<p>Trivalent rare-earth ions-activated phosphor-converted light-emitting diodes (pc-LEDs) are being explored as a leading light source in the lighting field. In this study, Gd₂O₃:4 at.% Eu³⁺, xTm³⁺ (x = 0.5–3.0 at.%) nanophosphors were synthesized and systematically analyzed to investigate their structural, morphological, and photoluminescent characteristics. X-ray diffraction confirmed a highly crystalline single-phase cubic Gd₂O₃ structure (Ia-3, ICDD 43-1014), with increasing Tm³⁺ concentration enhancing crystallinity and enlarging crystallite sizes from 3.4 to 7.3&#xa0;nm. Rietveld refinement yielded excellent agreement factors (GoF ≈ 1), verifying successful Eu³⁺ and Tm³⁺ substitution at Gd³⁺ lattice sites and a gradual expansion of unit cell volume. FT-IR spectra revealed characteristic Gd-O vibrations and minor carbonate-related modes from atmospheric CO₂ adsorption, with no impurity signatures, further confirming phase purity. TEM analysis showed uniformly distributed nanoparticles (~ 37.5&#xa0;nm) with clear lattice fringes consistent with cubic Gd₂O₃, supported by SAED indexing and EDS elemental mapping. Photoluminescence studies demonstrated strong blue emissions from Tm³⁺ (¹D₂ → ³F₄, ¹G₄ → ³H₆) and intense red emissions from Eu³⁺, dominated by the hypersensitive ⁵D₀ → ⁷F₂ transition at 613&#xa0;nm. Emission intensity increased with Tm³⁺ codoping up to 2.0 at.% before concentration quenching occurred. Sm³⁺ codoping further revealed characteristic excitation and emission features, with the strongest excitation at 408&#xa0;nm assigned to the ⁶H_5/2 →⁶P_3/2 transition. Overall, the results confirm the successful synthesis of phase-pure, highly crystalline Gd₂O₃-based codoped nanophosphors with tunable multicolor emission, demonstrating their strong potential for solid-state lighting applications.</p>