Upconversion luminescence and favorable temperature sensing performance of eulytite-type Sr₃Y(PO₄)₃:Yb³⁺/Ln³⁺ phosphors (Ln=Ho, Er, Tm)

Abstract

Phase-pure eulytite-type Sr₃Y_0.88(PO₄)₃:0.10Yb³⁺,0.02Ln³⁺ upconversion (UC) phosphors (Ln = Ho, Er, Tm) were synthesized <i>via</i> gel-combustion and subsequent calcination at 1250°C. Their UC luminescence, temperature-dependent fluorescence intensity ratio of thermally and/or non-thermally coupled energy levels, and performance of optical temperature sensing were systematically investigated. The phosphors typically exhibit green, orange-red and blue luminescence under 978 nm NIR laser excitation for Ln = Er, Ho and Tm, respectively, which were discussed from two- and three-photon processes. The 524 nm green (Er³⁺), 657 nm red (Ho³⁺) and 476 nm blue (Tm³⁺) main emissions were analyzed to have average decay times of ~52 ± 2, 260.6 ± 0.7 and 117 ± 1 μs, respectively. It was shown that (1) the Er³⁺ doped phosphor has a better overall performance of temperature sensing with thermally coupled ²H_11/2 and ⁴S_3/2 energy levels, whose maximum absolute (<i>S</i> _A) and relative (<i>S_R</i> ) sensitivities are ~5.07 × 10^-3 K^-1 at 523 K and ~1.16% at 298 K, respectively; (2) the Ho³⁺ doped phosphor shows maximum <i>S</i> _A and <i>S_R</i> values of ~0.019 K^-1 (298-573 K) and 0.42% at 573 K for the non-thermally coupled energy pairs of ⁵F₅/(⁵F₄,⁵S₂) and ⁵I₄/⁵F₅, respectively; (3) the Tm³⁺ doped phosphor has a maximum <i>S</i> _A of ~12.74 × 10^-3 K^-1 at 573 K for the non-thermally coupled ³F_2,3/¹G₄ energy levels and a maximum <i>S_R</i> of ~1.74% K^-1 at 298 K for the thermally coupled ³F_2,3/³H₄ levels. Advantages of the current phosphors in optical temperature sensing were also revealed by comparing with other typical UC phosphors.