Accurate Determination of the Bandgap Energy of the Rare-Earth Niobate Series.

We report diffuse reflectivity measurements in InNbO, ScNbO, YNbO, and eight rare-earth niobates. A comparison with established values of the bandgap of InNbO and ScNbO shows that Tauc plot analysis gives erroneous estimates of the bandgap energy. Conversely, accurate results are obtained considering excitonic contributions using the Elliot-Toyozawa model. The bandgaps are 3.25 eV for CeNbO, 4.35 eV for LaNbO, 4.5 eV for YNbO, and 4.73-4.93 eV for SmNbO, EuNbO, GdNbO, DyNbO, HoNbO, and YbNbO. The fact that the bandgap energy is affected little by the rare-earth substitution from SmNbO to YbNbO and the fact that they have the largest bandgap are a consequence of the fact that the band structure near the Fermi level originates mainly from Nb 4d and O 2p orbitals. YNbO, CeVO, and LaNbO have smaller bandgaps because of the contribution from rare-earth atom 4d, 5d, or 4f orbitals to the states near the Fermi level.