We use a layered solution crystal growth method to synthesize high-quality single crystals of two different benzylammonium lead iodide perovskite-like organic/inorganic hybrids. The well-known (C6H5CH2NH3)2PbI4 phase is obtained in the form of bright orange platelets, with a structure comprised of single ⟨100⟩-terminated sheets of corner-sharing PbI6 octahedra separated by bilayers of the organic cations. The presence of water during synthesis leads to formation of a novel minority phase that crystallizes in the form of nearly transparent, light yellow bar-shaped crystals. This phase adopts the monoclinic space group P21/n and incorporates water molecules, with structural formula (C6H5CH2NH3)4Pb5I14·2H2O. The crystal structure consists of ribbons of edge-sharing PbI6 octahedra separated by the organic cations. Density functional theory calculations including spin–orbit coupling show that these edge-sharing PbI6 octahedra cause the band gap to increase with respect to corner-sharing PbI6 octahedra in (C6H5CH2NH3)2PbI4. To gain systematic insight, we model the effect of the connectivity of PbI6 octahedra on the band gap in idealized lead iodide perovskite-derived compounds. We find that increasing the connectivity from corner-, via edge-, to face-sharing causes a significant increase in the band gap. This provides a new mechanism to tailor the optical properties in organic/inorganic hybrid compounds.

High-quality single crystals of perovskite-like (CH3NH3)3Bi2I9 hybrids have been synthesized, using a layered-solution crystal-growth technique. The large dielectric constant is strongly affected by the polar ordering of its constituents. Progressive dipolar ordering of the methylammonium cation upon cooling below 300 K gradually converts the hexagonal structure (space group P63/mmc) into a monoclinic phase (C2/c) at 160 K. A well-pronounced, ferrielectric phase transition at 143 K is governed by in-plane ordering of the bismuth lone pair that breaks inversion symmetry and results in a polar phase (space group P21). The dielectric constant is markedly higher in the C2/c phase above this transition. Here, the bismuth lone pair is disordered in-plane, allowing the polarizability to be substantially enhanced. Density functional theory calculations estimate a large ferroelectric polarization of 7.94 μC/cm2 along the polar axis in the P21 phase. The calculated polarization has almost equal contributions of the methylammonium and Bi3+ lone pair, which are fairly decoupled.

Vacancies exist throughout nature and determine the physical properties of materials. By manipulating the density and distribution of vacancies, it is possible to influence their physical properties such as band-gap, conductivity, magnetism, etc. This can generate exciting applications in the fields of water treatment, energy storage, and physical devices such as resistance-change memories. In this review, we focus on recent progress in vacancy engineering for the design of materials for energy harvesting applications. A brief discription of the concept of vacancies, the way to create and control them, as well as their fundamental properties, is first provided. Then, emphasis is placed on the strategies used to tailor vacancies for metal–insulator transitions, electronic structures, and introducing magnetism in non-magnetic materials. Finally, we present representative applications of different structures with vacancies as active electrode materials of lithium or sodium ion batteries, catalysts for water splitting, and hydrogen evolution.

We use a layered solution crystal growth technique to synthesize high-quality single crystals of phenylalkylammonium lead iodide organic/inorganic hybrid compounds. Single-crystal X-ray diffraction reveals low-dimensional structures consisting of inorganic sheets separated by bilayers of the organic cations. The shortest alkyls yield two-dimensional structures consisting of inorganic sheets of corner-sharing PbI6-octahedra. However, the longer alkyls induce both corner- and face-sharing of the PbI6-octahedra, and form new compounds. Density functional theory calculations including spin–orbit coupling show quantum confinement in two dimensions for the shorter alkyls, and in one dimension for the longer alkyls, respectively. The face-sharing PbI6-octahedra create a confinement leading to effectively one-dimensional behavior. These confinement effects are responsible for the observed peak shifts in photoluminescence for the different phenylalkylammonium lead iodide hybrids. Our results show how the connectivity of the octahedra leads to confinement effects that directly tune the optical band gap.

Transition metal sulfides exhibit chemical and physical properties that are of much scientific and technological interest and can largely be attributed to their covalent bonding of 3d electrons. Hierarchical structures of these materials are suited for a broad range of applications in energy storage, as biological scaffold, and as sensors. In this work, hierarchical SnS2 structures have been synthesized and show excellent photocatalytic performance for the production of H2 under blue light (450 nm) irradiation. A combination of high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy indicates the formation of layered SnS2/SnS superstructures with a lattice mismatch between the two alternating layers. This indicates the presence of S vacancies and results in a drastic decrease of the band gap by 0.3 eV compared to bulk SnS2. This strategy of self-narrowing of the band-gap demonstrates its great potential for the design of new materials with visible light reactivity. Finally, we have extended this strategy to the synthesis of other transition metal sulfides (Ni3S4, CuS, CuS@C, and FeS2) with similar hierarchical structures, which have potential applications such as supercapacitors and electrode materials for sodium/lithium ion batteries.

Fe7Se8 exists as a hexagonal NiAs-like crystal structure with a large number of ordered intrinsic vacancies. It is an ideal candidate for studying the effect of defects on properties such as magnetism and electrical transport. In this work, highly crystalline Fe7Se8 with the 3c crystal structure was synthesized by a solid-state reaction. Sharp changes in the magnetization at 100 K confirm a rotation of the spins from the ab plane to the c axis with decreasing temperature. We observe an interesting metal–insulator transition at the same temperature as the spin-direction changes. We propose that locked spins in the grain boundaries induce electron localization and result in the metal–insulator transition. Electron localization is confirmed by X-ray photoelectron spectroscopy of the Fe 2p peaks, which exhibit two characteristic satellite peaks. This mechanism is also verified by comparing it with the properties of the 4c-Fe7Se8 crystal structure.

The marcasite structure FeSe2−δ was synthesized using a simple solvothermal method. Systematic study of the electrical transport properties shows that the transport is dominated by variable-range hopping (VRH), with a changeover from Mott VRH at higher temperature to Efros-Shklovskii VRH for temperatures lower than the width of the Coulomb gap. This also confirms the presence of a Coulomb gap in the density of states at the Fermi energy. We observe that Yttrium doping increases the electrical conductivity dramatically without significantly reducing the Seebeck coefficient. This results in remarkably high power factors for thermoelectric performance in the regime where the mean hopping energy shifts from defect dominated to Coulomb repulsion dominated. High resolution transmission electron microscopy, in combination with theoretical calculations, proves the narrowing of the band gap by introducing Se vacancies. This leads to a good conductivity and is responsible for the excellent thermoelectric performance. The formation of nanoclusters, resulting from Se vacancies, is responsible for a dense system of stacking faults and the generally reported weak ferrimagnetism. This also determines the transition between the different electrical transport mechanisms and contributes to the improved thermoelectric performance.

•Single crystalline YMnO3 shows two different relaxation features between 200 and 450 K.•Frequency, thickness and electrode dependence were used for characterization.•Low temperature relaxation has Maxwell–Wagner type relaxation characteristics.•Higher temperature relaxation effects are ascribed to hopping of charge carriers.We have investigated the origin of the dielectric relaxation in YMnO3 single crystals. Two distinct dielectric relaxation features were observed at low (200⩽T⩽373K) and high (300⩽T⩽450K) temperatures. Analysis of our detailed frequency, electrode and thickness dependent dielectric measurements and ac conductivity data as well as the use of single crystals allow us to get a comprehensive picture of these relaxations. The low temperature relaxation is attributed to the Maxwell–Wagner type effects originating from the dipoles at the surface while the high temperature one is suggested to originate from hopping of charge carriers resulting from the second ionization of oxygen vacancies.

High-purity Fe3S4 (greigite) microcrystals with octahedral shape were synthesized via a simple hydrothermal method using a surfactant. The as-prepared samples have the inverse spinel structure with high crystallinity. The saturation magnetization (Ms) reaches 3.74 μB at 5 K and 3.51 μB at room temperature, which is larger than all reported values thus far. Electrical transport measurements show metallic behavior with a resistivity 40 times lower than in any previous report. The potential use of greigite as an anode in lithium-ion batteries was investigated by cyclic voltammery and galvanostatic discharge–charge cycling on as-prepared samples. The discharge capacity was 1161 mAh/g in the first cycle and 563 mAh/g in the 100th cycle. This excellent electrochemical performance can be attributed to the high purity, crystallinity, and favorable morphology of the products.

We investigate the structural, magnetic, and dielectric properties of the organic–inorganic hybrid material CuCl4(C6H5CH2CH2NH3)2 and demonstrate that spontaneous ferroelectric order sets in below 340 K, which coexists with ferromagnetic ordering below 13 K. We use X-ray diffraction to show that the electric polarization results from the spatial ordering of hydrogen bonds that link the organic block comprised of phenylethylammonium cations to the inorganic copper chloride block. The hydrogen bond ordering is driven by buckling of the corner-linked copper chloride octahedra. Because the magnetic exchange pathways are also determined by this octahedral buckling, a potentially large magnetoelectric coupling is induced. Our results imply that such hybrids form a new family of multiferroic materials.Keywords: ferroelectric; ferromagnetic; metal-oxide framework; multiferroic; organic−inorganic hybrid;

We have studied the magnetoelectric coupling of the frustrated triangular antiferromagnet iron jarosite using Raman spectroscopy, dielectric measurements and specific heat. Temperature dependent capacitance measurements show an anomaly in the dielectric constant at TN. Specific heat data indicate the presence of a low frequency Einstein mode at low temperature. Raman spectroscopy confirms the presence of a new mode below TN that can be attributed to folding of the Brillouin zone. This mode shifts and sharpens below TN. We evaluate the strength of the magnetoelectric coupling using the symmetry unrestricted biquadratic magnetoelectric terms in the free energy.Graphical abstractSketch of two connected triangles formed by Fe3+ spins (red arrows) in the hexagonal basal plane of potassium iron jarosite. An applied magnetic field (H) below the antiferromagnetic ordering temperature induces shifts of the hydroxy ligands, giving rise to local electrical dipole moments (blue arrows). These electric displacements cancel out in pairwise fashion by symmetry. Ligand shifts are confined to the plane and shown by shadowing.Highlights► Evidence has been found for spin–lattice coupling in iron jarosite. ► A new optical Raman mode appears below TN and shifts with temperature. ► The magnetodielectric coupling is mediated by superexchange. ► Symmetry of Kagome magnetic lattice causes local electrical dipole moments to cancel.

Molecular recognition between two species of Au55 clusters bearing complementary hydrogen-bonding groups provides a facile route to accomplish solution-based assembly of two-component nanoparticle aggregates.

Vacancies exist throughout nature and determine the physical properties of materials. By manipulating the density and distribution of vacancies, it is possible to influence their physical properties such as band-gap, conductivity, magnetism, etc. This can generate exciting applications in the fields of water treatment, energy storage, and physical devices such as resistance-change memories. In this review, we focus on recent progress in vacancy engineering for the design of materials for energy harvesting applications. A brief discription of the concept of vacancies, the way to create and control them, as well as their fundamental properties, is first provided. Then, emphasis is placed on the strategies used to tailor vacancies for metal–insulator transitions, electronic structures, and introducing magnetism in non-magnetic materials. Finally, we present representative applications of different structures with vacancies as active electrode materials of lithium or sodium ion batteries, catalysts for water splitting, and hydrogen evolution.

Transition metal sulfides exhibit chemical and physical properties that are of much scientific and technological interest and can largely be attributed to their covalent bonding of 3d electrons. Hierarchical structures of these materials are suited for a broad range of applications in energy storage, as biological scaffold, and as sensors. In this work, hierarchical SnS2 structures have been synthesized and show excellent photocatalytic performance for the production of H2 under blue light (450 nm) irradiation. A combination of high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy indicates the formation of layered SnS2/SnS superstructures with a lattice mismatch between the two alternating layers. This indicates the presence of S vacancies and results in a drastic decrease of the band gap by 0.3 eV compared to bulk SnS2. This strategy of self-narrowing of the band-gap demonstrates its great potential for the design of new materials with visible light reactivity. Finally, we have extended this strategy to the synthesis of other transition metal sulfides (Ni3S4, CuS, CuS@C, and FeS2) with similar hierarchical structures, which have potential applications such as supercapacitors and electrode materials for sodium/lithium ion batteries.

Molecular recognition between two species of Au55 clusters bearing complementary hydrogen-bonding groups provides a facile route to accomplish solution-based assembly of two-component nanoparticle aggregates.