| Abstract: | Some Aspects of the Structural Chemistry of the Rare Earth Trifluorides The preparation and characterization of the rare earth trifluorides crystallizing in the hexagonal LaF3-type and the orthorhombic YF3-type is described. The lattice parameters were derived from GUINIER powder data of high precision. These lattice parameters served to calculate the effective ionic radii of the tripositive rare earth ions for coordination numbers 8 and 9. It is shown that both structure types can be stabiblized by the formation of nonstoichiometric compounds of the general formula (Me, RE)Fx (Me ? Eu2+, Sr2+ and RE ? Eu3+, Gd3+) with x varying approximately from 2,77 to 2,94 for the hexagonal and from 2,94 to 3,00 for the orthorhombic nonstoichiometric phases. For the pure trifluorides and the nonstoichiometric phases a critical radius ratio of 0.830±0.008 was found for the changeover from the LaF3- to the YF3-structure. |
