Na(Gd,RE)(WO4)2 double tungstates (RE=Eu/Yb-Er): crystallization from a novel layered hydroxide precursor and favorable luminescence for optical thermometry

Abstract

Rare earth double tungstates Na(Gd,RE)(WO 4 ) 2 (RE = Eu/Yb-Er) were successfully obtained from layered rare earth hydroxide precursors via sacrificial reaction at 200°C with molar ratio WO 4 2-/RE 3+ = 2.5 for 24 h reaction without calcination and the employment of any organic reagents. The temperature sensing performances of Na(Gd,RE)(WO 4 ) 2 (RE = Eu/Yb-Er) were comparatively investigated by analyzing the temperature-dependent down-conversion (DC) and upconversion (UC) luminescence spectra. The DC phosphors Na(Gd,Eu)(WO 4 ) 2 were found to be well capable of sensing temperature via fluorescence lifetime (FL) mode. The UC phosphors Na(Gd,Yb,Er)(WO 4 ) 2 were capable of sensing temperature via both FL mode and fluorescence intensity ratio (FIR) mode through thermally coupled 2 H 11/2 , 4 S 3/2 levels (I 520 /I 552 ), and non thermally coupled 2 H 11/2 , 4 F 9/2 energy levels (I 520 /I 670 ). The UC phosphors Na(Gd,Yb,Er)(WO 4 ) 2 have maximum absolute sensitivity (S A ) and maximum relative sensitivity (S R ) of 174 × 10 −4 K −1 (548 K) and 80 × 10 −4 K −1 (298 K) for the thermally coupled 2 H 11/2 , 4 S 3/2 levels and maximum S A and S R of ~260 × 10 −4 K −1 (300-550 K) and 96 × 10 −4 K −1 (298 K) for the non-thermally coupled 2 H 11/2 , 4 F 9/2 energy levels.