Ruddlesden–Popper perovskite sulfides A3B2S7: A new family of ferroelectric photovoltaic materials for the visible spectrum

Perovskite ferroelectric materials exhibit the novel ferroelectric photovoltaic effect, where photon-excited electron–hole pairs can be separated by ferroelectric polarization. Especially, semiconducting ferroelectric materials with small band gaps (E[subscript g]) have been extensively studied for applications in solar energy conversion. Traditional route for creating semiconducting ferroelectrics requires cation doping, where E[subscript g] of the insulating perovskite ferroelectric oxides are reduced via substitution of certain cations. But cation doping tends to reduce the carrier mobility due to the scattering, and usually lead to poor photovoltaic efficiency. In the present work, based on first-principles calculations, we propose and demonstrate a new strategy for designing stoichiometric semiconducting perovskite ferroelectric materials. Specifically, we choose the parent non-polar semiconducting perovskite sulfides AB S[subscript 3] with Pnma symmetry, and turn them into ferroelectric Ruddlesden–Popper A[subscript 3]B[subscript 2]S[subscript 7] perovskites with spontaneous polarizations. Our predicted Ruddlesden–Popper Ca[subscript 3]Zr[subscript 2]S[subscript 7] and other derived compounds exhibit the room-temperature stable ferroelectricity, small band gaps (E[subscript g] < 2.2 eV) suitable for the absorption of visible light, and large visible-light absorption exceeding that of Si.National Basic Research Program of China (973 Program) (Contract 2012CB619402)National Natural Science Foundation (China) (Contract 11574244)China. Ministry of Education (Program for Innovative Research Team in University. Contract IRT13034)National Science Foundation (U.S.) (Grant DMR-1410636

A Nanoscale Shape Memory Oxide

Stimulus-responsive shape memory materials have attracted tremendous research interests recently, with much effort focused on improving their mechanical actuation. Driven by the needs of nanoelectromechnical devices, materials with large mechanical strain particularly at nanoscale are therefore desired. Here we report on the discovery of a large shape memory effect in BiFeO3 at the nanoscale. A maximum strain of up to ~14% and a large volumetric work density can be achieved in association with a martensitic-like phase transformation. With a single step, control of the phase transformation by thermal activation or electric field has been reversibly achieved without the assistance of external recovery stress. Although aspects such as hysteresis, micro-cracking etc. have to be taken into consideration for real devices, the large shape memory effect in this oxide surpasses most alloys and therefore demonstrates itself as an extraordinary material for potential use in state-of-art nano-systems.Comment: Accepted by Nature Communication

Piezoelectricity enhancement in Dion-Jacobson RbBiNb

We use first-principles calculations to study the structural, ferroelectric and piezoelectric properties of the recently synthesized Dion-Jacobson RbBiNb2O7, a novel layered-perovskite piezoelectrics with extremely high Curie temperature TC. We show that ferroelectric RbBiNb2O7 crystalizes in orthorhombic Pmc21 phase, exhibiting in-plane spontaneous polarization. We well reproduce the major experimental results for RbBiNb2O7. We further propose that under 5% volume-expansion–induced negative pressure, ${\sim}3\times$ increase of dielectric permittivity and ${\sim}2\times$ increase of piezoelectricity can be achieved in RbBiNb2O7. The decomposed piezoelectricity analysis reveals that the activation of piezoelectric response of cation Rb by negative pressure can lead to large piezoelectricity enhancement. Based on our calculations, we demonstrate that negative pressure is a promising way to optimize the performance of RbBiNb2O7 as high-TC piezoelectrics

Ruddlesden-Popper perovskite sulfides A 3 B 2 S 7 : A new family of ferroelectric photovoltaic materials for the visible spectrum-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

a b s t r a c t Perovskite ferroelectric materials exhibit the novel ferroelectric photovoltaic effect, where photon-excited electron-hole pairs can be separated by ferroelectric polarization. Especially, semiconducting ferroelectric materials with small band gaps ( ) E g have been extensively studied for applications in solar energy conversion. Traditional route for creating semiconducting ferroelectrics requires cation doping, where E g of the insulating perovskite ferroelectric oxides are reduced via substitution of certain cations. But cation doping tends to reduce the carrier mobility due to the scattering, and usually lead to poor photovoltaic efficiency. In the present work, based on first-principles calculations, we propose and demonstrate a new strategy for designing stoichiometric semiconducting perovskite ferroelectric materials. Specifically, we choose the parent non-polar semiconducting perovskite sulfides ABS 3 with Pnma symmetry, and turn them into ferroelectric Ruddlesden-Popper A B 3 2 S 7 perovskites with spontaneous polarizations. Our predicted Ruddlesden-Popper Ca 3 Zr 2 S 7 and other derived compounds exhibit the roomtemperature stable ferroelectricity, small band gaps ( &lt; ) E 2.2 eV g suitable for the absorption of visible light, and large visible-light absorption exceeding that of Si

Study of Phase Stability and Hydride Diffusion Mechanism of BaTiO₃ Oxyhydride from First-Principles

First-principles calculations were performed to study structural, electronic and hydride diffusion properties of BaTiO₃ oxyhydride. In agreement with experiment (<i>Nat. Mater.</i> <b>2012</b>, 11, 507 and <i>J. Am. Chem. Soc.</i> <b>2012</b>, 134, 8782), we find that the incoming H species occupy the anion vacancy sites left by oxygen, forming the stable hydride anions H^–1. As a result of the electron doping introduced by H species, both interstitial H and hydride anion H^–1 can induce metallicity and eliminate ferroelectricity in BaTiO₃. We further clarify the role of the migration of the interstitial H in determining the hydrogen diffusion properties of the oxyhydrides. A low diffusion barrier was predicted, responsible for high hydrogen diffusion mobility observed in experiment. Based on our results, we demonstrate that BaTiO₃ oxyhydride can be used as a mixed electron/hydride conductor, displaying the promising applications as the electrolytes for solid-oxide fuel cells

Strong Sliding Ferroelectricity and Interlayer Sliding Controllable Spintronic Effect in Two-Dimensional HgI₂ Layers

Exploration of two-dimensional (2D) sliding ferroelectric (FE) materials with experimentally detectable ferroelectricity and value-added novel functionalities is highly sought for the development of 2D “slidetronics”. Herein, based on first-principles calculations, we identify the synthesizable van der Waals (vdW) layered crystals HgX2 (X = Br and I) as a new class of 2D sliding ferroelectrics. Both HgBr2 and HgI2 in 2D multilayered forms adopt the preferential stacking sequence, leading to room temperature stable out-of-plane (vertical) ferroelectricity that can be reversed via the sliding of adjacent monolayers. Owing to strong interlayer coupling and interfacial charge rearrangement, 2D HgI2 layers possess strong sliding ferroelectricity up to 0.16 μC/cm2, readily detectable in experiment. Moreover, robust sliding ferroelectricity and interlayer sliding controllable Rashba spin texture of FE-HgI2 layers enable potential applications as 2D spintronic devices such that the electric control of electron spin detection can be realized at the 2D regime

Interplay of Cation Ordering and Ferroelectricity in Perovskite Tin Iodides: Designing a Polar Halide Perovskite for Photovoltaic Applications

Owing to its ideal semiconducting band gap and good carrier-transport properties, the fully inorganic perovskite CsSnI₃ has been proposed as a visible-light absorber for photovoltaic (PV) applications. However, compared to the organic–inorganic lead halide perovskite CH₃NH₃PbI₃, CsSnI₃ solar cells display very low energy conversion efficiency. In this work, we propose a potential route to improve the PV properties of CsSnI₃. Using first-principles calculations, we examine the crystal structures and electronic properties of CsSnI₃, including its structural polymorphs. Next, we purposefully order Cs and Rb cations on the A site to create the double perovskite (CsRb)­Sn₂I₆. We find that a stable ferroelectric polarization arises from the nontrivial coupling between polar displacements and octahedral rotations of the SnI₆ network. These ferroelectric double perovskites are predicted to have energy band gaps and carrier effective masses similar to those of CsSnI₃. More importantly, unlike nonpolar CsSnI₃, the electric polarization present in ferroelectric (CsRb)­Sn₂I₆ can effectively separate the photoexcited carriers, leading to novel ferroelectric PV materials with potentially enhanced energy conversion efficiency