Local structural features and composition of the Bi4Ge3O12:Dy³⁺crystals: Effect of doping concentration

Bismuth germanate Bi4Ge3O12 single crystals doped with 0.1 and 1.0 wt% Dy2O3 have been successfully grown by the Czochralski method. The actual compositions of the Dy3+-doped Bi4Ge3O12 crystals have been refined for the first time using single-crystal and synchrotron powder X-ray diffraction. Different local structural behavior of the Dy3+ and Bi3+ ions in the Bi4Ge3O12 crystal matrix depending on the initial Dy3+ content has been revealed by X-ray absorption spectroscopy. In the crystal with the refined composition (Bi3.996(11)Dy0.004)Ge3O12 (0.1 wt% Dy2O3), the octahedron Dy3+O7 with a split vertex and vacancy octahedron Bi3+O5 are formed in adjacent octahedra. In the crystal with the refined composition (Bi3.952(47)Dy0.048)Ge3O12 (1.0 wt% Dy2O3), the semi-octahedron Dy3+O5 and distorted octahedron Bi3+O6 are realized. A character of distribution of the residual electron density showed a probable decrease in the symmetry of Dy3+-doped Bi4Ge3O12 crystals that limits the possible maximum Dy3+ content in the Bi4Ge3O12 crystal matrix. Noticeable fluctuations were not revealed in the absorption and luminescence spectra of Dy3+-doped Bi4Ge3O12 crystals.