The mother compound, NaBiO3, had go

The mother compound, NaBiO3, had good photocatalytic activity for phenol decomposition under visible-light irradiation;(14) however, β-NaBiO3 exhibited no photocatalytic activity under the same experimental conditions as those shown in Figure 3. Figure 4 indicates optical absorption spectra for NaBiO3 and β-NaBiO3. The band-gap energies were estimated from the dependence of (αhν)2 on energy and hν (Tauc plot) on the assumption of direct transitions.(14b) The Tauc plot estimation of the band-gap energy for polycrystalline samples was reported to give accurate values for monazite-type oxides.(15) The band-gap energies for NaBiO3 and β-NaBiO3 were calculated to be 2.4 and 1.7 eV, respectively. This difference corresponds to the change of color during the high-pressure treatment. The disappearance of photocatalytic activity is thought to be caused by a decrease of the band-gap energy. This result is confirmed with the density of states (DOS) calculation, as shown in Figure S2. The calculated band-gap energy in β-NaBiO3 is smaller than that in NaBiO3. A similar band-gap collapse by high-pressure phase transition is observed in NaSbO3(7) and InTaO4.(16)