Abstract: RE-doped YPO4 (RE = Sm, Dy, Er, Eu, Ce, and Nd) powder samples are synthesised via high-temperature solid-state sintering at temperatures up to 1300 °C, and their structural and photoluminescent properties are thoroughly characterised. X-ray and neutron diffraction studies confirmed the formation of a single-phase xenotime (tetragonal) YPO4 structure. The short-range structure consists of YO8 and PO4 units. Raman spectroscopy revealed peaks corresponding to the symmetric and asymmetric stretching vibrational modes of P-O bonds in the PO4 units, indicating that RE doping does not significantly alter the short-range structural framework. The incorporation of RE ions into the YPO4 matrix enables emission across the entire UV-visible spectrum. Specifically, Eu3+ (excited at 697 nm) and Sm3+ (excited at 404 nm) produce red emission, while Dy3+ exhibits blue emission when excited at 365 nm. Er3+-doped samples show red and green upconversion (UC) emission upon 980 nm excitation. Additionally, Nd3+ displays mid-infrared emission under 753 nm excitation, whereas Ce3+ doping results in blue emission under UV irradiation at 263 nm. Fluorescence lifetime measurements indicate that Er3+- and Nd3+-doped samples exhibit lifetimes below 5 µs and 8.6(3) µs, respectively. In contrast, Dy3+, Sm3+, and Eu3+ show significantly longer lifetimes, ranging from 0.34(1) to 2.75(1) ms.
