Kinetically Determined Crystal Structures of Undoped and La3+-Doped LnF3

Abstract

A series of lanthanide fluoride nanoparticles were prepared with a simple colloidal approach at 75 °C. All the nanoparticles are highly water-dispersible with sizes in the range of 3−10 nm. The light lanthanide fluoride salts, LaF3, CeF3, and NdF3, have the same trigonal crystal structure as the corresponding bulk materials. However, for the fluoride salts of the heavy Dy, Ho, Er, and Yb, nonstoichiometric cubic structures of NaxLnyFz composition were identified. Particularly, the middle GdF3 and EuF3 nanoparticles have both trigonal and orthorhombic crystal phases instead of a single orthorhombic phase as for the corresponding bulk materials, which is attributed to kinetically formed products. Study of La3+-doped GdF3 nanoparticles showed that 15% La3+ doping is sufficient for GdF3 nanoparticles to crystallize completely in the trigonal phase, i.e., the same as LaF3, which is dramatically different than 50% La3+ doping for the bulk. Finally, on the basis of the thermodynamic cycle for the preparation of the doped materials, calculations carried out explain well the published experimental results for the bulk. However, this approach failed to explain the results of the nanoparticles. Evidence is provided that the formation of kinetic products is responsible.