Optical temperature sensing and upconversion luminescence in Er³⁺/Yb³⁺ co-doped BNT ferroelectric ceramic

In the present work, a series of Er³⁺/Yb³⁺ co-doped Bi_0.5-x-yEr_xYb_yNa_0.5TiO₃ ferroelectric ceramic is prepared using traditional solid state technique to investigate the structural, optical and sensing properties. The XRD analysis confirms the rhombohedral geometry and no extra peaks shows the solubility of the dopant ions. SEM images exhibited a dense microstructure with well-defined grain boundaries. The FTIR vibrational bands observed at 550 cm⁻¹ and 820 cm⁻¹ depict the typical characteristics of perovskite structure caused by the expansion of the octahedral group of Ti–O bonds. The Tauc plot displays the energy band gaps (E_g) in a range of 2.93 eV to 2.88 eV as a function of the Yb³⁺concentration. The photoluminescence spectra were measured at two wavelengths of excitation at 489 nm and 980 nm for all the BNT ceramic compositions. It has been observed that the two intense green bands and one visible red band appeared at 530 nm, 549 nm and 662 nm, respectively. The dependency of pump power on UCL spectra is observed with varying pump powers at 980 nm excitation. The two photons that are involved in the UCL process are confirmed by this investigation. The time-resolved fluorescence spectroscopy reveals that the efficiency in energy transfer between the dopant ions increased for all the co-doped BNT ceramic compositions. The absolute (S_ab) and relative (S_r) sensitivity of BE3Y3 ceramic composition are 0.54% K⁻¹ and 1.24% K⁻¹ at 523 K and 303 K, respectively.