Optical thermometry based on thermolabile intrinsic polarons in Tm3+ and Yb3+ co-doped congruent lithium niobate single crystal

•A FIR thermometry strategy based on structure variation induced by thermolabile polarons in Tm:Yb:LN is proposed.•The abnormal ‘U’ shape variation of upconversion emission with increasing temperature is investigated.•The maximum absolute and relative sensitivities reach 3.7% K−1 and 1.25% K−1 respectively.