Enhanced f and d state upconversion of Ln3+ and Cr3+ by codoping zinc and cadmium in gallogermanates

(ZnxCd(1-x))(3)Ga2GeO8:Cr3+, Yb3+, Ln(3+) (Ln = Er, Ho, Tm) phosphors are developed in comparison to the reported Zn3Ga2GeO8:Cr3+, Yb3+, Er3+ phosphor. In these zinc and cadmium codoped gallogermanates, both f and d state upconversion of Ln(3+) and Cr3+ ions are much stronger than those of zinc or cadmium singly doped samples under 980 nm excitation. Also, the enhancement of ultraviolet light irradiated Cr3+ persistent luminescence is observed in codoped substrates. Through fluorescence kinetic analysis, it is concluded that not only is there energy transfer from Ho3+ to Cr3+ ions but energy back transfer from Cr3+ to Ho3+ ions. The former one leads to the upconversion of Cr3+ and the latter to the red upconversion of Ho3+ ions. The change of phonon energy and symmetry of substrates due to codoping cadmium ions, should be responsible for the upconversion enhancements. Their combined effects yield an optimal value of Zn/Cd ratio for the highest upconversion efficiency.