Structure–luminescence relationship in Eu3+-doped Sr3La2(Ge3O9)2 phosphors

A series of Sr3La2-xEux(Ge3O9)2 (x = 0.1–1.8) cyclogermanates has been prepared by the citrate method. The powder XRD study has shown that the compounds crystallize in the monoclinic space group C2/c, Z = 4. An increase in the dopant content causes a morphotropic phase transition: at x = 0.1–0.7 the samples are isostructural to Sr3La2(Ge3O9)2, while at x = 0.9–1.8 the germanates have a crystal structure of Sr3Eu2(Ge3O9)2. In all compounds, strontium and rare earth ions occupy three symmetry independent cation sites and form layers alternating along the [101¯] direction. The relationships between the crystal structure and the luminescence characteristics: the energy of the charge transfer transition, the values of 4f–4f/CTB and 5D0 → 7F2/5D0 → 7F1 intensity ratios have been revealed. CIE chromaticity coordinates for Sr3La1.4Eu0.6(Ge3O9)2 and Sr3La0.8Eu1.2(Ge3O9)2 phosphors under 393 nm excitation have been measured.