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Re-entrant relaxor ferroelectric behaviour is identified in BiFeO3–BaTiO3 ceramics, yielding anomalous thermal variations in structural distortion and functional properties.

BiFeO3–BaTiO3 (BF–BT) ceramics exhibit great potential for diverse applications in high temperature piezoelectric transducers, temperature-stable dielectrics and pulsed-power capacitors. Further optimization of functional properties for different types of applications can be achieved by modification of processing parameters or chemical composition. In the present work, the influence of pentavalent niobium substitution for trivalent ferric ions on the structure, microstructure and dielectric properties of 0.7BF–0.3BT ceramics was investigated systematically. Doping with niobium led to incremental reductions in grain size (from 7.0 to 1.3 µm) and suppression of long-range ferroelectric ordering. It was found that core-shell type microstructural features became more prominent as the Nb concentration increased, which were correlated with the formation of distinct peaks in the dielectric permittivity–temperature relationship, at ~470 and 600 °C, which were attributed to the BT-rich shell and BF-rich core regions, respectively. Nb-doping of BF–BT ceramics yielded reduced electronic conductivity and dielectric loss, improved electrical breakdown strength and enhanced dielectric energy storage characteristics. These effects are attributed to the charge compensation of pentavalent Nb donor defects by bismuth vacancies, which suppresses the formation of oxygen vacancies and the associated electron hole conduction mechanism. The relatively high recoverable energy density (Wrec = 2.01 J cm−3) and energy storage efficiency (η = 68%) of the 0.7BiFeO3–0.3BaTiO3 binary system were achieved at 75 °C under an electric field of 15 kV mm−1. This material demonstrates the greatest potential for applications in energy storage capacitors and temperature-stable dielectrics.

Perovskite solar cells continue to attract interest due to their facile preparation and high power conversion efficiencies. However, the highest efficiency perovskite solar cells inevitably contain lead, which raises concerns over contamination of drinking water when a solar module is broken and then flooded. We previously showed that conventional synthetic hydroxyapatite (HAP) nanoparticles could capture some of the lead from broken solar cells, but the amount of lead released was well above the safe drinking water level. Here, we modify the HAP synthesis to prepare new spherical-HAP (s-HAP) nanoparticles with a 60% increase in the Pb absorption capacity. We blend s-HAPs with TiO2 nanoparticles to construct mixed scaffolds and investigate their effect on (FAPbI3)0.97(MAPbBr3)0.03 solar cell performance and lead capture. Replacement of 80% of the TiO2 nanoparticles with s-HAP causes the power conversion efficiency to increase from 18.61% to 20.32% as a result of decreased charge carrier recombination. Lead contamination of water from devices subjected to simulated hail damage followed by flooding is shown to decrease exponentially with increasing s-HAP content. The lead concentration in water after 24 h is below the US safe water drinking limit.

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