Actually i was far from your field. Seen REE having tetrad effect in solid and liquid. And due its laser capablities and effective doped glass system via Racah formula development , i believe REE such as Gd, Nd, Ce especially and etc. might be have potential for up conversion or down conversion or reversible both reaction process. However your are inclined to see and find many papers and books published by Dr. Glenn T.Seaborg and Prof Salam. Very interesting.