Second-Coordination-Sphere Cation Substitution as a Tool for Controlling Phase Transitions and Performance of the Luminescence Thermometry

TL;DR

This study shows that substituting Na+ for Li+ in Eu3+-doped LiYO2 shifts phase transition temperatures, enabling better control of luminescent thermometry performance, but also reduces sensitivity due to structural disorder.

Contribution

It introduces a cost-effective method of tuning phase transition temperatures via second-coordination-sphere cation substitution, affecting thermometric sensitivity and phase transition nature.

Findings

Na+ substitution shifts phase transition temperature

Lower transition temperature reduces thermometric sensitivity

Structural disorder and lattice strain increase with Na+ incorporation

Abstract

Despite the exceptionally high relative sensitivities achieved by luminescent thermometers based on first-order structural phase transitions, their principal limitation lies in the inherently narrow thermal operating range associated with the transition temperature. In this work, we demonstrate that partial substitution of Li+ by Na+ ions in the second coordination sphere of Eu3+ ions in LiYO2 enables a substantial shift of the phase transition temperature, thereby allowing controlled optimization of the thermometric performance. This approach represents a significantly more cost-effective and efficient strategy for tuning the phase transition temperature compared with the previously proposed substitution of Y3+ by other lanthanide ions. Importantly, we show that lowering the transition temperature through Na+ incorporation simultaneously introduces static compositional disorder and local lattice strain. As a consequence, the enthalpy difference between the competing structural phases decreases, and the cooperativity of the lattice distortion is reduced, indicating a gradual weakening of the first-order character of the phase transition. Our results demonstrate that such structural modifications, while effective in shifting the transition temperature, inevitably lead to a reduction in the relative sensitivity of phase-transition-based luminescent thermometers.