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Single crystals of Ho₂O₃-doped Y₂O₃ stabilized ZrO₂ (YSZ) with different Y₂O₃ and Ho₂O₃ contents were grown by the optical floating zone method. XRD and Raman spectra were measured and showed that crystal samples all had tetragonal structures. Measurements of positron annihilation lifetime spectra indicated that the increase in Y₂O₃ concentration led to the increases of defects and mean positron lifetime, which enhanced the scattering of light and reduced the luminous intensity and the quantum yield (QY) of the crystal. Under the excitation at 446 nm, photoluminescence (PL) spectra of Ho₂O₃_-doped YSZ crystals showed emission peaks at 540, 551, 670, and 757 nm corresponding to Ho³⁺ transitions from ⁵S₂, ⁵F₄, ⁵F₅, and ⁵I₄ excited states to the ⁵I₈ ground state, respectively. At low Ho₂O₃-doped concentrations (0.10–0.50 mol%), the overall emission intensity increased with Ho₂O₃ contents, reached the maximum value at 0.50 mol%, then decreased with higher Ho₂O₃ contents, probably as a result of increased non-radiative relaxation caused by increased interactions between Ho³⁺ ions. Quenching of the PL occurred at Ho₂O₃ concentrations > 0.5 mol% and due to the electric dipole–dipole interaction. The calculated chromaticity coordinates (CIE) were approximately (0.307, 0.683) and the color purity achieved 99.6%. The results showed that Ho₂O₃: YSZ crystals were suitable for green light-emitting devices.