Dynamics of energy absorption versus crystallization in Bi4Ge3O12 (BGO) amorphous materials

Theoretical and experimental determinations show that the intrinsic luminescence of BGO materials is strongly influenced by GeO₄⁻ tetrahedra around Bi³⁺ ions. Besides Bi³⁺ transitions, the excitonic spectra mask the BGO bandgap. Theoretical computations give the bandgap at 5.19 eV compared with the experimental one at 5.17 eV in BGO crystals and a significant decrease at 3.26 eV in the amorphous materials. This is due to the high disordering of the GeO₄⁻ tetrahedral which also reduces the refractive index of the amorphous materials. Formation of excitonic states and their overlap with the Bi³⁺ transitions suggests that the energy absorption takes place via the p-electrons of the O₂⁻ ions and is then transferred to the Bi³⁺p-electrons (excited states) close to the conduction band. Light emission appears after de-excitation to the ground state.