Exploring calibration-free luminescence thermometry with Mn⁴⁺ anti-Stokes/Stokes emission

Mn⁴⁺-activated phosphors offer a unique possibility of calibration-free primary luminescence thermometry using the ratio of anti-Stokes to Stokes (AS/S) vibronic emission lines of the ²E → ⁴A₂ transition. Previous work used the AS/S emission of Mn⁴⁺ for thermometry but always relying on fitting procedures with effective vibrational energies that deviate from actual energies. Here, we explore a strategy without prior temperature calibration using two well-known Mn⁴⁺-doped hosts, Mg₁₄Ge₅O₂₄ (MGO) and K₂SiF₆ (KSF). In both Mn⁴⁺ occupies octahedral sites, replacing Ge⁴⁺ or Si⁴⁺, resulting in a weak zero-phonon line (ZPL) and strong vibronic Stokes and anti-Stokes lines. From the temperature-dependent emission spectra the AS/S ratios ν₆'/ν₆, ν₄'/ν₄, ν₃'/ν₃ were determined to serve as input for luminescence intensity ratio (LIR) analysis. For the well-resolved vibronic lines of Mn⁴⁺ in KSF it is feasible to realize temperature readout without temperature calibration - a capability rarely achieved in luminescence thermometry. In contrast, for Mn⁴⁺ in MGO vibronic lines exhibit significant overlap, complicating single-phonon-mode modeling. Even spectral deconvolution methods do not allow for reliable calibration-free temperature sensing. This work establishes a generalizable approach to optical thermometry relying on anti-Stokes/Stokes vibronic lines, offering a path towards calibration-free temperature sensing.