Luminescence properties of PrNbO4 and EuNbO4 orthoniobates and investigation of their structural phase transition by high-temperature Raman spectroscopy

This work reports the photoluminescence (PL) properties at room temperature for PrNbO4 and EuNbO4 orthoniobates synthesized by solid-state reaction. An accurate colorimetric study was carried out for these materials, with determination of the most important parameters for the development of optical devices: emission coordinates, color purity, and emitted color temperature. The emission spectra showed the characteristic lines for the rare earths Pr3+ and Eu3+, with the 3P0 → 3F2 and 5D0 → 7F2 emissions being the most intense for PrNbO4 and EuNbO4, respectively. The samples emitted in the orange (Pr3+) and reddish-orange (Eu3+) regions, having the emission coordinates (0.5723, 0.4053) for PrNbO4 and (0.5868, 0.3938) for EuNbO4. The samples exhibited color purities higher than 90%, so they could be considered promising red phosphors for application in LEDs. This work also demonstrates the suitability of high-temperature Raman spectroscopy (HT-Raman) as a good alternative method for analysis of the monoclinic-tetragonal phase transition in orthoniobates. In situ HT-Raman analyses in the temperature range 25–700 °C showed that the PrNbO4 material changed from the C2/c (#15) space group to I41/a (#88), at ~600 °C. On the other hand, the EuNbO4 ceramic showed no alteration of the crystal structure, in the temperature range studied. The observed structural phase transition from fergusonite-type to scheelite-type was reversible and could be confirmed by Raman measurements during the cooling process. The main contribution of this work is the approach used to determine the PL properties of the PrNbO4 sample, for which there have been no previous reports in the literature, to the best of our knowledge. Furthermore, the RT-Raman technique was successfully used, for the first time, to investigate the structural phase transitions of rare earth orthoniobates.