| Summary: | Tetragonal Er<sub>0.5</sub>Nb<sub>0.5</sub>O<sub>2</sub> and monoclinic ErNbO<sub>4</sub> micro- and nanoparticles were prepared by the citrate sol–gel method and heat-treated at temperatures between 700 and 1600 °C. ErNbO<sub>4</sub> revealed a spherical-shaped crystallite, whose size increased with heat treatment temperatures. To assess their optical properties at room temperature (RT), a thorough spectroscopic study was conducted. RT photoluminescence (PL) spectroscopy revealed that Er<sup>3+</sup> optical activation was achieved in all samples. The photoluminescence spectra show the green/yellow <sup>2</sup>H<sub>11/2</sub>, <sup>4</sup>S<sub>3/2</sub>→<sup>4</sup>I<sub>15/2</sub> and red <sup>4</sup>F<sub>9/2</sub>→<sup>4</sup>I<sub>15/2</sub> intraionic transitions as the main visible recombination, with the number of the crystal field splitting Er<sup>3+</sup> multiplets reflecting the ion site symmetry in the crystalline phases. PL excitation allows the identification of Er<sup>3+</sup> high-energy excited multiplets as the preferential population paths of the emitting levels. Independently of the crystalline structure, the intensity ratio between the green/yellow and red intraionic transitions was found to be strongly sensitive to the excitation energy. After pumping the samples with a resonant excitation into the <sup>4</sup>G<sub>11/2</sub> excited multiplet, a green/yellow transition stronger than the red one was observed, whereas the reverse occurred for higher excitation photon energies. Thus, a controllable selective excited tunable green to red color was achieved, which endows new opportunities for photonic and optoelectronic applications.
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