Anion Effect on the Thermometric Properties of Near-Infrared Emitting Mn⁵⁺-Activated Sr₅(PO₄)₃X (X = F, Cl, Br) Apatite Phosphors

Near-infrared emitting Mn⁵⁺-activated phosphors are recently recognized as promising thermal sensors for biological applications. However, guidelines to predict suitable hosts with optimal temperature sensing ability are still missing. In this view, the indirect effect of the different halides on the 3d² electrons of the Mn⁵⁺ ions in different Sr₅(PO₄)₃X apatites (X = F, Cl, and Br) is deeply investigated through a combined experimental and theoretical approach. The temperature dependence of the PL and decay curves allows to simultaneously get information about the electron-phonon coupling and the potential application as optical thermometers, showing promising results in terms of relative sensitivity. Boltzmann-based and lifetime-based thermometry ensure a unique reliability, and the simultaneous use of the ratio between the emission originating from the ³T₁ and ¹E excited states and the split components of the ¹E (E₁ and E₂) state allows to enlarge the temperature range of applicability of the thermometers from cryogenic to high temperature. The local [PO₄]³⁻ tetrahedral geometry in the halide apatites drives their thermometric properties, showing interesting results in terms of sensitivity, particularly for the bromide Sr₅(PO₄)₃Br:Mn⁵⁺ phosphor. Moreover, the high quantum efficiencies estimated at room temperature support the potential use in real applications of this family of NIR phosphors.