Electro- and photon-induced cooling in BNT-BT-SBET relaxors with in situ optical temperature sensing

A novel lead-free luminescent ferroelectric (FE) ceramic, ${{\rm Bi}_{0.5}}{{\rm Na}_{0.5}}{{\rm TiO}_3} {-} {0.{06\; \rm BaTiO}_3} {-} {0.{055\;\rm Sr}_{0.7}}{{\rm Bi}_{0.18}}{{\rm Er}_{0.02 \,\square\, 0.1}}$Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-0.055Sr_0.7Bi_0.18Er_0.02◻0.1${{\rm TiO}_3}$TiO₃ (BNT-BT-SBET), is developed with an adiabatic temperature change ($\Delta T$ΔT) of 0.7 K under an electric field ($E$E) of 60 kV/cm at room temperature, an anti-Stokes fluorescence cooling, and a maximum optical $T$T sensitivity of ${0.0055}\;{{\rm K}^{ - 1}}$0.0055K^-1 at 522 K. Interestingly, the electrocaloric response reaches a saturation at permittivity shoulder $T$T of 100°C; meanwhile, the maximized emission intensity of $^2{{\rm H}_{11/2}}{ \to ^4}{{\rm I}_{15/2}}$²H_11/2→⁴I_15/2 occurs. $T$T- and $E$E-tunable enhancement of $^2{{\rm H}_{11/2}}{ \to ^4}{{\rm I}_{15/2}}$²H_11/2→⁴I_15/2 emission intensity is due to the population inversion from the $^4{{\rm S}_{3/2}}$⁴S_3/2 to $^2{{\rm H}_{11/2}}$²H_11/2 states caused by an incoherent regime consisting of FE phase and polar nanoregions in a relaxor matrix.