Deep Seek to the Excited States: A Thermally‐Activated Jahn–Teller Effect Achieves Superior Sensitivity for Thermometry at Cryogenic Temperature

Abstract Boltzmann equilibrium is not always achieved at the lower temperature limit for a typical ratiometric thermometer with thermally coupled excited states. The universal approach to broaden the dynamic temperature range of all Boltzmann‐type thermometers still remains a challenge. Herein, a bismuth‐based halide single crystal (DMA ₄ BiCl ₇ : Sb ³⁺ , DMA is referred to as dimethylammonium) shifts the onset temperature by ≈200 K. It also provides trustworthy experimental evidence in observing a thermally‐driven process from ³ P ₁ state to A _T minima. The action of both spin‐orbit coupling and the dynamic Jahn–Teller effect represented by large effective Huang‐Rhys factors ( S ₁ = 32.44, S ₂ = 42.51) is responsible for maximizing the opposite emission changes at different temperatures and overcomes the thermodynamic limitations of the traditional thermally coupled energy levels. A high relative sensitivity ( S _r ) value of 18.9 % K ⁻¹ is obtained at 77 K. It finds application potential in extending the temperature window to cover the cryogenic range from 77 K to 277 K with constantly high accuracy (temperature uncertainty δT = 0.065 K at 77 K). Moreover, a new way is developed to visualize temperature mapping via grayscale imaging tools in determining the temperature distribution of a real sample is demonstrated. anti‐thermal quenching; hybrid metal halide; temperature mapping; thermally activated; thermometer