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

We report diffuse reflectivity measurements in InNbO 4 , ScNbO 4 , YNbO 4 , and eight rare-earth niobates. A comparison with established values of the bandgap of InNbO 4 and ScNbO 4 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 , 4.35 eV for LaNbO 4 , 4.5 eV for YNbO 4 , and 4.73-4.93 eV for SmNbO 4 , EuNbO 4 , GdNbO 4 , DyNbO 4 , HoNbO 4 , and YbNbO 4 . The fact that the bandgap energy is affected little by the rare-earth substitution from SmNbO 4 to YbNbO 4 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 4 , CeVO 4 , and LaNbO 4 have smaller bandgaps because of the contribution from rare-earth atom 4d, 5d, or 4f orbitals to the states near the Fermi level.