Zn2−aGeO4:aRE and Zn2Ge1−aO4:aRE (RE = Ce3+, Eu3+, Tb3+, Dy3+): 4f–4f and 5d–4f transition luminescence of rare earth ions under different substitution
Generally, luminescent properties of rare earth ions doped host can be tuned by controlling the host composition, that is, when substituted for different cations of host, the rare earths ions can present different characteristics. In this research, Zn2−xGeO4:xCe3+ and Zn2Ge1−xO4:xCe3+ can show two different emission bands due to the 5d–4f transitions of Ce3+ ions. Moreover, the CIE chromaticity coordinates and the luminescence photographs of Zn2−xGeO4:xCe3+ and Zn2Ge1−xO4:xCe3+ exhibit different characteristics. The emission bands appear blueshift and redshift due to the combination of crystal field splitting and nephelauxetic effect. However, for Zn2−aGeO4:aR and Zn2Ge1−aO4:aR (R = Eu3+, Tb3+, Dy3+), the 4f–4f characteristic transitions of Tb3+, Eu3+ and Dy3+ are observed, and the emission and excitation peaks are not influenced by the crystal field.