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

Abstract

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 (Bi_3.996(11)Dy_0.004)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 (Bi_3.952(47)Dy_0.048)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.