Mn^<b>5+</b>-Activated Highly Efficient NIR-II Emission Phosphor Ba_<b>10</b>(PO_<b>4</b>)_<b>6</b>F_<b>2</b>:Mn^<b>5+</b> for Compact Light Sources and Optical Fiber Temperature Sensors

The development of highly efficient near-infrared-II (NIR-II) emission phosphors is crucial for advancing compact light sources and their emerging applications. However, the discovery of target phosphors remains a major challenge. Herein, a high-efficiency apatite phosphor, Ba10(PO4)6F2:Mn5+, peaking at 1171 nm with a full width at half-maximum of 11 nm, is synthesized under an oxygen atmosphere. Benefiting from the crystallographic site confinement and oxygen partial pressure control in stabilizing pentavalent manganese, the phosphor exhibits a record internal quantum efficiency (IQE) of 84.2% and external QE of 56%, alongside good thermal stability. By combining Ba10(PO4)6F2:Mn5+ with a red light-emitting diode (LED) chip, a phosphor-converted compact LED with an NIR-II output power of 40.86 mW at 300 mA and a high photoelectric conversion efficiency of 6.82% is presented, demonstrating applications in nondestructive testing, etc. Furthermore, the different temperature-dependent behaviors of the zero-phonon line emission (1171 nm) and vibrational sidebands (1224/1289 nm) of Mn5+ in Ba10(PO4)6F2:Mn5+ enable the fabrication of a high-efficiency fluorescence intensity ratio optical fiber temperature sensor, exhibiting a maximum relative sensitivity value of 0.518% K–1 at 373 K. This work offers some insights into Mn5+-activated high-efficiency NIR-II emission phosphors and their potential for temperature sensing and compact light source applications.