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The wide bandgap of typical ferroelectric perovskites (with AB[O.sub.3] composition) is due to the fundamental characteristics of the metaloxygen A-Oand B-O bonds. The excitation across the bandgap is essentially a [...], Ferroelectrics have recently attracted attention as a candidate class of materials for use in photovoltaic devices, and for the coupling of light absorption with other functional properties (1-7). In these materials, the strong inversion symmetry breaking that is due to spontaneous electric polarization promotes the desirable separation of photo-excited carriers and allows voltages higher than the bandgap, which may enable efficiencies beyond the maximum possible in a conventional p-n junction solar cell (2,6,8-10). Ferroelectric oxides are also stable in a wide range of mechanical, chemical and thermal conditions and can be fabricated using low-cost methods such as sol-gel thin-film deposition and sputtering (3,5). Recent work (3,5,11) has shown how a decrease in ferroelectric layer thickness and judicious engineering of domain structures and ferroelectric-electrode interfaces can greatly increase the current harvested from ferroelectric absorber materials, increasing the power conversion efficiency from about [10.sup.-4] to about 0.5 per cent. Further improvements in photovoltaic efficiency have been inhibited by the wide bandgaps (2.7-4 electronvolts) of ferroelectric oxides, which allow the use of only 8-20 per cent of the solar spectrum. Here we describe a family of single-phase solid oxide solutions made from low-cost and non-toxic elements using conventional solid-state methods: [[Knb[O.sub.3]].sub.1-x][[Ba[Ni.sub.1/2][Nb.sub.1/2][O.sub.3-δ].sub.x] (KBNNO). These oxides exhibit both ferroelectricity and a wide variation ofdirect bandgaps in the range 1.1-3.8 electronvolts. In particular, the x= 0.1 composition is polar at room temperature, has a direct bandgap of 1.39 electronvolts and has a photocurrent density approximately 50 times larger than that of the classic ferroelectric (Pb,La)(Zr,Ti)[O.sub.3] material. The ability of KBNNO to absorb three to six times more solar energy than the current ferroelectric materials suggests a route to viable ferroelectric semiconductor-based cells for solar energy conversion and other applications.

Ferroelectric oxides have been demonstrated to be promising in developing emerging photovoltaic technologies because of the various mechanisms that allow above‐band‐gap photovoltages and higher efficiencies. However, the wide band gaps of conventional ferroelectric oxides (2.7‐4 eV) limit their utilization of the solar spectrum. Here the phase stability, absorption properties, ferroelectric, and photovoltaic responses of Ni‐ and Ni‐Nb‐substituted BaTiO3 were explored to evaluate their potential as visible‐light‐absorbing photovoltaic materials. Although the acceptor substitution of Ni2+ stabilized a hexagonal 6H polymorph in both systems, post‐annealing treatments allowed restoration of a tetragonal 3C phase for Ba(Ti0.99Ni0.01)O2.99 and (0.9)BaTiO3‐(0.1)Ba(Ni1/2Nb1/2)O2.75. The oxygen vacancies accompanying the Ni and Ni‐Nb substitutions significantly lower the optical band gap of BaTiO3 to ~1.5 eV and the visible light absorption can be systematically tuned between 380 and 1000 nm by varying the Ni:Nb ratio. Room temperature ferroelectricity was observed in Ba(Ti0.99Ni0.01)O2.99 with a saturation polarization = 18 μC/cm2 and remnant polarization = 1 μC/cm2. The Ni‐Nb substituted composition (0.9)BaTiO3‐(0.1)Ba(Ni1/2Nb1/2)O2.75 shows a ferroelectric response with a remnant polarization of 5 μC/cm2 at 77 K, which gradually decreases as temperature increases. Both compositions exhibit ferroelectrically switchable photoresponses under an AM 1.5 G sunlight simulator; the highest switchable steady‐state current of 8 nA/cm2 observed for (0.9)BaTiO3‐(0.1)Ba(Ni1/2Nb1/2)O2.75 exceeds those reported in previous studies of BaTiO3 ceramic samples (J Solid State Chem, 1975; 12: 193; Jpn J Appl Phys, 2013; 52: 09KF03). [ABSTRACT FROM AUTHOR]

We report a strong thickness dependence of the complex frequency-dependent optical dielectric function ̃∈(ω) over a spectral range from 1.24 to 5 eV in epitaxial CaMnO3(001) thin films on SrTiO3(001), LaAlO3(001), and SrLaAlO4(001). A doubling of the peak value of the imaginary part of ̃∈(ω) and spectral shifts of 0.5 eV for a given magnitude of absorption are observed. On the basis of experimental analyses and first-principles density functional theory calculations, contributions from both surface states and epitaxial strain to the optical dielectric function of CaMnO3 are seen. Its evolution with thickness from 4 to 63 nm has several regimes. In the thinnest, strain-coherent films, the response is characterized by a significant contribution from the free surface that dominates strain effects. However, at intermediate and larger thicknesses approaching the bulk-like film, strain coherence and partial strain relaxation coexist and influence ̃∈(ω). [ABSTRACT FROM AUTHOR]

Wereport how a low vacuum pressure process followed by a few-minuteannealing enables epitaxial stabilization, producing high-quality,phase-pure, single-crystalline epitaxial, and misfit dislocation-freeBiFeO3(001) thin films on SrTiO3(001) at ∼450°C less than current routes. These results unambiguously challengethe widely held notion that atomic layer deposition (ALD) is not appropriatefor attaining high-quality chemically complex oxide films on perovskitesubstrates in single-crystalline epitaxial form, demonstrating applicabilityas an inexpensive, facile, and highly scalable route. [ABSTRACT FROM AUTHOR]

Structural degradation of oxide electrodes during the electrocatalytic oxygen evolution reaction (OER) is a major challenge in water electrolysis. Although the OER is known to induce changes in the surface layer, little is known about its effect on the bulk of the electrocatalyst and its overall phase stability. Here, we show that under OER conditions, a highly active SrCoO 3- x electrocatalyst develops bulk lattice instability, which results in the formation of molecular O 2 dimers inside the bulk and nanoscale amorphization induced via chemo-mechanical coupling. Using high-resolution resonant inelastic X-ray scattering and first-principles calculations, we unveil the potential-dependent evolution of lattice oxygen inside the perovskite and demonstrate that O 2 dimers are stable in a densely packed crystal lattice, thus challenging the assumption that O 2 dimers require sufficient interatomic spacing. We also show that the energy cost of local atomic rearrangements in SrCoO 3- x becomes very low under the OER conditions, leading to an unusual amorphization under intercalation-induced stress. As a result, we propose that the amorphization energy can be calculated from the first principles and can be used to assess the stability of electrocatalysts. Our study demonstrates that extreme oxidation of electrocatalysts under OER can intrinsically destabilize the lattice and result in bulk anion redox and disorder, suggesting why some oxide materials are unstable and develop a thick amorphous layer under water electrolysis conditions.

We report on the growth of polycrystalline BiFeO 3 thin films on SiO 2 /Si(001) and Pt(111) substrates by atomic layer deposition using the precursors ferrocene, triphenyl-bismuth, and ozone. By growing alternating layers of Fe 2 O 3 and Bi 2 O 3 , we employ a superlattice approach and demonstrate an efficient control of the cation stoichiometry. The superlattice decay and the resulting formation of polycrystalline BiFeO 3 films are studied by in situ X-ray diffraction, in situ X-ray photoelectron spectroscopy, and transmission electron microscopy. No intermediate ternary phases are formed and BiFeO 3 crystallization is initiated in the Bi 2 O 3 layers at 450 °C following the diffusion-driven intermixing of the cations. Our study of the BiFeO 3 formation provides an insight into the complex interplay between microstructural evolution, grain growth, and bismuth oxide evaporation, with implications for optimization of ferroelectric properties., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)