Extending the optical temperature sensing range of Cr³⁺ by synchronously tuning ²E and ⁴T₂ emission

Due to the different sensitivities of the ²E and ⁴T₂ energy levels of Cr³⁺ to the local environment, Cr³⁺-doped fluorescent materials appear as excellent candidates for highly sensitive temperature sensing based on luminescence intensity ratio technology. However, a way to broaden the strict Boltzmann temperature measuring range is rarely reported. In this work, through Al³⁺ alloying strategy, a series of SrGa_12-AlO₁₉:0.5%Cr³⁺(x = 0, 2, 4, and 6) solid-solution phosphors were synthesized. Remarkably, the introduction of Al³⁺ can play a role in regulating the crystal field around Cr³⁺ and the symmetry of [Ga/AlO₆] octahedron, realizing the synchronous tuning of ²E and ⁴T₂ energy levels when the temperature changes in a wide range, achieving the purpose of increasing the intensity difference of ²E → ⁴A₂ and ⁴T₂ → ⁴A₂ transitions, so as to extend the temperature sensing range. Among all samples, SrGa₆Al₆O₁₉:0.5%Cr³⁺ showed the widest temperature measuring range from 130 K to 423 K with S_a of 0.0066 K^-1 and S_r of 1% K^-1@130 K. This work proposed a feasible way to extend the temperature sensing range for transition metal-doped LIR-mode thermometers.