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 (S
R = 1.8%/K at 180 K for
1%Yb3+), green to red emission intensities ratio (S
R = 1.5%/K at 305 K for 2%Yb3+),
and single band ratiometric approach (S
R = 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.