Molten salt shielding synthesis of Er3+/Yb3+ co-doped La9W6O30Cl phosphors for optical temperature sensing

Oxychloride phosphors that contain active ions coordinated with both oxygen and chloride ions have garnered significant attention, but the application of oxychloride phosphors is yet limited because of their demanding synthesis condition of vacuum or reducing atmosphere. Herein, we synthesize a new serial oxychlorides of La9W6O30Cl (LWOC) and Er3+/Yb3+, Ho3+/Yb3+, Tm3+/Yb3+ doped LWOC through molten salt shielding method (MSS) in air, which is more efficient compared to the traditional method in vacuum. The compound of LWOC crystallizes in the tetragonal system space group P42/nmc (No. 137) with cell parameters a = b = 10.0445(1) Å, c = 12.6826(4) Å, V = 1279.57(5) Å3 and Z = 11. The asymmetric unit of LWOC includes three La atoms, one unique W atom, two disordered W atoms with occupancy factors of 0.16(5) and 0.84(5), respectively, one Cl atom and six O atoms. Under the excitation of 980 nm laser, all the Er3+/Yb3+, Ho3+/Yb3+ and Tm3+/Yb3+ co-doped LWOC phosphors have excellent upconversion (UC) luminescence with bright visible light emission. A fluorescence intensity ratio (FIR) type temperature sensor is developed by using LWOC: Er3+, Yb3+ phosphor as sensing media, which has a maximum relative temperature sensitivity (Sr) of 0.90 % K−1 at 300 K.