Investigation on anomalous thermal enhancement and temperature sensing properties of Zn 3 Mo 2 O 9 :Yb 3+ /RE 3+ (RE = Er/Ho) phosphors.

In this work, Yb 3+ /RE 3+ (RE = Er/Ho) co-doped Zn 3 Mo 2 O 9 phosphors were synthesized by high-temperature solid-state reactions. Under 980 nm excitation, the upconversion (UC) luminescence thermal enhancement was obtained for Zn 3 Mo 2 O 9 :Yb 3+ /RE 3+ phosphors. The green emission intensity of the Zn 3 Mo 2 O 9 :Yb 3+ /Er 3+ sample was increased 5 times from 373 to 573 K. The red emission intensity of the Zn 3 Mo 2 O 9 :Yb 3+ /Ho 3+ sample was enhanced 7.92 times. The anomalous thermal enhancement of UC emission was induced by the negative thermal expansion (NTE) of the Zn 3 Mo 2 O 9 host. The energy transfer rate from the sensitizer (Yb 3+ ) to the activator (RE 3+ ) was enhanced because of the lattice contraction and distortion for NTE materials. Compared with the UC emission of Er 3+ single doped Zn 3 Mo 2 O 9 sample, the luminescence thermal enhancement was absent, which contributed to proving the physical mechanism. The temperature sensing properties of the Zn 3 Mo 2 O 9 :Yb 3+ /Er 3+ and Zn 3 Mo 2 O 9 :Yb 3+ /Ho 3+ samples were also investigated based on the fluorescence intensity ratio (FIR) technology. The absolute sensitivity ( S A ) and relative sensitivity ( S R ) of Zn 3 Mo 2 O 9 :Yb 3+ /Er 3+ phosphor reached 0.0060 K -1 and 0.72% K -1 , which is based on the thermal coupling levels ( 2 H 11/2 , 4 S 3/2 ) FIR of Er 3+ ions. In addition, the S A and S R of Zn 3 Mo 2 O 9 :Yb 3+ /Ho 3+ phosphor reached 0.0119 K -1 and 0.86% K -1 , that is based on the non-thermal coupling levels ( 5 S 2 / 5 F 4 , 5 F 5 ) FIR of Ho 3+ ions. The research results indicate that the Zn 3 Mo 2 O 9 host shows NET. The Yb 3+ /RE 3+ co-doped Zn 3 Mo 2 O 9 phosphors are good materials for highly sensitive optical temperature measurement, which can be used to develop thermally enhanced ratiometric optical thermometers.