Ionic-Radius Mismatch as a Structural Lever for Tuning Phase Transitions and Luminescent Thermometry

Abstract

The widespread implementation of luminescence thermometers requires a comprehensive understanding of the structural factors governing their thermometric performance. Establishing such structure-property relationships is essential for the rational design of sensing materials with application-tailored characteristics. This is particularly relevant for phase-transition-based luminescence thermometers, which offer exceptionally high relative sensitivities. The systematic analysis of K3Lu(PO4)2:Eu3+ demonstrates that introducing co-dopant ions with a controlled ionic-radius mismatch provides an effective strategy for tailoring phase-transition characteristics. This approach enables both the phase-transition temperature and thermal operating range to be controlled. Specifically, the transition temperature shifts from 210 K for K3Lu(PO4)2:Eu3+ to 310 K for K3Lu(PO4)2:Eu3+,10%La3+, while the operating range broadens from 30 to 60 K. Importantly, linear correlations between the ionic-radius mismatch parameter, Ω, and the phase-transition temperature, enthalpy, and entropy provide a quantitative framework for controlling the thermodynamics of the transition through compositional engineering. Beyond luminescence thermometry, these relationships establish a general strategy for designing materials exhibiting first-order phase transitions with tailored thermodynamic characteristics, opening opportunities for their optimization across a broad range of functional applications.