2016 started early this year, check out this new publication about valence instability in europium and terbium phosphor and how it is studied.

Rodrigues, L.C.V., Hölsä, J., Brito, H.F., Maryško, M., Matos, J.R., Paturi, P., Rodrigues, R.V., and Lastusaari, M., Magneto-optical studies of valence instability in europium and terbium phosphors, J. Lumin. 170 (2016) 701-706.
Read online: http://dx.doi.org/10.1016/j.jlumin.2015.08.018