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This work is dedicated to the crystallization and luminescent properties of a prospective Ca₂YMgScSi₃O₁₂:Ce (CYMSSG:Ce) micropowder (MP) phosphor converter (pc) for a white light–emitting LED (WLED). The set of MP samples was obtained by conventional solid-phase synthesis using different amounts of B₂O₃ flux in the 1–5 mole percentage range. The luminescent properties of the CYMSSG:Ce MPs were investigated at different Ce³⁺ concentrations in the 1–5 atomic percentage range. The formation of several Ce³⁺ multicenters in the CYMSSG:Ce MPs was detected in the emission and excitation spectra as well as the decay kinetics of the Ce³⁺ luminescence. The creation of the Ce³⁺ multicenters in CYMSSG:Ce garnet results from: (i) the substitution by the Ce³⁺ ions of the heterovalent Ca²⁺ and Y³⁺ cations in the dodecahedral position of the garnet host; (ii) the inhomogeneous local environment of the Ce³⁺ ions when the octahedral positions of the garnet are replaced by heterovalent Mg²⁺ and Sc³⁺ cations and the tetrahedral positions are replaced by Si⁴⁺ cations. The presence of Ce³⁺ multicenters significantly enhances the Ce³⁺ emission band in the red range in comparison with conventional YAG:Ce phosphor. Prototypes of the WLEDs were also created in this work by using CYMSSG:Ce MP films as phosphor converters. Furthermore, the dependence of the photoconversion properties on the layer thickness of the CYMSSG:Ce MP was studied as well. The changes in the MP layer thickness enable the tuning of the white light thons from cold white/daylight to neutral white. The obtained results are encouraging and can be useful for the development of a novel generation of pcs for WLEDs.