| Summary: | Rare earth doped lithium fluorides are a class of materials with a wide variety of optical applications, but the hazardous reagents used in their synthesis often restrict the amount of product that can be created at one time. In this work, 10%Yb<sup>3+</sup>:LiLuF<sub>4</sub> (Yb:LLF) crystals have been synthesized through a safe and scalable polyethylene glycol (PEG)-assisted hydrothermal method. A combination of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and photoluminescence (PL) measurements were used to characterize the obtained materials. The influence of reaction temperature, time, fluoride source, and precursor amount on the shape and size of the Yb:LLF crystals are also discussed. Calibrated PL spectra of Yb<sup>3+</sup> ions show laser cooling to more than 15 K below room temperature in air and 5 K in deionized water under 1020 nm diode laser excitation measured at a laser power of 50 mW.
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