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

ABSTRACT Rare earth double tungstates Na(Gd,RE)(WO4)2 (RE = Eu/Yb-Er) were successfully obtained from layered rare earth hydroxide precursors via sacrificial reaction at 200°C with molar ratio WO4 2-/RE3+ = 2.5 for 24 h reaction without calcination and the employment of any organic reagents. The temperature sensing performances of Na(Gd,RE)(WO4)2 (RE = Eu/Yb-Er) were comparatively investigated by analyzing the temperature-dependent down-conversion (DC) and up-conversion (UC) luminescence spectra. The DC phosphors Na(Gd,Eu)(WO4)2 were found to be well capable of sensing temperature via fluorescence lifetime (FL) mode. The UC phosphors Na(Gd,Yb,Er)(WO4)2 were capable of sensing temperature via both FL mode and fluorescence intensity ratio (FIR) mode through thermally coupled 2H11/2,4S3/2 levels (I 520/I 552), and non thermally coupled 2H11/2,4F9/2 energy levels (I 520/I 670). The UC phosphors Na(Gd,Yb,Er)(WO4)2 have maximum absolute sensitivity (SA ) and maximum relative sensitivity (SR ) of 174 × 10−4 K−1 (548 K) and 80 × 10−4 K−1 (298 K) for the thermally coupled 2H11/2,4S3/2 levels and maximum SA and SR of ~260 × 10−4 K−1 (300–550 K) and 96 × 10−4 K−1 (298 K) for the non-thermally coupled 2H11/2,4F9/2 energy levels.