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The lead-free halide perovskite Cs₃Bi₂Br₉ is a promising semiconductor material for room-temperature X-ray detection due to its excellent properties. However, material purity and crystal quality still limit the use of Cs₃Bi₂Br₉ crystals as detectors. In this work, we present a highly efficient purification method using continuous vacuum extraction to sublimate BiBr₃ precursors for Cs₃Bi₂Br₉. Impurity analysis via inductively coupled plasma mass spectroscopy showed that the purification method successfully removed most of the impurities in BiBr₃ precursors and improved the purity by at least one order of magnitude. Centimeter-sized Cs₃Bi₂Br₉ single crystals were grown by the vertical Bridgman method. The improved properties after purification were confirmed by UV-Vis-NIR spectra, infrared transmittance, and current–voltage (I–V) measurements. The results showed that the average transmittance of Cs₃Bi₂Br₉ crystals significantly increased from 62% to 75% in the 0.5–20 μm spectral range. Additionally, the resistivity increased by nearly three orders of magnitude from 5.0 × 10⁹ Ω·cm to 2.2 × 10¹² Ω·cm, meaning the material will have low leakage currents and be suitable for developing applications for room temperature radiation detection.