Monophase Ce:Gd₃Al_5−xGa_xO₁₂ (Ce:GAGG) with x = 0.5–3.0 and 50 vol% Al₂O₃−containing composite phosphor ceramics (PCs) were prepared in a pure oxygen atmosphere. The effects of Ga³⁺ substitution on their phase formation, microstructure, and luminescence properties were systematically investigated. For Ce:GAGG series samples, no additional phases were identified, and the distribution of Ga between the octahedral (Al/Ga)₂ (0, 0, 0) and tetrahedral (Al/Ga)₃ (0.375, 0, 0.25) sites of the garnet phase was clarified. For Al₂O₃−Ce:GAGG composites, the exchange of Al and Ga elements between the phases of garnet Ga(Al,Ga)G and oxide (Al,Ga)₂O₃ was revealed, and the transformation of (Al,Ga)₂O₃ from the α- to κ-phase (x ≥ 2.0) with the formation of elongated grains and their partial melting (x = 3) is shown. This was also reflected in a less pronounced shift of the photoluminescence peak (PL) toward shorter wavelengths for the composite series in comparison with the monophase series: with an increase in x to 3.0, the shift in the position of the PL peak of intensity by 18 nm for Al₂O₃−Ce:GAGG was equivalent to that for Ce:GAGG at x = 1.5. A phosphorescence phenomenon was found at x = 2.5 and 3.0 for monophasic Ce:GAGG compositions. Under the excitation of 1 W 450 nm LDs in reflection mode, 0.4 mm-thick Al₂O₃−Ce:GAGG with x = 0.5–1.5 had an optimum correlated color temperature of 5400–6300 K, a luminous efficiency of 123–145 lm∙W⁻¹, and a color rendering index (Ra = 69–64). The obtained PCs showed high application potential in solid-state laser lighting.

Ultrafine-grained Al₂O₃–rare earth:yttrium aluminium garnet (Al₂O₃–RE:YAG) (RE = Ce; Ce+Gd) composite ceramics were obtained for the first time by reactive spark plasma sintering (SPS) using commercially available initial oxide powders. The effect of key sintering parameters (temperature, dwell time, and external pressure (P_load)) on densification peculiarities, structural-phase states, and luminescent properties of composites was studied comprehensively. Differences in phase formation and densification between Ce-doped and Ce,Gd-codoped systems were shown. Parameters of reactive SPS, at which there is partial melting with the formation of near-eutectic zones of the Al₂O₃–YAG system/coexistence of several variations of the YAG-type phase, were established. Pure corundum–garnet biphasic ceramics with an optimal balance between microstructural and luminescence performance were synthesized at 1425 ℃/30 min/30–60 MPa. The external quantum efficiency (EQE) of the phosphor converters reached 80.7% and 72% with close lifetime of ~63.8 ns, similar to those of commercial Ce:YAG materials, which is promising for further applications in the field of high-power white light-emitting diodes (WLEDs) and laser diodes (LDs).