Intervalence charge transfer in Pr3+- and Tb3+-doped double tungstate crystals KRE(WO4)2 (RE = Y, Gd, Yb, Lu)

First experimental evidences of intervalence charge transfer (IVCT) transitions in Pr3+- and Tb3+-doped double tungstate crystals KRE(WO4)2 (RE = Y, Gd, Yb, Lu) are reported. The IVCT bands are located nearly at the same energies for Pr3+ and Tb3+ in a given crystal and these energies are consistent with an empirical model recently developed for Pr3+ ions in titanates, vanadates, niobates and tantalates. These data made it possible to locate the 4fn states of trivalent lanthanides in KREW crystals by applying the “double seat” empirical model proposed by Dorenbos.