On the structures and luminescence properties of Eu³⁺ -doped Li₂ CaGeO₄ , Ca₂ GeO₄ , and Ca₅ Ge₃ O₁₁ compounds

Powder samples of Li₂ CaGeO₄ , Ca₂ GeO₄ , and Ca₅ Ge₃ O₁₁ doped by 0.5, 1, 2, 3, 4 and 5 at% Eu³⁺ relative to the Ca²⁺ , were prepared using a conventional solid-state synthesis technique. X-ray diffraction (XRD) analyses confirmed obtaining the pure phases at all dopant concentrations. In parallel, single crystals of the three compounds with the experimentally found optimal Eu³⁺ concentration were grown using a flux method. Structural investigation on the single crystals were done with a special attention to the form of the Ca-O polyhedron, the mean Ca-O distance, the Ca-Ca distance in the structure, the distortion degree of the polyhedron, as well as the Eu-Ca substitution site. The main spectral characteristics were analyzed and several relationships between the structural and spectra features were found. The optimal dopant concentration was 3 at% for Ca₂ GeO₄ and 4at% for Ca₅ Ge₃ O₁₁ and Li₂ CaGeO₄ . Commission Internationale de l'éclairage coordinates of the samples showed emission colours in the red region close to the standard red coordinates and slightly influenced by the active ion concentration. The obtained results showed that europium-doped Li₂ CaGeO₄ , Ca₂ GeO₄ , and Ca₅ Ge₃ O₁₁ could be used as red phosphors.