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

A series of Sr₃La_2-xEu_x(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 [ 10 1 bar ] 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 Sr₃La_1.4Eu_0.6(Ge₃O₉)₂ and Sr₃La_0.8Eu_1.2(Ge₃O₉)₂ phosphors under 393 nm excitation have been measured.