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Title: Visual Luminescence Thermometry Enabled by Phase-Transition-Activated Cross Relaxation of Tb3+ Ions

Authors: L. Marciniak*, M. Szymczak*

Journal:  Materials Horizons

DOI: 10.1039/D6MH00684A

The development of visual luminescent thermometers capable of exhibiting pronounced color changes in response to temperature variations requires the rational design of phosphors with high spectrally selective thermal sensitivity. In this work, we present a strategy based on phasetransition-induced activation of cross-relaxation processes in LiYO₂:Tb³⁺. The monoclinic-totetragonal structural phase transition modifies the point symmetry of Tb³⁺ ions in the host lattice, enhances the Stark effect, and enables energetic resonance required for efficient cross relaxation. Consequently, emission originating from the ⁵D₃ excited state is rapidly quenched relative to that from the ⁵D₄ level above approximately 300 K, resulting in a distinct temperature-dependent color change of the emitted light from blue to green. This mechanism yields exceptionally high chromaticity-coordinate-based sensitivities, reaching S_Rx,max = 0.40% K^-1 and S_Ry,max =0.72% K^-1 at 410 K. Furthermore, phase-transition-driven modifications of the Tb³⁺ emission spectral profile enable the realization of a multimode luminescent thermometer with a maximum relative sensitivity of S_Rmax =13% K^-1. The practical applicability of this system is demonstrated through an ON-OFF luminescent thermal switch and fully filter-free, dynamic two-dimensional thermal imaging using the blue and green channels of a standard digital camera, enabling intuitive visualization, remote readout, and temperature mapping under dynamic conditions.

This work was supported by the National Science Center (Poland) under 2022/45/B/ST5/01629 project.