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

Four types of bismuth oxides, NaBiO, NaBiO, α-BiO, and ε-BiO, were obtained by hydrothermal reactions using NaBiO·HO in NaOH solution. The crystal structure of a new phase (Na Bi)BiO ((NaBi)BiO) was determined by using single crystal X-ray diffraction data, and this compound was found to show a NaMnCl-related structure with a monoclinic system (space group, ) with the following lattice parameters: = 5.990 (2) Å, = 3.335 (2) Å, = 10.108 (2) Å, and β = 91.08 (3)°. The final -factors and were 0.041 and 0.090 (all data), respectively. The new phase was composed of mixed valence states of Bi (Bi and Bi, with a mean Bi valence of 4.30) with five distinct Bi sites, where two Bi (Bi1 and Bi2) fully occupied the distorted octahedral sites and three Bi (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 BiO octahedra formed one-dimensional chains via edge-sharing along the -axis, with the chains held by Bi/Na split sites. The structural feature except for the split distribution of Bi/Na was classified as a NaMnCl-type structure. DFT calculations based on a model discounting the split distribution of Bi/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 NaBiO, the hydrothermal reaction using NaBiO·HO in NaOH solution yielded micrometer-sized single crystals of an ilmenite-type NaBiO and two polymorphs of bismuth oxides with monoclinic (α-BiO) and orthorhombic (ε-BiO) structures, depending on the reaction temperature and NaOH concentration.