Modeling of a Single‐Band‐Ratiometric Sensor Based on Lattice Positive Thermal Expansion in Eu3+‐Activated Halide Perovskite Cs2NaEuCl6

Abstract Recently, single‐band ratiometric (SBR) thermometry has emerged as an innovative approach to traditional fluorescence thermometry, overcoming uncertainties associated with emission spectrum overlap or scattering while maintaining high spatial resolution and remote monitoring. This paper presents a novel Cs 2 NaEuCl 6 perovskite prepared through a slow‐cooling solution method. Additionally, it proposes a temperature sensor model that relies on the thermal quenching of charge‐transfer state absorption. Mechanical studies highlight the role of lattice positive thermal expansion in affecting Eu 3+ emission. Conversely, a significant emission enhancement is observed upon excitation corresponding to both the ground state and excited state absorption. The distinct luminescent behavior of this Eu 3+ ‐activated halide perovskite model makes it suitable for developing a highly sensitive SBR‐type sensor with a relative sensitivity ( S r ) exceeding 1.5% K −1 and temperature resolution (𝛿T) below 1 K at room temperature. Furthermore, it demonstrates the thermal stability during multiple heating‐cooling cycles. Finally, the practical applicability of the proposed SBR model is demonstrated by employing a self‐manufactured film sensor that enables precise real‐time temperature detection for electronic components. The work is regarded as a significant stride toward the development of cutting‐edge and exquisitely sensitive thermometers based on lanthanide‐based halide double perovskites.