystals

Abstract

Survey of spectroscopic studies of trivalent rare-earth (RE3+) ions at orthorhombic or monoclinic sites in various crystals, which are important in view of promising technological applications, has revealed a number of crystal field (CF) parameter (CFP) sets belonging to different ranges in the CF parameter space. This indicates that the major properties of orthorhombic and monoclinic CF Hamiltonians have not been fully realized in literature. This includes the existence of alternative, i.e. physically equivalent, CF parameter (CFP) sets and incorrectness of comparison of such alternative yet disparate CFP sets, which due to their intrinsic features cannot be directly compared. Proper methodology for dealing with such CFP sets has been outlined earlier. In this paper we consider implications of these properties for interpretation of (i) spectroscopic data for rare-earth ions obtained from several experimental techniques and (ii) the axis systems in which the fitted CFPs are supposedly expressed. Since these aspects are not realized by many researchers, incorrect conclusions concerning, e.g. the strength of CF and structural implications, have often been drawn. The standardization approach is employed for meaningful comparative analysis of the CFP sets for RE3+ ions at monoclinic sites in selected laser crystals. The following ion-host systems are studied: Nd3+: LuAlO3, Nd3+: BaY2F8, Eu3+: Gd2O3, Eu3+, Pr3+, Tb3+, Ho3+, Tm3+, Nd3+, Sm3+, Dy3+, Er3+: Y2O3, Er3+: Er2O3, Pr3+, Sm3+, Gd3+, Er3+: LaF3, Er3+: YAlO3. A representative sample of CFP sets is analyzed, standardized, and presented in a unified way. The alternative CFP sets provided here may be utilized in the multiple correlated fitting technique to improve reliability of final fitted CFPs. The present approach may facilitate future spectroscopic determination of CFPs as well as increase reliability of fitted and theoretical CFPs for fN (RE3+) ions at lower symmetry sites.