Abstract: BPO₄ crystal has attracted considerable interest as a nonlinear optical （NLO） material due to its extremely short ultraviolet （UV） cutoff edge，strong powder second-harmonic generation （SHG） response，and high physicochemical stability. In this work，high-quality BPO₄ single crystals were successfully grown via the top-seeded solution growth （TSSG） method employing a Li₂O-B₂O₃ flux，and their crystalline quality，optical properties，and mechanical properties were systematically investigated. The Li₂O-B₂O₃ flux system exhibits low volatility and remains transparent at high temperatures，enabling real-time monitoring of the growth process. The as-grown crystals show a narrow rocking curve with a full width at half maximum （FWHM） of 0.008°，confirming the efficacy of this flux in producing high quality crystals. Raman and infrared spectroscopy are carefully elucidated to confirm the ion group in the BPO₄ crystal. Optical transmission measurements reveal that the crystal possesses a broad transparent range，with UV and IR cutoff edges extending to 133 and 4 270 nm，respectively，underscoring its potential for deep-UV applications. Furthermore，nanoindentation tests demonstrate its robust mechanical stability，yielding a hardness of approximately 12.75 GPa and a reduced modulus of about 153.37 GPa. In summary，Li₂O-B₂O₃ flux exhibits low volatility and high solution transparency，proving to be an excellent medium for the growth of high-quality BPO₄ crystals. The combination of an extremely short UV cutoff edge，wide spectral transparency，and favorable mechanical properties positions BPO₄ as a promising candidate for next-generation deep-UV NLO materials.

Key words: BPO₄ single crystal; nonlinear optical crystal; top-seeded solution growth method; flux system; optical property; mechanical property