Comparative analysis of crystal-field parameters for rare-earth ions at monoclinic sites inAB(WO4)2 crystals:I. Tm3+ inKGd(WO4)2 andKLu(WO4)2, andHo3+ andEr3+ ionsin KGd(WO4)2

Abstract

The crystal-field (CF) parameters determined by various authors for rare-earth ions at monoclinic sitesin AB(WO4)2 crystals are reanalyzed using a methodology incorporating several approaches, namely standardization,multiple-correlated fitting technique and closeness of CFP sets. In Part I recent spectroscopic data forTm3+ ions inKGd(WO4)2 (KGdW) andKLu(WO4)2 (KLuW),and Ho3+ and Er3+ ions in KGdW, which were interpreted using the free-ion (FI) and CF parameter (CFP)sets, are thoroughly revisited. Our reanalysis enables clarification of several doubtfulaspects involved in the previous studies. The initial CFPs for fitting, calculatedusing the simple overlap model (SOM), differ markedly from the fitted CFPs forTm3+ ions in KGdW and KLuW. An inspection of the pertinent CFP sets reveals deeper intrinsicdifferences between the model and fitted CFPs. The model CFPs and the fitted CFPs forRE3+ ions in both KGdW and KLuW crystals turn out to be non-standard. Importantly, themodel and fitted CFP sets for Tm-KLuW belong to disparate regions of the CFP space andthus are intrinsically incompatible, i.e. such sets should not be directly compared. Thus theCFP sets reported in the literature require reconsideration in view of the intrinsicproperties of monoclinic CF Hamiltonians previously not taken into account.Standardization of the originally non-standard CFP sets is carried out to ensure directcomparability of the CFP sets in question with other literature data. The correlatedalternative CFP sets are calculated for each original set to facilitate future applications ofthe multiple correlated fitting technique, which enables improving overall reliabilityof the fitted CFPs. The closeness of the standardized CFP sets is assessed in aquantitative way. Our considerations indicate also the importance of proper definitionsof the axis system used in the CFP model calculations and provide argumentsfor the nominal meaning of the axis systems assigned to the fitted CFPs. Theconsistent methodology proposed here may be considered as a general framework foranalysis of CF levels and CFP modelling for rare-earth and transition-metal ionsat monoclinic symmetry sites in crystals. CFP sets for other rare-earth ions inAB(WO4)2 crystals will be reanalyzed in Part II.