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

We report diffuse reflectivity measurements in InNbO₄, ScNbO₄, YNbO₄, and eight rare-earth niobates. A comparison with established values of the bandgap of InNbO₄ and ScNbO₄ 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 CeNbO₄, 4.35 eV for LaNbO₄, 4.5 eV for YNbO₄, and 4.73-4.93 eV for SmNbO₄, EuNbO₄, GdNbO₄, DyNbO₄, HoNbO₄, and YbNbO₄. The fact that the bandgap energy is affected little by the rare-earth substitution from SmNbO₄ to YbNbO₄ 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. YNbO₄, CeVO₄, and LaNbO₄ have smaller bandgaps because of the contribution from rare-earth atom 4d, 5d, or 4f orbitals to the states near the Fermi level.