The unusual temperature dependence of the Eu$^{2+}$ fluorescence   lifetime in CaF$_2$ crystals
  Fluorescence lifetimes of Eu$^{2+}$ doped in CaF$_2$ are measured at various temperatures between 4K and 450K. The lifetime increases with between 100K and 300K, in contrast to the usual lifetime-temperature dependence. At higher or lower temperatures the lifetime decreases with increasing temperature. The phenomenon is explained by simulation of the energy levels and lifetimes of low-lying $4f^65d$ states involved in the fluorescence. 
C.K. Duan and A. Mejerink and R.J. Reeves and M.F. Reid