Er3+/Yb3+ codoped phosphor Ba3Y4O9 with intense red upconversion emission and optical temperature sensing behavior.

The Ba₃Y₄O₉ host matrix with a low cutoff phonon energy of 585 cm⁻¹ is first applied to upconversion (UC) luminescence (UCL) by codoping Er³⁺/Yb³⁺. The new phosphor shows an intense red UC emission which is 6.8-fold and 5.9-fold stronger than that of Y₂O₃ and β-NaYF₄, respectively, under 980 nm GaAs laser diode (LD) excitation at a low density. A broad absorption band centered at 976 nm is observed, meaning a high adaptability to the GaAs LD required in actual applications. The optical thermometry behaviors based on the temperature dependent fluorescence intensity ratios of thermally coupled green UC bands ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 as well as the thermally coupled red UC emission bands originating from the Stark sublevels of ⁴F_9/2 manifold have been explored. The results show that green emissions are suitable for temperatures above 350 K with the maximum sensitivity of 0.00248 K⁻¹ at 563 K and the red emissions are appropriate for temperatures below 350 K with the maximum sensitivity of 0.00371 K⁻¹ at 143 K in our experimental range, indicating their complementary temperature sensing ranges. Moreover, a new method is proposed for evaluating the radiative lifetime of the Er^{3+ 4}F_9/2 state based on the analysis of photoluminescence (PL) spectra and fluorescence decay curves. The radiative lifetime of 879 μs for the red emitting level in Ba₃Y₄O₉ is achieved. Thereby, the emission efficiency of ⁴F_9/2 in the new UCP is a little higher than that in Y₂O₃. Our results imply that Ba₃Y₄O₉:Er³⁺/Yb³⁺ is a promising UCP, which could be applied to wide scope optical thermometry using a dual-color scheme. [ABSTRACT FROM AUTHOR]