SmF3 in neat (no solvent) prefluorination of NH4MnF3 (mixed with SmF3) at about 100°C with N2-diluted F2 followed by treatment with pure F2 at 250°C and 200 bar, heating the sealed mixt. to 650°C and maintaining the temp. for 21-28 d; cooling the mixt. at 40°C/d;