Electronic Structures and Luminescence Properties of YNbO4 and YNbO4:Bi

The luminescence properties of fergusonite phosphors, YNbO4 and YNbO4:Bi, were investigated with photoluminescence (PL) measurements and first-principles calculations. The various absorption spectra of these phosphors have been interpreted using the calculated partial density of states in the framework of density functional theory. We were able to determine the charge-transfer gap of YNbO4, 4.3 eV, that agrees well with our experimental measurement. From our analysis of PL spectra and calculations, we have investigated the Bi effect in YNbO4:Bi, which shows an excitation peak at a longer wavelength than YNbO4. We have found that the charge transfer from oxygen to bismuth is the main reason for the peak position shift in the excitation spectra.