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

(Zn_xCd_1-x)₃Ga₂GeO₈:Cr³⁺, Yb³⁺, Ln³⁺ (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 Ln³⁺ 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.