Dual-mode luminescent thermometry in LiYO2:Nd3+,Yb3+ enabled by structural phase transition and phonon-assisted energy transfer

In this work, a dual-mode luminescent thermometer operating via both ratiometric and lifetime-based readout strategies was developed, enabled by the coexistence of two thermally driven effects: a structural phase transition in LiYO2 and a phonon-assisted energy transfer from Yb3+ to Nd3+. As demonstrated, changes in the shape of the emission band of Yb3+ ions corresponding to the 2F5/2 -> 2F7/2 electronic transition, induced by the phase transition, enabled the design of a ratiometric thermometer with a maximum relative sensitivity (SR) of 3.1% K^-1 for LiYO2 doped with 10% Yb3+ and 1% Nd3+ at 290 K. In contrast, the temperature-dependent Yb3+ -> Nd3+ energy transfer facilitated the development of a lifetime-based thermometer with a maximum SR of 1.5% K^-1 for 20% Nd3+ at 378 K. In both approaches, tuning the Nd3+ concentration allowed modulation of both the sensitivity and the temperature at which the maximum SR occurred. This was achieved by shifting the phase transition temperature and increasing the probability of interionic energy transfer, respectively. Notably, the temperature ranges corresponding to the maximum SR for the ratiometric and lifetime modes were distinct, effectively broadening the thermal operating window of the sensor. Additionally, it was shown that LiYO2 doped with Nd3+ and Yb3+ can also be used as a temperature sensor through the ratio of luminescence intensities recorded at two different time gates. Furthermore, the results confirmed that the phonon-assisted energy transfer process plays the dominant role in shaping the luminescence kinetics, surpassing the influence of the structural phase transition. Overall, this study identifies LiYO2:Nd3+,Yb3+ as a promising candidate for multimodal luminescent temperature sensing applications.