Upconversion luminescence and optical thermometry based on non-thermally-coupled levels of Ca9Y(PO4)7: Tm3+, Yb3+ phosphor

Ca9Y(PO4)7: Tm3+, Yb3+ upconversion phosphors were prepared by a high-temperature solid-state method, and their crystal structure, upconversion emission spectra, and temperature-sensing properties were studied. The upconversion temperature-sensing behavior based on the luminescence intensity ratio (LIR) from the non-thermally coupled levels (NTCLs) of Tm3+ in the temperature range of 323–823 K was also studied systematically. Under 980-nm excitation, there were three emission bands peaked at 471, 644, and 690 nm. Compared with the 3F3→3H6 transition with its peak at 690 nm, the 1G4→3F4 transition with its peak at 644 nm, and the 1G4→3H6 transition with its peak at 471 nm exhibit opposite temperature-dependent properties. The Sr-max and Sa-max of LIR (I690nm/I644nm) were 1.04%/K at 523 K and 0.0807 K−1 at 823 K, while those of LIR (I690nm/I471nm) were 1.07%/K at 473 K and 0.0113 K−1 at 823 K, respectively. Thus, the temperature measurement performance of the NTCL-based phosphors was obviously better than that based on thermally coupled levels. Our results imply the potential applications of Ca9Y(PO4)7: Tm3+, Yb3+ upconversion phosphors in temperature sensing.