In water based solutions the lanthanium salts will quickly hydrolyze and form hydroxides and carbonates. Best thing to do is to use an aprotic (solvates the kation) polar solution like acetone, THF, ethylacetate, DMF or DMSO. They all have high dipole moments and I think you have to experiment a bit which solvent works best. Since acetone is readily available I would start with this solvent. Of course be cautious with the highly flamable nature of acetone.