A general trend of rare earth ions in the KRE(WO4)2 double tungstates (RE = Y, Yb, Gd, Lu)

Crystal-field energy levels of Er3+ ions doped in KYb(WO4)2, KY(WO4)2 and KLu(WO4)2 laser crystals were fitted to the parameters of semi-empirical Hamiltonian including J-mixing. A simulation of the energy level scheme is carried out. The crystal-field effect is introduced through the 14 non-zero crystal-field parameters allowed by C2 symmetry site occupied by the rare earth. Very satisfactory correlations were obtained between 58, 64 and 15 calculated and experimental Stark energy levels, with an r.m.s. of 8.7 cm−1, 9.3 cm−1 and 2.6 cm−1 for Er3+ ions doped in KYb(WO4)2, KY(WO4)2 and KLu(WO4)2 respectively. These results are compared with those reported for other rare earth ions in the same hosts and with Er3+ ions in KGd(WO4)2 crystal. Furthermore, based on the extended electrostatic point charge model, interesting results are obtained for Yb3+ ions doped KYb(WO4)2, KY(WO4)2, KLu(WO4)2 and KGd(WO4)2 crystals. The concordance between the crystal-field strength parameters indicates the consistence of our analysis. A general trend is then proposed for the rare earth ions in the KRE(WO4)2 double tungstates.