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Photoferroelectrics offer unique opportunities to explore light energy conversion based on their polarization-driven carrier separation and above-bandgap voltages. The problem associated with the wide bandgap of ferroelectric oxides, i.e., the vanishingly small photoresponse under visible light, has been overcome partly by bandgap tuning, but the narrowing of the bandgap is, in principle, accompanied by a substantial loss of ferroelectric polarization. In this article, we report an approach, ‘gap-state’ engineering, to produce photoferroelectrics, in which defect states within the bandgap act as a scaffold for photogeneration. Our first-principles calculations and single-domain thin-film experiments of BiFeO 3 demonstrate that gap states half-filled with electrons can enhance not only photocurrents but also photovoltages over a broad photon-energy range that is different from intermediate bands in present semiconductor-based solar cells. Our approach opens a promising route to the material design of visible-light-active ferroelectrics without sacrificing spontaneous polarization. Overcoming the optical transparency of wide bandgap of ferroelectric oxides by narrowing its bandgap tends to result in a loss of polarization. By utilizing defect states within the bandgap, Matsuo et al. report visible-light-active ferroelectrics without sacrificing polarization.

Titanium oxide materials have multiple functions such as photocatalytic and photovoltaic effects. Ferroelectrics provide access to light energy conversion that delivers above-bandgap voltages arising from spatial inversion symmetry breaking, whereas their wide bandgap leads to poor absorption of visible light. Bandgap narrowing offers a potential solution, but this material modification suppresses spontaneous polarization and, hence, sacrifices photovoltages. Here, we report successive-redox mediated ferrophotovoltaics that exhibit a robust visible-light response. Our single-crystal experiments and ab initio calculations, along with photo-luminescence analysis, demonstrate that divalent Fe 2+ and trivalent Fe 3+ coexisted in a prototypical ferroelectric barium titanate BaTiO 3 introduce donor and acceptor levels, respectively, and that two sequential Fe 3+ /Fe 2+ redox reactions enhance the photogenerated power not only under visible light but also at photon energies greater than the bandgap. Our approach opens a promising route to the visible-light activation of photovoltaics and, potentially, of photocatalysts. Photovoltaic response from normal ferroelectric materials generates large voltages but low current due to poor absorption of visible light. Noguchi et al. dope ferrous and ferric ion couple into barium titanate crystals to enhance the photogeneration at photon energies both below and above bandgap.

Sodium niobate (NaNbO 3 ) is a potential material for lead-free dielectric ceramic capacitors for energy storage applications because of its antipolar ordering. In principle, a reversible phase transition between antiferroelectric (AFE) and ferroelectric (FE) phases can be induced by an application of electric field ( E ) and provides a large recoverable energy density. However, an irreversible phase transition from the AFE to the FE phase usually takes place and an AFE-derived polarization feature, a double polarization ( P )- E hysteresis loop, does not appear. In this study, we investigate the impact of chemically induced hydrostatic pressure ( p chem ) on the phase stability and polarization characteristics of NaNbO 3 -based ceramics. We reveal that the cell volume of Ca-modified NaNbO 3 [(Ca x Na 1−2 x V x )NbO 3 ], where V is A-site vacancy, decreases with increasing x by a positive p chem . Structural analysis using micro-X-ray diffraction measurements shows that a reversible AFE–FE phase transition leads to a double P - E hysteresis loop for the sample with x  = 0.10. DFT calculations support that a positive p chem stabilizes the AFE phase even after the electrical poling and provides the reversible phase transition. Our study demonstrates that an application of positive p chem is effective in delivering the double P - E loop in the NaNbO 3 system for energy storage applications.

Controlling and manipulating defects in materials provides an extra degree of freedom not only for enhancing physical properties but also for introducing additional functionalities. In ferroelectric oxides, an accumulation of point defects at specific boundaries often deteriorates a polarization-switching capability, but on the one hand, delivers interface-driven phenomena. At present, it remains challenging to control oxygen vacancies at will to achieve a desirable defect structure. Here, we report a practical route to designing oxygen-vacancy distributions by exploiting the interaction with transition-metal dopants. Our thin-film experiments combined with ab-initio theoretical calculations for BiFeO 3 demonstrate that isovalent dopants such as Mn 3+ with a partly or fully electron-occupied e g state can trap oxygen vacancies, leading to a robust polarization switching. Our approach to controlling oxygen vacancy distributions by harnessing the vacancy-trapping capability of isovalent transition-metal cations will realize the full potential of switchable polarization in ferroelectric perovskite oxides.

Spontaneous polarization ( P s ) in ferroelectrics has provided the impetus to develop piezoelectric devices such as sensors, actuators and diagnostic imaging transducers. Widely used lead-based perovskites exhibit a composition-driven phase diagram involving a transition region, known as a morphotropic phase boundary, where the ferroelectric structure changes dramatically and the piezoelectric activity is maximal. In some perovskites, ferroic polarization coexists with nonpolar rotations of octahedra, suggesting an unprecedented phase diagram. Here, we show morphotropic phase boundaries, where ‘ferrielectric’ appears as a bridging phase between ferroelectrics with rhombohedral and tetragonal symmetries in Bi 1/2 Na 1/2 TiO 3 -based perovskites. Neutron diffraction analysis demonstrates that the intermediate ferrielectric displays a small P s resulting from up and down polarizations coupled with an in-phase TiO 6 rotation. Our ab initio calculations indicate that a staggered Bi-O conformation at an appropriate chemical pressure delivers the ferrielectric-mediated phase boundaries, which provides a promising platform for (multi)ferroic materials with enhanced physical properties.

Various types of small-scale wastewater treatment systems are widely used in rural areas, and life-cycle assessment (LCA) should be performed to evaluate their environmental performance. In this study, septic systems were first classified into five categories based on their wastewater treatment performance. Effluent samples from actual systems were collected, and their water qualities were determined. A model to evaluate the environmental load from the septic systems using LCA methods was then established. The water-quality values obtained were input to the model, and the life-cycle environmental costs of the classified septic systems were calculated. The mean environmental load of the effluent during the operation stage was 37.6%, confirming that evaluation of an effluent discharge inventory using LCA, inspection, and water-quality monitoring to improve operations is critical for reducing the environmental load. The operation stage accounts for over 99% of the involved eutrophication, biological toxicity, and toxic chemicals, which are strongly related to the quality of the effluent. Evaluation of the effluent discharge inventory using LCA is of great significance, even for small-scale wastewater treatment systems. The set of procedures developed in this study can be used to calculate comprehensive environmental impacts at wastewater treatment plants.

Cultivated strawberry is the most widely consumed fruit crop in the world, and therefore, many breeding programs are underway to improve its agronomic traits such as fruit quality. Strawberry cultivars were vegetatively propagated through runners and carried a high risk of infection with viruses and insects. To solve this problem, the development of F 1 hybrid seeds has been proposed as an alternative breeding strategy in strawberry. In this study, we conducted a potential assessment of genomic selection (GS) in strawberry F 1 hybrid breeding. A total of 105 inbred lines were developed as candidate parents of strawberry F 1 hybrids. In addition, 275 parental combinations were randomly selected from the 105 inbred lines and crossed to develop test F 1 hybrids for GS model training. These populations were phenotyped for petiole length, leaf area, Brix, fruit hardness, and pericarp color. Whole-genome shotgun sequencing of the 105 inbred lines detected 20,811 single nucleotide polymorphism sites that were provided for subsequent GS analyses. In a GS model construction, inclusion of dominant effects showed a slight advantage in GS accuracy. In the across population prediction analysis, GS models using the inbred lines showed predictability for the test F 1 hybrids and vice versa, except for Brix. Finally, the GS models were used for phenotype prediction of 5,460 possible F 1 hybrids from 105 inbred lines to select F 1 hybrids with high fruit hardness or high pericarp color. These F 1 hybrids were developed and phenotyped to evaluate the efficacy of the GS. As expected, F 1 hybrids that were predicted to have high fruit hardness or high pericarp color expressed higher observed phenotypic values than the F 1 hybrids that were selected for other objectives. Through the analyses in this study, we demonstrated that GS can be applied for strawberry F 1 hybrid breeding.

The photovoltaic (PV) effect in polar materials offers great potential for light-energy conversion that generates a voltage beyond the bandgap limit of present semiconductor-based solar cells. Ferroelectrics have received renewed attention because of the ability to deliver a high voltage in the presence of ferroelastic domain walls (DWs). In recent years, there has been considerable debate over the impact of the DWs on the PV effects, owing to lack of information on the bulk PV tensor of host ferroelectrics. In this article, we provide the first direct evidence of an unusually large PV response induced by ferroelastic DWs—termed ‘DW’-PV effect. The precise estimation of the bulk PV tensor in single crystals of barium titanate enables us to quantify the giant PV effect driven by 90° DWs. We show that the DW-PV effect arises from an effective electric field consisting of a potential step and a local PV component in the 90° DW region. This work offers a starting point for further investigation into the DW-PV effect of alternative systems and opens a reliable route for enhancing the PV properties in ferroelectrics based on the engineering of domain structures in either bulk or thin-film form.

We report the robust ferroelectric properties of (1 −  x )(Bi 1/2 Na 1/2 )TiO 3 – x (Bi 1/2 K 1/2 )TiO 3 ( x  = 33%) single crystals grown by a top-seeded solution growth process under a high oxygen-pressure (0.9 MPa) atmosphere. The sample exhibit a large remanent polarization of 48 μC/cm 2 and a sizeable piezoelectric strain constant of 460 pm/V. Neutron powder diffraction structural analysis combined with first-principles calculations reveals that the large ferroelectric polarization comparable to PbTiO 3 stems from the hybridization between Bi-6 p and O-2 p orbitals at a moderately negative chemical pressure.

Superlattice-structured epitaxial thin films composed of Mn(5%)-doped BiFeO3 and BaTiO3 with a total thickness of 600 perovskite (ABO3) unit cells were grown on single-crystal SrTiO3 substrates by pulsed laser deposition, and their polarization and dielectric properties were investigated. When the layers of Mn-BiFeO3 and BaTiO3 have over 25 ABO3 unit cells (N), the superlattice can be regarded as a simple series connection of their individual capacitors. The superlattices with an N of 5 or less behave as a unified ferroelectric, where the BaTiO3 and Mn-BiFeO3 layers are structurally and electronically coupled. Density functional theory calculations can explain the behavior of spontaneous polarization for the superlattices in this thin regime. We propose that a superlattice formation comprising two types of perovskite layers with different crystal symmetries opens a path to novel ferroelectrics that cannot be obtained in a solid solution system.