Phase-pure eulytite-type Sr₃Y_0.88(PO₄)₃:0.10Yb³⁺,0.02Ln³⁺ upconversion (UC) phosphors (Ln = Ho, Er, Tm) were synthesized via 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 (S_A) and relative (S_R) 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 S_A and S_R 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 S_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 S_R 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. Graphical abstract Phase-pure eulytite-type Sr₃Y_0.88(PO₄)₃:0.10Yb³⁺,0.02Ln³⁺ upconversion (UC) phosphors (Ln=Ho, Er, Tm) were synthesized via gel-combustion, whose property and mechanism of UC luminescence and the performance of optical temperature sensing with thermally and/or non-thermally coupled energy levels were thoroughly investigated.

This review article summarizes the recent achievements in stabilization of the metastable lattice of gadolinium aluminate garnet (Gd3Al5O12, GAG) and the related developments of advanced optical materials, including down-conversion phosphors, up-conversion phosphors, transparent ceramics, and single crystals. Whenever possible, the materials are compared with their better known YAG and LuAG counterparts to demonstrate the merits of the GAG host. It is shown that novel emission features and significantly improved luminescence can be attained for a number of phosphor systems with the more covalent GAG lattice and the efficient energy transfer from Gd3+ to the activator. Ce3+ doped GAG-based single crystals and transparent ceramics are also shown to simultaneously possess the advantages of high theoretical density, fast scintillation decay, and high light yields, and hold great potential as scintillators for a wide range of applications. The unresolved issues are also pointed out.