Ratiometric Luminescent Thermometers with a Customized Phase-Transition-Driven Fingerprint in Perovskite Oxides

The development of noncontact thermometers with self-control to specific temperatures to be used as control markers with an additional degree of reliability is a challenge in the field of thermal sensors. Herein, a strategy exploiting the wide tunability of an intrinsic feature of oxide perovskites such as the phase-transition temperature to design a new class of ratiometric luminescent thermometers is introduced. The structural and optical response to the thermal stimuli of LaGaO₃:Nd³⁺ system is used as a prototype to show the unprecedented opportunity to combine the processes of two different regimes in the same compound, leading to a reliable optical thermal sensor with an intrinsic tell-tale sign at specific temperatures. High relative sensitivity, low temperature uncertainty, and good reproducibility, together with the need for a single calibration curve irrespective of the phase-transition temperature and the doping effects, attest the goodness of the thermometric performances. This work demonstrates the control of the phase-transition (orthorhombic ↔ rhombohedral) temperature, T_c, of lanthanum gallate in the 400-700 K range by carefully doping the perovskite structure, as a proof of concept for the design of customized thermometers characterized by a spectral shape change acting as a self-fingerprint for the T_c.