| Summary: | This work is devoted to the crystallization and investigation of the optical properties of single crystalline films (SCFs) of Ce<sup>3+</sup>-doped Y<sub>3−x</sub>Ca<sub>x</sub>Al<sub>5−y</sub>Si<sub>y</sub>O<sub>12</sub> garnet, where the content of Ca<sup>2+</sup> and Si<sup>4+</sup> cations varied in the x = 0.13–0.52 and y = 0.065–0.5 ranges, respectively. The SCF samples were grown using the liquid phase epitaxy technique onto Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> substrates from the melt solution with equimolar Ca and Si content using PbO-B<sub>2</sub>O<sub>3</sub> flux. However, the Ca and Si concentration in Y<sub>3−x</sub>Ca<sub>x</sub> Al<sub>5−y</sub>Si<sub>y</sub>O<sub>12</sub>:Ce SCFs is not equal: the Ca<sup>2+</sup> content was systematically larger than that of Si<sup>4+</sup>, and the Ca<sup>2+</sup> excess is compensated for by the Ce<sup>4+</sup> ion formation. The absorption, scintillation, and luminescent properties of Y<sub>3−x</sub>Ca<sub>x</sub>Al<sub>5−y</sub>Si<sub>y</sub>O<sub>12</sub>:Ce SCFs with different Ca/Si concentrations were investigated and compared with the sample of YAG:Ce SCF. Due to the creation of Ce<sup>4+</sup> ions, the as-grown Y<sub>3−x</sub>Ca<sub>x</sub>Al<sub>5−y</sub>Si<sub>y</sub>O<sub>12</sub>:Ce SCFs show relatively low light yield (LY) under α–particle excitation but a fast scintillation response with a decay time in the ns range. After SCF annealing in the reducing (N<sub>2</sub> + H<sub>2</sub>) atmosphere at T > 1000 °C, the recharging of Ce<sup>4+</sup>→Ce<sup>3+</sup> ions occurs. Furthermore, the samples annealed at 1300 °C SCF possess an LY of about 40% in comparison with the reference YAG:Ce SCF and scintillation decay kinetics much closer to that of the SCF counterpart. Due to Ca<sup>2+</sup> and Si<sup>4+</sup> alloying, the Ce<sup>3+</sup> emission spectra in Y<sub>3−x</sub>Ca<sub>x</sub>Al<sub>5−y</sub>Si<sub>y</sub>O<sub>12</sub> SCFs are extended to the red range in comparison with the spectra of YAG:Ce SCF. Such an extension is caused by the Ce<sup>3+</sup> multicenter formation at the substitutions of both Y<sup>3+</sup> and Ca<sup>2+</sup> dodecahedral positions in the hosts of these mixed garnets.
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