Synthesis, Growth, and Luminescence Properties of Rare Earth Borates KSrY(BO₃)₂: Tb³⁺ and Tb⁴⁺

KSrY(BO3)2: Tb3+ and Tb4+ solid solutions have been obtained using the solid-state synthesis and top-seeded solution growth method from the KF flux. Comparing the two types of Tb spectra 3d3/2 TbO2 (Tb4+) and Na3Tb(BO3)2 (Tb3+) with annealing in air of the KSrTb(BO3)2 crystal, X-ray photoelectron spectroscopy (XPS) revealed that the crystal contains about 15% of Tb4+. The entire Y/Tb series has polymorphic phase transitions that occur at temperatures between 550 and 600 °C. Synthesis and subsequent treatment methodology have an impact on the resulting Tb3+/Tb4+ ratio in the sample. The best luminescent properties were measured on the composition KSrY0.9Tb0.1(BO3)2, which was annealed at 700 °C under a hydrogen flow and cooled slowly.