In response to the ongoing quest for new, highly sensitive upconverting luminescent thermometers, this article introduces, for the first time, upconverting luminescent thermometers based on thermally induced structured phase transitions. As demonstrated, the transition from the low-temperature monoclinic to the high-temperature tetragonal structures of LiYO2:Yb3+,Er3+ induces multifaceted modification in the spectroscopic properties of the examined material, influencing the spectral positions of luminescence bands, energy gap values between thermally coupled energy levels, and the red-to-green emission intensities ratio. Moreover, as illustrated, both the color of the emitted light and the phase transition temperature (from 265 K, for LiYO2:Er3+, 1%Yb3+, to 180 K, for 10%Yb3+), and consequently, the thermometric parameters of the luminescent thermometer can be modulated by the concentration of Yb3+ sensitizer ions. Establishing a correlation between the phase transition temperature and the mismatch of ion radii between the host material and dopant ions allows for smooth adjustment of the thermometric performance of such a thermometer following specific application requirements. Three different thermometric approaches were investigated using thermally coupled levels (SR = 1.8%/K at 180 K for 1%Yb3+), green to red emission intensities ratio (SR = 1.5%/K at 305 K for 2%Yb3+), and single band ratiometric approach (SR = 2.5%/K at 240 K for 10%Yb3+). The thermally induced structural phase transition in LiYO2:Er3+,Yb3+ has enabled the development of multiple upconverting luminescent thermometers. This innovative approach opens avenues for advancing the field of luminescence thermometry, offering enhanced relative thermal sensitivity and adaptability for various applications.
Keywords: Er3+; Yb3+; luminescent thermometry; phase transition; relative sensitivity; upconversion.