Development of VUV-excited phosphors based on correlation between quantum cutting and acceptor-site arrangement within coordination

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We study quantum cutting via downconversion for the couple of R_1^<3+>-R_2^<3+>, R_1^<3+>-R_2^<3+>-R_3^<3+> (R_1=Gd, R_2=Eu, Tb, R_3=Er), and R_2^<3+>-R_2^<3+> (R_2=Tb), and investigate the dynamics and mechanisms of the energy transfer of their systems. In this research the results obtained are as follows.(1) In CsGd_2F_7: Er^<3+>, Tb^<3+> and CsGd_<2-x>, Y_xF_7: Er^<3+>, Tb^<3+> (x=0.2-0.4) mixed phosphors, the quantum cutting effect takes place upon VUV excitation in the 4f_<10> 5d levels of Er^<3+>. The dependence of the cross-relaxation efficiency ηCR on the Er^<3+> and Tb^<3+> concentrations are examined. The cross-relaxation efficiency ηCR decreases for CsGd_2F_7: Er^<3+>, Tb^<3+> as Er^<3+> concentration increases. With increasing Tb^<3+> concentration, the cross-relaxation efficiency ηCR CsGd_2F_7: Er^<3+> Tb^<3+> passes through a maximum. The cross-relaxation efficiency ηCR for CsGd_<2-x> Y_xF_7: Er^<3+> (1at.%), Tb^<3+> (0.2at.%) mixed crystals decreases with increasing Y^<3 … More +> concentration. We have also investigated the dynamics of the quantum cutting in CsGd_2F_7: Er^<3+>, Tb^<3+> by means of measurements of time-resolved luminescence spectra. The experimentally observed decay curves are explained by the two energy-transfer processes. The present analysis provides more insight in the dynamics of down conversion processes in the quantum cutting model.(2) Visible quantum cutting upon VUV and short-UV excitations in the 4f^75d states of Tb^<3+> in CsGd_2F_7: Tb^3 and KGd_3F_10: Tb^<3+> CsGd_2F_7: Tb^<3+>, via downconversion mechanism has been observed. The dependence of the cross-relaxation efficiency ηCR on the Tb^<3+> concentrations is examined. The cross-relaxation efficiency ηCR for CsGd_2F_7: Tb^<3+> KGd_3F_10: Tb^<3+> and KGd_3F_10: Tb^<3+> fluorides increases with increasing Tb^<3+> concentration upon VUV and short-UV excitations at 172 nm and 212 nm. We have obtained a green-emitting quantum cutting phosphor CsGd_2F_7: Tb_<3+> for which the calculated ηCR achieves 0.53 and 0.64 for VUV excitations. The time-resolved emission spectra of Tb^<3+> in CsGd_2F_7: Tb^<3+> suggest the a two-step energy transfer process by cross relaxation and direct energy transfer from one Tb^<3+> to a neighbouring Tb^<3+> and/or a neighbouring Gd^<3+>, -downconversion mechanism as proposed in the quantum cutting model.(3) Visible quantum cutting through downconversion is observed for the Gd^<3+>-Eu^<3+> couple in KLiGdF_5: Eu^<3+> (KLGF: Eu^<3+>) In this Gd^<3+>-based fluoride, visible quantum cutting, occurs upon VUV excitation of Gd^<3+> at the 6G_J level via two-step energy transfer from Gd^<3+> to Eu^<3+> by cross-relaxation and sequential transfer of the remaining excitation energy. The dependence of the efficiency of cross-relaxation on the Eu^<3+> doping concentration is discussed in terms of the probability of energy transfer, and it is found that KLGF crystal doped with 2 at.% Eu^<3+> exhibits the maximum cross-relaxation efficiency of 0.4. Based on thr number of nearest neighbors around Gd^<3+> ions and the ^5D_0 emission intensity, energy transfer from excited Gd^<3+> ions appears to extend to the fourth-nearest Eu^<3+> neighbors in the quantum cutting process.(4)Ce^<3+> doped SrMgF_4 and BaMgF_4 crystals have the absorption and luminescence spectra in the VUV and UV ranges, respectively, so that wide pumping and tuning ranges are expended for laser operation. The persistent spectral hole burning (PSHB) in KLaF_4: Sm^<2+> has been observed at 12K. UV or visible excitation for Eu^<2+-> doped Ba_2SiO_4 and Ba_3SiO_5 crystals produces broadband luminescence in the green and yellow regions, respectively. The temperature dependence of the long-lasting phosphorescence suggest that electrons and holes produced by UV excitation move back to Eu^<2+> sites in the crystals through hopping and tunneling, and recombine radiatively at Eu^<2+>. Less