Strong Emissions from Er/Tm/Yb Codoped Monoclinic YNbO₄ Single Crystals under 980 nm Excitations

High-quality monoclinic YNbO4 and YNbO4:Er0.06Tm0.6Yb2 single crystals were successfully grown for the first time using optical floating zone technique. For comparison, cubic yttria-stabilized zirconia (YSZ) and YSZ:Er0.06Tm0.6Yb2 single crystals were also prepared. This work presents the first comparative study on differences in up- and down-conversion photoluminescence (UCPL and DCPL) characteristics between monoclinic YNbO4:Er0.06Tm0.6Yb2 and cubic YSZ:Er0.06Tm0.6Yb2 crystals. The YNbO4 crystal exhibits a significantly reduced oxygen vacancy concentration compared to YSZ. The YNbO4 crystal shows a transmittance of 81%, exceeding the 75% observed for YSZ, indicating superior transparency. The absorption spectra of YNbO4:Er0.06Tm0.6Yb2 and YSZ:Er0.06Tm0.6Yb2 display absorption peaks at 360, 378, 461, 520, 683, 784, and 980 nm. Under 980 nm excitation, the UCPL spectra of YNbO4:Er0.06Tm0.6Yb2 and YSZ:Er0.06Tm0.6Yb2 exhibit emission peaks at 365, 479, 535, 555, 655, 673, and 805 nm, where the former shows remarkably higher emission intensity than the latter. Under 360 nm excitation, the DCPL spectra of YNbO4:Er0.06Tm0.6Yb2 and YSZ:Er0.06Tm0.6Yb2 show emission peak at 457 nm, with the former displaying significantly higher emission intensity than the latter. The stronger emission intensities observed in the UCPL and DCPL spectra of YNbO4:Er0.06Tm0.6Yb2 compared to YSZ:Er0.06Tm0.6Yb2 can be attributed to differences in oxygen vacancy concentration, optical transmittance, and crystal symmetry.