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Hybrid organic-inorganic perovskite (HOIP) ferroelectrics are attracting considerable interest because of their high performance, ease of synthesis, and lightweight. However, the intrinsic thermodynamic origins of their ferroelectric transitions remain insufficiently understood. Here, we identify the nature of the ferroelectric phase transitions in displacive [(CH

We present a theoretical model of methylhydrazinium CH3NH2NH2+ molecular cation ordering in perovskite [CH3NH2NH2][M(HCOO)3] (M = Mn, Mg, Fe, and Zn) formate frameworks, which exhibit two structural phase transitions. The proposed model is constructed by analyzing the available structural information and mapping the molecular cation states on a three-dimensional simple cubic lattice. The model includes the short-range Ising and Potts interactions between the dipolar CH3NH2NH2+ cations. We study the model using the Monte Carlo simulations as well as by the density functional theory calculations. The simulations indicate that our model accurately describes the methylhydrazinium cation arrangement in all three structural phases of the compounds. The calculated temperature dependences of the heat capacity and electric polarization are in good agreement with the experimental data. The simulations also allow us to obtain the characteristic energies of the molecular cation interactions for all members of the [CH...

It is shown that there are deposition conditions in reactive sputtering of TiO2 in which the anatase and rutile crystals can coherently arrange in a convenient way that predominantly exposes the anatase facets, a configuration that is expected to favor the photocatalytic activity of the films. The latter process plays at present an important role in environmental conservation related to energy generation and water cleaning. Therefore, our results present an experimental path for obtaining highly photoreactive TiO2 films.

Biological ferroelectric materials have great potential in biosensing and disease diagnosis and treatment. Glycine crystals form the simplest bioferroelectric materials, and here we investigate the polarizations of its β- and γ-phases. Using density functional theory, we predict that glycine crystals can develop polarizations even larger than those of conventional inorganic ferroelectrics. Further, using systematic molecular dynamics simulations utilizing polarized crystal charges, we predict the Curie temperature of γ-glycine to be 630 K, with a required coercive field to switch its polarization states of 1 V·nm–1, consistent with experimental evidence. This work sheds light on the microscopic mechanism of electric dipole ordering in biomaterials, helping in the material design of novel bioferroelectrics.

In this work, we focus on [(CH3)2NH2]PbI3, a member of the [AmineH]PbI3 series of hybrid organic–inorganic compounds, reporting a very easy mechanosynthesis route for its preparation at room temperature. We report that this [(CH3)2NH2]PbI3 compound with 2H-perovskite structure experiences a first-order transition at ≈250 K from hexagonal symmetry P63/mmc (HT phase) to monoclinic symmetry P21/c (LT phase), which involves two cooperative processes: an off-center shift of the Pb2+ cations and an order–disorder process of the N atoms of the DMA cations. Very interestingly, this compound shows a dielectric anomaly associated with the structural phase transition. Additionally, this compound displays very large values of the dielectric constant at room temperature because of the appearance of a certain conductivity and the activation of extrinsic contributions, as demonstrated by impedance spectroscopy. The large optical band gap displayed by this material (Eg = 2.59 eV) rules out the possibility that the observ...

A displacive-type mechanism, which accounts for the occurrence of ferroelectricity in most inorganic ferroelectrics, is rarely found in molecule-based ferroelectrics. Its role is often covered by the predominant order–disorder one. Herein, we report a lone-pair-electron-driven displacive-type ferroelectric organic–inorganic hybrid compound, [H2dmdap][SbCl5] (1; dmdap = N,N-dimethyl-1,3-diaminopropane). The structure of 1 features a typical zigzag chain of [SbCl5]∞ containing cis-connected anionic octahedra. The compound undergoes a second-order paraelectric–ferroelectric phase transition at 143 K (P21/c ↔ Pc) with a saturation polarization of 1.36 μC·cm–2 and a coercive field of 3.5 kV·cm–1 at 119 K. Theoretical study discloses the ferroelectricity mainly originating from the relative displacements of the Sb and Cl ions in the crystal lattice, which are driven by the 5s2 lone-pair electrons of the SbIII center. Furthermore, on the basis of analysis, possible routes are suggested to enhance ferroelectric p...

There is increasing interest in the development of new magnetic polymeric carriers for biomedical applications such as trigger-controlled drug release, magnetic hyperthermia (MH) for the treatment of cancer, and as contrast agents in magnetic resonance imaging (MRI). This work describes the synthesis of sub-micrometer and magnetic polymer nanocomposite capsules (MPNCs) by combining in one single platform the biodegradable polymer poly-e-caprolactone (PCL) and different concentrations of ∼8 nm oleic acid (OA)-functionalized magnetite nanoparticles (Fe3O4@OA), employing the oil-in-water emulsion/solvent evaporation method. The MPNCs showed a significant increase in particle size from ∼400 to ∼800 nm as the magnetic loading in the organic–inorganic hybrids increases from 1.0% to 10%. The MPNCs presented high incorporation efficiency of Fe3O4@OA nanoparticles, good colloidal stability, and super-paramagnetic properties. Interestingly, electron microscopy results showed that the Fe3O4@OA nanoparticles were pre...

The report describes the Ostwald ripening process in a nanocomposite comprising gold nanoparticles and TiO2 semiconductor under electrochemical conditions. The phenomenon is considered in relation to previous observations on the Ostwald ripening process in metallic nanostructures. Possible processes involved are discussed, and a mechanism is proposed based on the size dependence of the electrochemical parameters of gold nanostructures.

We report on the hybrid inorganic-organic ammonium compound [NH4][Cd(HCOO)3], which displays a most unusual framework structure: instead of the expected 4(9)·6(6) topology, it shows an ABX3 perovskite architecture with the peculiarity and uniqueness (among all the up-to-date reported hybrid metal formates) that the Cd ions are connected only by syn-anti formate bridges, instead of anti-anti ones. This change of the coordination mode of the formate ligand is thus another variable that can provide new possibilities for tuning the properties of these versatile functional metal-organic framework materials. The room-temperature crystal structure of [NH4][Cd(HCOO)3] is noncentrosymmetric (S.G.: Pna21) and displays a polar axis. DFT calculations and symmetry mode analysis show that the rather large polarization arising from the off-center shift of the ammonium cations in the cavities (4.33 μC/cm(2)) is partially canceled by the antiparallel polarization coming from the [Cd(HCOO)3](-) framework, thus resulting in a net polarization of 1.35 μC/cm(2). As shown by second harmonic generation studies, this net polarization can be greatly increased by applying pressure (Pmax = 14 GPa), an external stimulus that, in turn, induces the appearance of new structural phases, as confirmed by Raman spectroscopy.

We perform density functional theory calculations on a recently synthesized metal-organic framework (MOF) with a perovskite-like topology ABX3, i.e., [CH3CH2NH3]Mn(HCOO)3, and predict a multiferroic behavior, i.e., a coexistence of ferroelectricity and ferromagnetism. A peculiar canted ordering of the organic A-cation dipole moments gives rise to a ferroelectric polarization of ~2 μC/cm(2). Starting from these findings, we show that by choosing different organic A cations, it is possible to tune the ferroelectric polarization and increase it up to 6 μC/cm(2). The possibility of changing the magnitude and/or the canting of the organic molecular dipole opens new routes toward engineering ferroelectric polarization in the new class of multiferroic metal-organic frameworks.

A new orthorhombic phase, MnCrO4, isostructural with MCrO 4 (M = Mg, Co, Ni, Cu, Cd) was prepared by evaporation of an aqueous solution, (NH4)2Cr2O7 + 2 Mn(NO 3)2, followed by calcination at 400 C. It is characterized by redox titration, Rietveld analysis of the X-ray diffraction pattern, Cr K edge and Mn K edge XANES, ESR, magnetic susceptibility, specific heat and resistivity measurements. In contrast to the high-pressure MnCrO4 phase where both cations are octahedral, the new phase contains Cr in a tetrahedral environment suggesting the charge balance Mn2+Cr 6+O4. However, the positions of both X-ray absorption K edges, the bond lengths and the ESR data suggest the occurrence of some mixed-valence character in which the mean oxidation state of Mn is higher than 2 and that of Cr is lower than 6. Both the magnetic susceptibility and the specific heat data indicate an onset of a three-dimensional antiferromagnetic order at TN ≈ 42 K, which was confirmed also by calculating the spin exchange interactions on the basis of first principles density functional calculations. Dynamic magnetic studies (ESR) corroborate this scenario and indicate appreciable short-range correlations at temperatures far above T N. MnCrO4 is a semiconductor with activation energy of 0.27 eV; it loses oxygen on heating above 400 C to form first Cr 2O3 plus Mn3O4 and then Mn 1.5Cr1.5O4 spinel. © 2013 American Chemical Society.

In this research, high resolution transmission electron microscopy (HRTEM) and high angle annular dark field–scanning transmission electron microscopy (HAADF-STEM) studies of ceria(IV) oxide CeO2 nanocrystals (NCs) synthesized by a hydrothermal/two phase process were conducted. The synthesis route affords the possibility of controlling the shape of the CeO2 NCs by changing the oleic acid/cerium ([OLA]/[Ce3+]) ratio. At a relatively low [OLA]/[Ce3+] ratio of 4, a polyhedral NC morphology was obtained with {111} and {200} termination facets. Increasing the [OLA]/[Ce3+] ratio to 8, while maintaining a constant reaction time and temperature during the synthesis, truncated cube-like CeO2 NCs with {200}, {220}, and {111} termination facets was generated. These morphologies were identified by HRTEM and HAADF-STEM characterization. Fourier transform infrared (FT-IR) analysis and thermogravimetric analysis (TGA) confirm the presence of chemically bonded oleic acid (OLA) on the CeO2 NC surface. It indicates that th...

MoS2 and WS2 emerge as important materials due to their layered structure, either as molecular sheets or as onion-like inorganic fullerenes. The prominent anisotropy within the materials requires understanding of the layer stacking sequence in order to understand the difference of physical properties of these structures from the bulk materials. Traditional Rietveld refinement in this regime proves to be inadequate to describe the crystallographic structure of these fullerene particles and nanotubes since the results contradict the direct evidence by high-resolution transmission electron microscopy. Another direct method of structure analysis, known as Debye Function Analysis, is used to determine these special features in the X-ray powder diffraction. It is demonstrated that the experimental diffraction intensities are substantially distorted compared to bulk material and that line shapes and intensities are consistent with a random stacking of trigonal prismatic MS2 layers rather than a mixture of hexago...

We explored the possibility of engineering the reactivity of a stepped rhodium surface for dissociation of carbon monoxide with the help of density functional theory calculations. Our results indicate that decorating the Rh step edges with late transition metals allows one to tune the local reaction barrier for nearly 2.5 eV. In the case of Ir@Rh(533), we predict at the same time a weaker adsorption in the final state and a lower dissociation barrier, which might help to overcome the poisoning problem in a realistic application.

Modeling of nanocrystals supported by advanced morphological and chemical characterization is a unique tool for the development of reliable nanostructured devices, which depends on the ability to synthesize and characterize materials on the atomic scale. Among the most significant challenges in nanostructural characterization is the evaluation of crystal growth mechanisms and their dependence on the shape of nanoparticles and the distribution of doping elements. This paper presents a new strategy to characterize nanocrystals, applied here to antimony-doped tin oxide (Sb-SnO(2)) (ATO) by the combined use of experimental and simulated high-resolution transmission electron microscopy (HRTEM) images and surface energy ab initio calculations. The results show that the Wulff construction can not only describe the shape of nanocrystals as a function of surface energy distribution but also retrieve quantitative information on dopant distribution by the dimensional analysis of nanoparticle shapes. In addition, a novel three-dimensional evaluation of an oriented attachment growth mechanism is provided in the proposed methodology. This procedure is a useful approach for faceted nanocrystal shape modeling and indirect quantitative evaluation of dopant spatial distribution, which are difficult to evaluate by other techniques.

The adsorption and dissociation of carbon monoxide were studied with plane-wave density functional theory on flat Rh(111), stepped and kinked Rh(553), and Ni-decorated Rh(553) surfaces. The theoretical results were compared to high-resolution X-ray photoelectron spectroscopy (HR-XPS) experiments. The most favorable CO adsorption sites for low coverages were identified by a systematic calculation of the adsorption energies, and their sequence of occupation as a function of CO exposure was determined experimentally in C 1s HR-XPS spectra via their characteristic surface core-level shifts. On the clean, stepped (553) surface, molecular CO is adsorbed more strongly on low-coordinated top sites at the step edge, but on the Ni-decorated surface, the binding is stronger at the terrace sites. The barrier for dissociation with respect to the gas phase is about 1 eV lower on the stepped Rh(553) surface than on the flat Rh(111) surface, implying a substantially higher reaction rate. The presence of kinks at the clean Rh(553) surface does not lead to a significant additional decrease of the dissociation barriers, resulting in dissociation energies just above the desorption threshold for both stepped and kinked surfaces, whereas the barrier can be additionally lowered by about 0.1 eV by decorating the step edges with Ni stripes. Whereas no dissociation of CO was observed by HR-XPS on the clean Rh(553) surface, a minor amount of CO dissociation was found on the Ni-decorated Rh surface, in agreement with the theoretical predictions. (Less)

The adsorption and desorption of carbon monoxide on Rh(553) was studied using temperature-programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and density functional theory (DFT) calculations. The results are discussed with respect to the previously reported results on the Rh(111) surface. TPD of carbon monoxide from Rh(553) is very similar to the TPD from Rh(111). This observation is confirmed by DFT calculations, which show that the adsorption geometries and adsorption energies are only slightly different on the two surfaces. Infrared spectroscopy shows that, at low coverages, on-top adsorption sites on the steps and on the terraces are occupied first. With increasing CO coverage also bridge sites on the steps are occupied, while bridge and hollow sites on the terraces are only occupied at the highest CO coverages. DFT calculations support the assignments of the IR peaks to the different adsorption sites.

In this work, we report the synthesis of titanium oxide nanocrystals, especially the rutile TiO 2 phase with nanorod morphology, by a method based on peroxotitanium complex decomposition. The results indicate that the anisotropic morphology reported for rutile TiO 2 nanocrystals is related to the oriented attachment process. Despite the predominance of rutile nanocrystals at longer treatment times, the nanocrystals were obtained also in the anatase type, according to the degradation time adopted. XANES results evidenced the absence of structural correlation between the peroxytitanium complex and phase evolution, and the coexistence of the two phases strongly suggests a correlation of the oriented attachment mechanism and the rutile phase stabilization.

The formation of base oil components by the hydroconversion of long-chain n-paraffins [i.e., n-octacosane (n-C28), n-hexatriacontane (n-C36), and n-tetratetracontane (n-C44)] on a 0.3% Pt/amorphous silica−alumina catalyst in a stirred microautoclave was investigated. The reaction pathway of hydroconversion, formation of base oil components, and kinetics were determined. The reaction can be described by a reaction network in which the conversion of n-paraffins occurs by three competitive reactions that directly lead to the formation of cracking products and two pseudocomponents: iso-Cn lube (i.e., lump of isoparaffins with sufficiently low pour points to make them suitable for a base oil) and iso-Cn nolube (i.e., lump of isoparaffins with pour points that are too high, making them unsuitable for a base oil). During reaction, the iso-Cn nolube fraction is converted into iso-Cn lube through subsequent hydroisomerization reactions. Both iso-Cn lube and iso-Cn nolube pseudocomponents further react to give cra...