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

Abstract

Phase-pure α-(La,Yb,Er) 2 W 2 O 9 upconversion (UC) phosphors were synthesized via room temperature coprecipitation and subsequent calcination at 1073-1273 K. The pH of the coprecipitation system and the RE 3+ to WO 4 2ratio were found crucial for the formation of the target α phase. Phase transformation from cubic β-(La,Yb,Er) 2 W 2 O 9 to triclinic α-(La,Yb,Er) 2 W 2 O 9 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 ( 4 S 3/2 → 4 I 15/2 ) 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 S A and S R values of 0.02694 K −1 (548 K) and 0.01699 K −1 (298 K) for the thermally coupled 2 H 11/2 and 4 S 3/2 energy levels in the 298-548 K temperature range.