Alloying Ga₂O₃ with Al₂O₃ or In₂O₃ enables control over the crystal structure and band gap, thus tuning the luminescence behaviour of dopant ions such as Cr³⁺ and Mn⁴⁺. While the effect of crystal field modification on Cr³⁺ luminescence in pristine β-Ga₂O₃-based host is relatively well established, the behaviours of Cr³⁺ and Mn⁴⁺ in solid solutions containing aluminium and indium were not as thoroughly studied. In this work, we synthesized and systematically investigated Mn⁴⁺- and Cr³⁺-doped oxide phosphors based on (Al_1-xGa_x)₂O₃, (Ga_1-xIn_x)₂O₃, and (Ga_1-xAl_x)₂O₃ compositions. Optical studies including photoluminescence (PL), PL excitation (PLE), diffuse reflection (DR), and PL decay kinetics were conducted in the 4-350 K range. For Cr³⁺-doped samples, persistent luminescence (PersL) and thermally stimulated luminescence (TSL) after UV excitation were measured up to 600 K. Structural characterization via X-ray diffraction confirmed phase composition and cation site preference. The results demonstrate that Cr³⁺ and Mn⁴⁺ ions form multiple luminescent centres, whose spectral features are determined by the local crystal field, modulated via host composition. Notably, Mn⁴⁺ emission in monoclinic (Al,Ga)₂O₃ reveals two distinct centres, associated with Al- and Ga-rich octahedra. In contrast, in pure Ga₂O₃ a third Mn⁴⁺ centre emerges, likely located in tetrahedral coordination. For Cr³⁺, spectral deconvolution of zero-phonon lines supports the presence of Al-, Ga- and In-associated sites, each exhibiting characteristic decay kinetics and temperature responses. These findings confirm the effectiveness of chemical pressure in tailoring luminescent properties of Ga₂O₃-based hosts. In particular, fine-tuning host composition enables optimization of these materials as near-infrared persistent phosphors and luminescence thermometers for contactless temperature sensing.

Acknowledgements
The work was supported by the Polish National Science Centre (project no. 2024/53/B/ST11/01108) and by the National Research Foundation of Ukraine (grant no. 2020.02/0373).

References
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