Coprecipitation synthesis of α-(La,Yb,Er)2W2O9 tungstate as a novel upconversion phosphor for optical thermometry

Phase-pure α-(La,Yb,Er)2W2O9 upconversion (UC) phosphors were synthesized via room temperature coprecipitation and subsequent calcination at 1073–1273 K. The pH of the coprecipitation system and the RE3+ to WO42- ratio were found crucial for the formation of the target α phase. Phase transformation from cubic β-(La,Yb,Er)2W2O9 to triclinic α-(La,Yb,Er)2W2O9 occurred when calcination temperature was above 873 K. The UC luminescence including power/temperature dependent emission, decay kinetics of the main emission, emission color, and underlying UC mechanism were systematically investigated. The phosphor exhibited bright green luminescence under 978 nm laser excitation. The main emission (4S3/2→4I15/2) was analyzed to decay in a single exponential manner and have a decay time of 153.86 μs at room temperature. A three-photon mechanism was suggested responsible for the observed UC luminescence through analyzing the UC intensity against excitation power via logarithmic data transformation. The performance of optical temperature sensing was evaluated and compared with other typical UC phosphors. It was shown that the phosphor has relatively high maximum SA and SR values of 0.02694 K−1 (548 K) and 0.01699 K−1 (298 K) for the thermally coupled 2H11/2 and 4S3/2 energy levels in the 298–548 K temperature range.