Visual luminescence thermometry enabled by phase-transition-activated cross relaxation of Tb3+ ions.

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 phase-transition-induced activation of cross-relaxation processes in LiYO2:Tb3+. The monoclinic-to-tetragonal 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 SRx,max = 0.40% K-1 and SRy,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 SRmax = 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.