Hydrothermal growths, optical features and first-principles calculations of sillenite-type crystals comprising discrete MO4 tetrahedra

Abstract

Highly crystalline single crystals of sillenite-type Bi₁₀Cd₃O₂₀ and Bi₁₂(Bi_xM_1−x)O₂₀ (M = B, Si, P, V, Mn, Fe, Ga, Ge) were successfully grown under mild hydrothermal conditions. Bulk crystals with smallest average size of 100 μm and preferable shapes can be obtained by optimizing respective growth conditions. The morphologies for Bi₂₅FeO₄₀ show regular evolutions along with different synthesis routes. Characterizations were performed by means of powder X-ray diffraction analyses, chemical analyses, optical spectra and first-principles calculations. It is found that all of the crystals possess certain transparency along with distinct absorptions in the visible region, corresponding to the discrete MO₄ tetrahedra in the crystal lattices. Band gap energies of the fabricated sillenite crystals vary from approximately 1.66 eV for Bi₁₂(Bi_0.375Mn_0.625)O₂₀ to 2.85 eV for Bi₁₂GeO₂₀, as estimated from the absorption data. The impacts of tetrahedrally coordinated transition-metal cations on their electronic structures were investigated by first-principles computational approaches.