STRUCTURAL AND OPTICAL PROPERTIES IN LANTHANIDE (LN3+= ER3+/YB3+/HO3+) DOPED BI0.5NA0.5TIO3 CERAMICS

Abstract

The development of lead-free ferroelectric materials has gained considerable attention due to their potential in energy storage, sensing, and optoelectronic applications. Among them, bismuth sodium titanate (Bi₀.₅Na₀.₅TiO₃, BNT) is a promising candidate owing to its favorable ferroelectric and structural properties. The functional performance of BNT can be significantly tailored through rare-earth doping, which modifies its electrical, luminescent, and sensing characteristics. This thesis outlines the effect of Er3+substituted bismuth sodium titanate ceramics with the chemical composition Bi0.5-xErxNa0.5TiO3 (x =0.00, 0.01, 0.02, 0.03, 0.04, and 0.05) were synthesized using conventional solid-state technique. The influence of Er3+ ions on structural, optical, ferroelectric, and temperature sensing properties have been investigated. The prepared ceramic powders were initially heated at a calcination temperature 850◦C to form single phase Bi0.5-xErxNa0.5TiO3 and finally sintered at temperature 1050◦C. Formation of pure phase composition with rhombohedral crystal structure is confirmed through X-ray diffraction studies. The typical FTIR bands near 540, 860, 910 cm-1 confirmed the presence of Ti–O stretching of octahedral groups in the perovskite structure. The decent squared shaped saturated P-E hysteresis are obtained under an electric field of 60 ≤E ≤70 kV/cm, and the loops become slimmer at higher Er3+ concentrations (x =0.04). The efficiency of energy storage density increases with Er3+ doping and an improved recoverable energy storage (Wr =2.73 J/cm3) and a higher efficiency (η =70.77%) are obtained for Er content, x =0.04. The photoluminescence spectra were recorded at two excitation wavelengths (488 nm and 980 nm). Two distinct green emission bands (529 nm and 550 nm) and one weak red emission band (670 nm) were observed at both excitation wavelengths. Increasing Er3+ content beyond (x >0.03) leads to significant quenching of light emission due to cross relaxation process and non- radiative relaxations. The pump power dependency revealed that two photons were involved in the light upconversion process. The time- viii resolved fluorescence spectroscopy confirmed the decrease in lifetime with increasing Er3+ concentration. The absolute and relative sensitivity of the prepared ceramic at Er3+ concentration (x =0.03) were found to be 0.47% K-1 at 523 K and 1.1% K-1 at 303 K, respectively. These optical and electrical properties open the possibility of realizing multifunctionality in the field of energy storage and opto- electronic applications. In continuation, a series of Er3+/Yb3+ co-doped Bi0.5-x-yErxYbyNa0.5TiO3 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 micro structure with well-defined grain boundaries. The FTIR vibrational bands observed at 550cm−1 and 820cm−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 (Eg) 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 (Sab) and relative (Sr) sensitivity of BE3Y3 ceramic composition are 0.54% K−1 and 1.24% K−1 at 523 K and 303 K, respectively. Further, the Ho3+ions are systematically inserted on the A-site of Bi0.5-xHoxNa0.5TiO3 lead-free ferroelectric ceramic via solid-state method. The x-ray diffraction spectra (XRD) shows the rhombohedral structure of Bi0.5- xHoxNa0.5TiO3. Fourier transform infrared (FTIR) spectroscopy shows two vibrational bands at 537 cm-1 and 832 cm-1, due to the stretching vibrations of Ti-O bonds in the octahedral units of the perovskite structure. The diffuse reflectance spectra (DRS) showed three bands at 454, 542, and 646 nm transit from 5I8→5G6, 5I8→5F4/5S2, and 5I8→5F5, respectively. The band gap varies and maximum for 0.03 concentration due to the local structural instability within ix the lattice by Ho3+ions. The photoluminescence (PL) emission spectra are traced under 452 nm, and one intense green band at 548 nm and two at 655 (orange) and 750 nm (red) were observed. In upconversion luminescence (UCL) spectra, four emission wavelengths were obtained at 490, 525, 552, and 660 nm. The orange emission band is highly intense, whereas other color bands are relatively weak. Pump power dependence on UCL spectra is analyzed using concentration (x =0.03). The decay profile for green and orange bands showed an average lifetime of 12.99 μs and 9.43 μs, respectively. The PE loops become slimmer, and Pr values decrease with increasing concentration. The energy storage efficiency (η%) of the ceramic is increasing with dopant concentration and comes out to be 90 %. The maximum absolute and relative sensitivity observed for the BNT (x =0.03) is 0.29 % K-1 and 0.22 % K-1 at 303 K, respectively. This study demonstrates the novel observation of orange emission in Ho3+doped Bi0.5Na0.5TiO3 ceramics without any sensitizers which is not usual behavior as Ho3+ typically exhibits green or red emission. This unusual luminescence highlights distinct site symmetry and energy level interactions within the lead-free BNT matrix, validating the uniqueness of the work. Lastly this study presents the application of BNT, the development and characterization of flexible piezoelectric nanocomposite films based on bismuth sodium titanate (BNT) embedded in a polyvinylidene fluoride (PVDF) matrix for efficient mechanical energy harvesting. BNT ceramic powder was synthesized via a conventional solid-state reaction route and subsequently incorporated into PVDF at varying concentrations (0 – 25wt% BNT) using a drop casting method. Structural analyses confirmed the successful formation of phase-pure rhombohedral BNT and enhancement of the electroactive β- phase in PVDF upon BNT addition. Morphological evaluation using FESEM revealed uniform dispersion of BNT up to 20wt% BNT, beyond which agglomeration was observed. FTIR and XRD studies substantiated the β-phase promotion in the composites, while polarization–electric field (P–E) loop measurements demonstrated improved ferroelectric behavior with increasing BNT content, peaking at 20wt% BNT. Notably, the piezoelectric voltage and current outputs were maximized at 20wt% BNT, registering 25V and 12μA, respectively, under mechanical excitation. A practical demonstration using the composite film to power LEDs confirmed its potential as a x flexible nanogenerator. These findings highlight the suitability of BNT/PVDF composites as promising candidates for next-generation, lead-free, wearable energy harvesting devices. In conclusion, Er³⁺,Er³⁺/Yb³⁺ and Ho³⁺doped Bi₀.₅Na₀.₅TiO₃ ceramics were successfully synthesized and systematically examined for their structural, ferroelectric, luminescent, and sensing characteristics. The results confirmed phase-pure rhombohedral structures, enhanced energy storage efficiency, and notable upconversion emissions with composition-dependent behavior. Particularly, Er³⁺ and Er³⁺/Yb³⁺ co-doping improved energy storage and temperature sensitivity, while Ho³⁺ substitution revealed an unusual orange emission, underscoring unique site symmetry effects. Furthermore, extending the study to BNT/PVDF nanocomposites demonstrated their effectiveness as flexible, lead-free piezoelectric generators with promising output performance. Overall, these findings establish BNT-based materials as multifunctional candidates for future energy storage, optoelectronic, and wearable energy-harvesting applications.