Hydrothermal Synthesis and Crystal Structure of a Mixed-Valence Bismuthate, Na₃Bi₃O₈

Four types of bismuth oxides, Na₃Bi₃O₈, NaBiO₃, α-Bi₂O₃, and ε-Bi₂O₃, were obtained by hydrothermal reactions using NaBiO₃·nH₂O in NaOH solution. The crystal structure of a new phase (Na₃ Bi³⁺)Bi₂⁵⁺O₈ ((Na_0.75Bi_0.25)₂BiO₄) was determined by using single crystal X-ray diffraction data, and this compound was found to show a Na₂MnCl₄-related structure with a monoclinic system (space group, Pm) with the following lattice parameters: a = 5.990 (2) Å, b = 3.335 (2) Å, c = 10.108 (2) Å, and β = 91.08 (3)°. The final R-factors R₁ and wR₂ were 0.041 and 0.090 (all data), respectively. The new phase was composed of mixed valence states of Bi (Bi³⁺ and Bi⁵⁺, with a mean Bi valence of 4.30) with five distinct Bi sites, where two Bi⁵⁺ (Bi1 and Bi2) fully occupied the distorted octahedral sites and three Bi³⁺ (Bi3, Bi4, and Bi5) were statistically distributed at the split sites with Na⁺ (Na3, Na4, and Na5). The Na6 site is fully occupied. The distorted Bi⁵⁺O₆ octahedra formed one-dimensional chains via edge-sharing along the b-axis, with the chains held by Bi³⁺/Na⁺ split sites. The structural feature except for the split distribution of Bi³⁺/Na⁺ was classified as a Na₂MnCl₄-type structure. DFT calculations based on a model discounting the split distribution of Bi³⁺/Na⁺ indicated that Bi 6s and O 2p orbitals form sp hybridization at the conduction band. This new mixed valence bismuth oxide exhibited photocatalytic activity for phenol degradation under visible light irradiation. In addition to Na₃Bi₃O₈, the hydrothermal reaction using NaBiO₃·nH₂O in NaOH solution yielded micrometer-sized single crystals of an ilmenite-type NaBiO₃ and two polymorphs of bismuth oxides with monoclinic (α-Bi₂O₃) and orthorhombic (ε-Bi₂O₃) structures, depending on the reaction temperature and NaOH concentration.