Upconversion luminescence of Sr₈MgY(PO₄)₇:Yb³⁺–Er³⁺/Ho³⁺ phosphors for optical thermometry

In this work, to develop new upconversion (UC) luminescent material for optical thermometry, the Sr₈MgY(PO₄)₇:Yb³⁺–Er³⁺/Ho³⁺ phosphors were explored, and their UC luminescence properties upon 980 nm excitation were investigated. The phase purity of the samples was checked by XRD patterns, which showed no secondary phase existed. For Yb³⁺–Er³⁺ codoped samples, three peaks including two green and one red emissions have been found, and the optimal Er³⁺ doping content was determined to be 3 mol% with regard to the green emissions. For Yb³⁺–Ho³⁺ codoped samples, the green and red emissions demonstrated different change with Ho³⁺ concentration, which could be owing to the cross-relaxation in Ho³⁺ ions. By studying the temperature-dependence of the emission intensities, it was suggested that several strategies can be used for temperature sensing, involving the I₅₂₁/I₅₄₈ and I₆₆₀/I₅₂₁ of Er³⁺ and I₆₅₈/I₅₄₈ of Ho³⁺. The absolute and relative sensitivities were evaluated and the Yb³⁺–Ho³⁺ codoped sample showed the higher sensitivities than Yb³⁺–Er³⁺ codoped sample. The above results indicated the present phosphors could be candidate luminescent materials for optical thermometry.