  In this work, we report diffuse reflectivity measurements in InNbO4, ScNbO4,
YNbO4, and eight different rare-earth niobates. From a comparison with the
established values of the band gap of InNbO4 and ScNbO4, we have found that the
broadly used Tauc plot analysis leads to erroneous estimates of the band-gap
energy of niobates. In contrast, accurate results are obtained considering
excitonic contributions using the Elliot-Toyozawa model. We have found that
YNbO4 and the rare-earth niobates are wide band-gap materials. The band-gap
energy is 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. An explanation
for the obtained results will be presented. The fact that the band-gap energy
is nearly not affected by the rare-earth substitution from SmNbO4 to YbNbO4 and
the circumstance that these are the compounds with the largest band gap are a
consequence of the fact that the band structure near the Fermi level originates
mainly from Nb 4d and O 2p orbitals. We hypothesize that YNbO4, CeVO4, and
LaNbO4 have smaller band gaps because of the contribution from rare-earth atom
4d or 5f states to the states near the Fermi level.
