On the standardization of crystal-field parameters and the multiple correlated fitting technique: Applications to rare-earth compounds

This work investigates the crystal-field parameter (CFP) sets for rare-eart h (RE) ions doped at orthorhombic and/or lower symmetry, i.e. monoclinic or triclinic sites. Two important questions are addressed: (1) How do you com pare CFP sets reported by different authors when there can be as many as si x numerically different equivalent sets? and (2) How do you distinguish bet ween global and local minima in the multi-parameter fittings? To answer the first question we propose to adopt the standardization of CFP sets. The or thorhombic and/or monoclinic standardization has been carried out for sever al illustrative CFP sets, including the contributions from various mechanis ms, for NdF3, RE2Te4O11, and RE3 + in garnet materials. It is shown that ad opting a well-defined standardization convention enables meaningful compari son of CFP sets taken from various sources. The analysis of literature data reveals also cases when the CF parameters may be misinterpreted due to the choice of a different representation in the multi-parameter fittings. To a nswer the second question we propose the multiple correlated fitting techni que (MCFT). Using experimental data for Nd3+ in YAG and LaF3, MCFT is emplo yed to probe for more reliable CFP sets. (C) 2000 Elsevier Science B.V. All rights reserved.