Ionisation of atoms determined by kappa refinement against 3D electron diffraction data

Conventional refinement strategies used for three-dimensional electron diffraction (3D ED) data disregard the bonding effects between the atoms in a molecule by assuming a pure spherical model called the Independent Atom model (IAM) and may lead to an inaccurate or biased structure. Here we show that it is possible to perform a refinement going beyond the IAM with electron diffraction data. We perform kappa refinement which models charge transfers between atoms while assuming a spherical model. We demonstrate the procedure by analysing five inorganic samples; quartz, natrolite, borane, lutecium aluminium garnet, and caesium lead bromide. Implementation of kappa refinement improved the structure model obtained over conventional IAM refinements and provided information on the ionisation of atoms. The results were validated against periodic DFT calculations. The work presents an extension of the conventional refinement of 3D ED data for a more accurate structure model which enables charge density information to be extracted.

Marcasite/pyrite nanocomposites confined in N,S-doped carbon nanoboxes for boosted alkali metal ion storage

FeS2 is a promising electrode material for alkali metal ion storage due to its high theoretical capacity. However, it still faces critical issues such as suboptimal rate and cycling performances owing to sluggish charge transport and significant volume variations. Herein, we constructed FeS2 (m-FeS2) and pyrite FeS2 (p-FeS2) nanocomposites embedded in N,S-doped carbon nanoboxes (m/p-FeS2@NSCN) to conquer such challenges. The microstructure design of nanoboxes effectively alleviates the stress caused by the volume expansion of FeS2 during lithiation processes, thereby improving the cycling stability of the FeS2 electrode. The marcasite/pyrite compositing design further increases the electronic conductivity of FeS2 and optimizes ion migration. As expected, the target m/p-FeS2@NSCN exhibits improved rate capability (595.5 mA h g-1 at 5.0 A g-1) and robust cycling stability (500 cycles without significant capacity decay at 0.1 A g-1) in lithium-ion batteries. Furthermore, m/p-FeS2@NSCN also shows excellent battery performances and potential application prospects in the field of sodium-ion batteries. It achieves a capacity of 355 mA h g-1 at 10.0 A g-1 and sustains 800 cycles without noticeable capacity decay at 0.5 A g-1. This work offers valuable guidance for rationally designing high-performance energy storage materials for alkali metal ion storage.

Unraveling the origin of the high photocatalytic properties of earth-abundant TiO2/FeS2 heterojunctions: insights from first-principles density functional theory

Herein, first-principles density functional theory calculations have been employed to unravel the interfacial geometries (composition and stability), electronic properties (density of states and differential charge densities), and charge carrier transfers (work function and energy band alignment) of a TiO2(001)/FeS2(100) heterojunction. Analyses of the structure and electronic properties reveal the formation of strong interfacial Fe-O and Ti-S ionic bonds, which stabilize the interface with an adhesion energy of -0.26 eV Å-2. The work function of the TiO2(001)/FeS2(100) heterojunction is predicted to be much smaller than those of the isolated FeS2(100) and TiO2(001) layers, indicating that less energy will be needed for electrons to transfer from the ground state to the surface to promote photochemical reactions. The difference in the work function between the FeS2(100) and TiO2(001) heterojunction components caused an electron density rearrangement at the heterojunction interface, which induces an electric field that separates the photo-generated electrons and holes. Consistently, a staggered band alignment is predicted at the interface with the conduction band edge and the valence-band edge of FeS2 lying 0.37 and 2.62 eV above those of anatase. These results point to efficient charge carrier separation in the TiO2(001)/FeS2(100) heterojunction, wherein photoinduced electrons would transfer from the FeS2 to the TiO2 layer. The atomistic insights into the mechanism of enhanced charge separation and transfer across the interface rationalize the observed high photocatalytic activity of the mixed TiO2(001)/FeS2(100) heterojunction over the individual components.

Preparation of Iron-Based Sulfides and Their Applications in Biomedical Fields

Recently, iron-based sulfides, including iron sulfide minerals and biological iron sulfide clusters, have attracted widespread interest, owing to their excellent biocompatibility and multi-functionality in biomedical applications. As such, controlled synthesized iron sulfide nanomaterials with elaborate designs, enhanced functionality and unique electronic structures show numerous advantages. Furthermore, iron sulfide clusters produced through biological metabolism are thought to possess magnetic properties and play a crucial role in balancing the concentration of iron in cells, thereby affecting ferroptosis processes. The electrons in the Fenton reaction constantly transfer between Fe²⁺ and Fe³⁺, participating in the production and reaction process of reactive oxygen species (ROS). This mechanism is considered to confer advantages in various biomedical fields such as the antibacterial field, tumor treatment, biosensing and the treatment of neurodegenerative diseases. Thus, we aim to systematically introduce recent advances in common iron-based sulfides.

Large scale dataset of real space electronic charge density of cubic inorganic materials from density functional theory (DFT) calculations

Driven by the big data science, material informatics has attracted enormous research interests recently along with many recognized achievements. To acquire knowledge of materials by previous experience, both feature descriptors and databases are essential for training machine learning (ML) models with high accuracy. In this regard, the electronic charge density ρ(r), which in principle determines the properties of materials at their ground state, can be considered as one of the most appropriate descriptors. However, the systematic electronic charge density ρ(r) database of inorganic materials is still in its infancy due to the difficulties in collecting raw data in experiment and the expensive first-principles based computational cost in theory. Herein, a real space electronic charge density ρ(r) database of 17,418 cubic inorganic materials is constructed by performing high-throughput density functional theory calculations. The displayed ρ(r) patterns show good agreements with those reported in previous studies, which validates our computations. Further statistical analysis reveals that it possesses abundant and diverse data, which could accelerate ρ(r) related machine learning studies. Moreover, the electronic charge density database will also assists chemical bonding identifications and promotes new crystal discovery in experiments.

Synthetic control over polymorph formation in the d-band semiconductor system FeS2

Pyrite, also known as fool's gold is the thermodynamic stable polymorph of FeS2. It is widely considered as a promising d-band semiconductor for various applications due to its intriguing physical properties. Marcasite is the other naturally occurring polymorph of FeS2. Measurements on natural crystals have shown that it has similarly promising electronic, mechanical, and optical properties as pyrite. However, it has been only scarcely investigated so far, because the laboratory-based synthesis of phase-pure samples or high quality marcasite single crystal has been a challenge until now. Here, we report the targeted phase formation via hydrothermal synthesis of marcasite and pyrite. The formation condition and phase purity of the FeS2 polymorphs are systematically studied in the form of a comprehensive synthesis map. We, furthermore, report on a detailed analysis of marcasite single crystal growth by a space-separated hydrothermal synthesis. We observe that single phase product of marcasite forms only on the surface under the involvement of H2S and sulphur vapor. The availability of high-quality crystals of marcasite allows us to measure the fundamental physical properties, including an allowed direct optical bandgap of 0.76 eV, temperature independent diamagnetism, an electronic transport gap of 0.11 eV, and a room-temperature carrier concentration of 4.14 × 1018 cm-3. X-ray absorption/emission spectroscopy are employed to measure the band gap of the two FeS2 phases. We find marcasite has a band gap of 0.73 eV, while pyrite has a band gap of 0.87 eV. Our results indicate that marcasite - that is now synthetically available in a straightforward fashion - is as equally promising as pyrite as candidate for various semiconductor applications based on earth abundant elements.

A Polymorphic FeS2 Cathode Enabled by Copper Current Collector Induced Displacement Redox Mechanism

In this contribution, we fabricated a composite consisting of two polymorphs of FeS₂, pyrite (P-FeS₂) and marcasite (M-FeS₂), for high-performance Li-FeS₂ battery. A series of electrochemical, microscopic, and spectroscopic characterizations indicate that the introduction of metastable M-FeS₂ into P-FeS₂ enables the four-electron reduction between FeS₂ and lithium to generate Fe and Li₂S, providing a high specific capacity of 894 mAh/g with specific energy over 1300 Wh/kg. Moreover, it is verified that the electrochemical irreversibility of this composite toward lithium storage is mainly rooted in the shuttle effect, caused by the elemental sulfur which is inevitably produced during the oxidation process of Li₂S and Fe. To tackle this issue, copper (Cu) current collector is adopted to chemically immobilize the soluble lithium polysulfides and fundamentally alter the reaction pathway. It is shown that compared with Fe, Li₂S prefers to react with Cu current collector to generate Cu₂S through the thermodynamically facile displacement reaction mechanism benefiting from the similar lattice framework between Cu₂S and Li₂S. Such displacement reaction without lattice reconstruction renders the composite superior rate capability (∼730 mAh/g@2 A/g) and long lifespan (89.7% capacity retention after 3200 cycles). Present work allows for the fabrication of high-performance electrodes based on metal chalcogenides.

Charge Density Analysis of Actinide Compounds from the Quantum Theory of Atoms in Molecules and Crystals

The nature of chemical bonding in actinide compounds (molecular complexes and materials) remains elusive in many respects. A thorough analysis of their electron charge distribution can prove decisive in elucidating bonding trends and oxidation states along the series. However, the accurate determination and robust analysis of the charge density of actinide compounds pose several challenges from both experimental and theoretical perspectives. Significant advances have recently been made on the experimental reconstruction and topological analysis of the charge density of actinide materials [Gianopoulos et al. IUCrJ, 2019, 6, 895]. Here, we discuss complementary advances on the theoretical side, which allow for the accurate determination of the charge density of actinide materials from quantum-mechanical simulations in the bulk. In particular, the extension of the Topond software implementing Bader's quantum theory of atoms in molecules and crystals (QTAIMAC) to f- and g-type basis functions is introduced, which allows for an effective study of lanthanides and actinides in the bulk and in vacuo, on the same grounds. Chemical bonding of the tetraphenyl phosphate uranium hexafluoride cocrystal [PPh4+][UF6-] is investigated, whose experimental charge density is available for comparison. Crystal packing effects on the charge density and chemical bonding are quantified and discussed. The methodology presented here allows reproducing all subtle features of the topology of the Laplacian of the experimental charge density. Such a remarkable qualitative and quantitative agreement represents a strong mutual validation of both approaches-experimental and computational-for charge density analysis of actinide compounds.

Production of Quasi-2D Platelets of Nonlayered Iron Pyrite (FeS2) by Liquid-Phase Exfoliation for High Performance Battery Electrodes

Over the past 15 years, two-dimensional (2D) materials have been studied and exploited for many applications. In many cases, 2D materials are formed by the exfoliation of layered crystals such as transition-metal disulfides. However, it has recently become clear that it is possible to exfoliate nonlayered materials so long as they have a nonisotropic bonding arrangement. Here, we report the synthesis of 2D-platelets from the earth-abundant, nonlayered metal sulfide, iron pyrite (FeS₂), using liquid-phase exfoliation. The resultant 2D platelets exhibit the same crystal structure as bulk pyrite but are surface passivated with a density of 14 × 10¹⁸ groups/m². They form stable suspensions in common solvents and can be size-selected and liquid processed. Although the platelets have relatively low aspect ratios (∼5), this is in line with the anisotropic cleavage energy of bulk FeS₂. We observe size-dependent changes to optical properties leading to spectroscopic metrics that can be used to estimate the dimensions of platelets. These platelets can be used to produce lithium ion battery anodes with capacities approaching 1000 mAh/g.

Prediction of a New Layered Polymorph of FeS2 with Fe3+S2-(S22-)1/2 Structure

The never-elucidated crystal structure of metastable iron disulfide FeS₂ resulting from the full deintercalation of Li in Li₂FeS₂ has been cracked thanks to crystal structure prediction searches based on an evolutionary algorithm combined with first-principles calculations accounting for experimental observations. Besides the newly layered C2/m polymorph of iron disulfide, two-dimensional dynamically stable FeS₂ phases are proposed that contain sulfides and/or persulfide S₂ motifs.

Chemical Bonding in Colossal Thermopower FeSb2

FeSb₂ exhibits a colossal Seebeck coefficient ( S ) and a record-breaking high thermoelectric power factor. It also has an atypical shift from diamagnetism to paramagnetism with increasing temperature, and the fine details of its electron correlation effects have been widely discussed. The extraordinary physical properties must be rooted in the nature of the chemical bonding, and indeed, the chemical bonding in this archetypical marcasite structure has been heavily debated on a theoretical basis since the 1960s. The two prevalent models for describing the bonding interactions in FeSb₂ are based on either ligand-field stabilization of Fe or a network structure of Sb hosting Fe ions. However, neither model can account for the observed properties of FeSb₂ . Herein, an experimental electron density study is reported, which is based on analysis of synchrotron X-ray diffraction data measured at 15 K on a minute single crystal to limit systematic errors. The analysis is supplemented with density functional theory calculations in the experimental geometry. The experimental data are at variance with both the additional single-electron Sb-Sb bond implied by the covalent model, and the large formal charge and expected d-orbital splitting advocated by the ionic model. The structure is best described as an extended covalent network in agreement with expectations based on electronegativity differences.

Charge densities in actinide compounds: strategies for data reduction and model building

The data quality requirements for charge density studies on actinide compounds are extreme. Important steps in data collection and reduction required to obtain such data are summarized and evaluated. The steps involved in building an augmented Hansen-Coppens multipole model for an actinide pseudo-atom are provided. The number and choice of radial functions, in particular the definition of the core, valence and pseudo-valence terms are discussed. The conclusions in this paper are based on a re-examination and improvement of a previously reported study on [PPh4][UF6]. Topological analysis of the total electron density shows remarkable agreement between experiment and theory; however, there are significant differences in the Laplacian distribution close to the uranium atoms which may be due to the effective core potential employed for the theoretical calculations.

The basis for reevaluating the reactivity of pyrite surfaces: spin states and crystal field d-orbital splitting energies of bulk, terrace, edge, and corner Fe(ii) ions

Pyrite, one of the most important minerals to catalyze redox reactions in nature and a bulk low-spin Fe mineral, needs to provide high-spin Fe on surfaces to moderate spin-forbidden transitions. Here, the spin state of pyrite is investigated using density functional theory (DFT) calculations on cluster and periodic models. The energies of clusters FexS2x (where x = 4, 8, 16, and 32) were calculated as a function of total spin and different up/down spin configurations. The undercoordinated Fe on surfaces, edges, and corners were found to provide intermediate and high-spin Fe necessary for catalysis. Generally, the lower the crystal field splitting energy (CFSE), Δ, for a particular Fe atom, the higher is the spin density. Pyrite bulk (3D) and surfaces (2D) (+ water to mimic aqueous systems) were examined. The calculated bulk band gap (0.95 eV) is in excellent agreement with previous reports. For the surface, a conducting state is predicted. The calculated CFSE for bulk Fe(ii) in pyrite (∼2.2 eV) agrees with previous CFT results; due to surface states, this CFSE decreases to ∼1 eV on terraces. This study highlights the importance of accurately describing the spin state of pyrite.

Fe0.36(4)Pd0.64(4)Se2: Magnetic Spin-Glass Polymorph of FeSe2 and PdSe2 Stable at Ambient Pressure

We report the synthesis and characterization of Fe_0.36(4)Pd_0.64(4)Se₂ with a pyrite-type structure. Fe_0.36(4)Pd_0.64(4)Se₂ was synthesized using ambient pressure flux crystal growth methods even though the space group Pa3 is high-pressure polymorph for both FeSe₂ and PdSe₂. Combined experimental and theoretical analysis reveal magnetic spin glass state below 23 K in 1000 Oe that stems from random Fe/Pd occupancies on the same atomic site. The frozen-in magnetic randomness contributes significantly to electronic transport. Electronic structure calculations confirm dominant d-electron character of hybridized bands and large density of states near the Fermi level. Flux-grown single crystal alloys in Pd-Fe-Se atomic system therefore open new pathway for exploring different polymorphs in crystal structures and their novel properties.

Structural stability and thermoelectric performance of high quality synthetic and natural pyrites (FeS2)

Single crystalline pyrite of high quality reveals good thermal- and bad electrical conductivities resulting in poor thermoelectric performance.

Giant thermopower in 'p' type OsX2 (X: S, Se, Te) for a wide temperature range: a first principles study

We report the electronic structure and thermoelectric (TE) properties of OsX₂ (X: S, Se, Te), and find a giant value of thermopower of magnitude 600 μV K^-1-800 μV K^-1 for a wide temperature range of 100 K-500 K for hole doping (at 10¹⁸ cm^-3), which is higher than the value found for well established TE materials. The optimized structural parameters are in good agreement with available experimental reports. The mechanical stability of all the compounds are confirmed from the computed elastic constants. The band gap of the investigated compounds is examined by several exchange correlation functionals, and TB-mBJ with modified parameters is found to be the best. The heavy valence bands stimulate the thermopower value for hole doping and light conduction bands intensifies the electrical conductivity values for electron doping, enabling both 'n' and 'p' type doping favourable for TE applications at higher concentrations (10²⁰ cm^-3), which brings out the device application. Our results unveil the possibility of TE applications for all the examined compounds for a wide temperature range (100 K-500 K), and OsS₂ specifically is quite alternative with the performing temperature ranging from 100 K-900 K.

Rubidium chalcogenido diferrates(III) containing dimers [Fe2 Q 6]6− of edge-sharing tetrahedra (Q=O, S, Se)

The two isotypic rubidium chalcogenido diferrates Rb12[Fe2 Q 6](Q 2)3 (Q=S/Se), which both form needles with green-metallic lustre, were synthesized from Rb2S, elemental iron, rubidium and sulfur (Q=S) or from the pure elements (Q=Se) at maximum temperatures of 500–800°C. Their triclinic crystal structures were determined by means of X-ray single crystal data (space group P1̅, a=863.960(10)/903.2(3), b=942.790(10)/982.1(3), c=1182.70(2)/1227.4(4) pm, α=77.4740(10)/77.262(6), β=71.5250(10)/71.462(6), γ=63.7560(10)/63.462(5)°, Z=1, R1=0.0308/0.0658 for Q=S/Se). The structures contain isolated dinuclear anions [FeIII 2 Q 6]6− composed of two edge-sharing [FeQ 4] tetrahedra (dFe −Q =223.4–232.3/236.2–244.8 pm), which are also found in the two polymorphs of the pure alkali diferrates Rb6[Fe2 Q 6]. The diferrate ions are arranged in layers running in the a/b plane around z=0. Inbetween (around z ≈ 1 2 $z \approx {1 \over 2}$ ), two crystallographically different disulfide/diselenide ions Q 2 2 − $Q_2^{2 - }$ (dQ −Q =211.1–213.4/237.9–241.1 pm), which are arranged in slightly puckered 36 nets, are intercalated. The intra-anionic distances and angles, the Rb coordination numbers and the molar volumes of these two ‘double-salts’ are in accordance with their corresponding reference compounds, Rb6[Fe2 Q 6] and Rb2 Q 2. In addition, the two polymorphs of Rb6[Fe2Se6], which are both isotypic with the sulfido analogous (Cs6[Ga2Se6]-type, monoclinic, space group P21 /c, a=827.84(5), b=1329.51(7), c=1074.10(6) pm, β=127.130(5)°, R1=0.0443 and Ba6[Al2Sb6]-type, orthorhombic, space group Cmce, a=1963.70(3), b=718.98(3), c=1348.40(7) pm, R1=0.0264) were prepared and characterized to complete the series of alkali diferrates(III) with oxido, sulfido and selenido ligands. The electronic band structures of the three Rb salts Rb6[Fe2 Q 6], which have been calculated within the GGA+U approach applying an AFM spin ordering in the dimers and appropriate Hubbard parameters, allow a comparison of the chemical bonding characteristics (e.g. covalency) and the magnetic properties (magnetic moments) within the series of chalcogenido ligands. An analysis of the spin densities enables a comparative consideration of the mechanisms crucial for the magnetic ordering in chalcogenido ferrates. Ultimately, the electronic structure of the new compound Rb12[Fe2S6](S2)3 nicely compares with those of the S2-free reference compound Rb6[Fe2S6].

Polymorphism in a high-entropy alloy

Polymorphism, which describes the occurrence of different lattice structures in a crystalline material, is a critical phenomenon in materials science and condensed matter physics. Recently, configuration disorder was compositionally engineered into single lattices, leading to the discovery of high-entropy alloys and high-entropy oxides. For these novel entropy-stabilized forms of crystalline matter with extremely high structural stability, is polymorphism still possible? Here by employing in situ high-pressure synchrotron radiation X-ray diffraction, we reveal a polymorphic transition from face-centred-cubic (fcc) structure to hexagonal-close-packing (hcp) structure in the prototype CoCrFeMnNi high-entropy alloy. The transition is irreversible, and our in situ high-temperature synchrotron radiation X-ray diffraction experiments at different pressures of the retained hcp high-entropy alloy reveal that the fcc phase is a stable polymorph at high temperatures, while the hcp structure is more thermodynamically favourable at lower temperatures. As pressure is increased, the critical temperature for the hcp-to-fcc transformation also rises.

Metastable Marcasite-FeS2 as a New Anode Material for Lithium Ion Batteries: CNFs-Improved Lithiation/Delithiation Reversibility and Li-Storage Properties

Marcasite (m-FeS₂) exhibits higher electronic conductivity than that of pyrite (p-FeS₂) because of its lower semiconducting gap (0.4 vs 0.7 eV). Meanwhile, as demonstrates stronger Fe-S bonds and less S-S interactions, the m-FeS₂ seems to be a better choice for electrode materials compared to p-FeS₂. However, the m-FeS₂ has been seldom studied due to its sophisticated synthetic methods until now. Herein, a hierarchical m-FeS₂ and carbon nanofibers composite (m-FeS₂/CNFs) with grape-cluster structure was designed and successfully prepared by a straightforward hydrothermal method. When evaluated as an electrode material for lithium ion batteries, the m-FeS₂/CNFs exhibited superior lithium storage properties with a high reversible capacity of 1399.5 mAh g^-1 after 100 cycles at 100 mA g^-1 and good rate capability of 782.2 mAh g^-1 up to 10 A g^-1. The Li-storage mechanism for the lithiation/delithiation processes of m-FeS₂/CNFs was systematically investigated by ex situ powder X-ray diffraction patterns and scanning electron microscopy. Interestingly, the hierarchical m-FeS₂ microspheres assembled by small FeS₂ nanoparticles in the m-FeS₂/CNFs composite converted into a mimosa with leaves open shape during Li⁺ insertion process and vice versa. Accordingly, a "CNFs accelerated decrystallization-recrystallization" mechanism was proposed to explain such morphology variations and the decent electrochemical performance of m-FeS₂/CNFs.

Periodic DFT+U investigation of the bulk and surface properties of marcasite (FeS2)

Marcasite FeS₂ and its surfaces properties have been investigated by Hubbard-corrected density functional theory (DFT+U) calculations.

Evaluating structure selection in the hydrothermal growth of FeS2 pyrite and marcasite

While the ab initio prediction of the properties of solids and their optimization towards new proposed materials is becoming established, little predictive theory exists as to which metastable materials can be made and how, impeding their experimental realization. Here we propose a quasi-thermodynamic framework for predicting the hydrothermal synthetic accessibility of metastable materials and apply this model to understanding the phase selection between the pyrite and marcasite polymorphs of FeS2. We demonstrate that phase selection in this system can be explained by the surface stability of the two phases as a function of ambient pH within nano-size regimes relevant to nucleation. This result suggests that a first-principles understanding of nano-size phase stability in realistic synthesis environments can serve to explain or predict the synthetic accessibility of structural polymorphs, providing a guideline to experimental synthesis via efficient computational materials design.

Synthesis, crystal and electronic structure of the new sodium chain sulfido cobaltates(II), Na3CoS3 and Na5[CoS2]2(Br)

The sulfido cobaltate(II) Na3CoS3 was synthesized from stoichiometric quantities of Na2S, elemental cobalt and sulfur at a maximum temperature of 1100°C. According to Na3CoS3=Na12[Co2S5(S2)][Co2S3(S2)] the orthorhombic structure of a new type (space group Cmc21, a=884.24(2), b=2177.38(5), c=1193.20(3) pm, Z=12, R1=0.0205) contains two different anions: i. dimers [Co2S5(S2)]8− of two edge-sharing [CoS4] tetrahedra with five sulfido and one η 1-disulfido ligand; ii. chains ∞ 1 [ Co 2 S 3 ( S 2 ) ] 4 − $_\infty ^1{[{\rm{C}}{{\rm{o}}_2}{{\rm{S}}_3}({{\rm{S}}_2})]^{4 - }}$ of [CoS4] tetrahedra connected via μ-sulfido (3×) besides μ-1,2-disulfido (1×) ligands. The second title compound, Na5[CoS2]2(Br), which has been likewise synthesized as a pure phase from stoichiometric quantities of Na2S, Co, S and NaBr, is isotypic to Na5[CoS2]2(S) (Na6PbO5 type; tetragonal, space group I4mm, a=914.58(7), c=625.59(5) pm; R1=0.0412). The structure contains linear chains ∞ 1 [ CoS 4 / 2 ] 2 − $_\infty ^1{[{\rm{Co}}{{\rm{S}}_{4/2}}]^{2 - }}$ running along the tetragonal c axis. In between, Br− ions are interspersed, which are coordinated by square pyramids of Na+ ions. The isotypic hydrogensulfide Na5[CoS2]2(SH) was obtained via the addition of NaSH. Several synthetic and structural arguments suggest that Na5[CoS2]2(SH) and the previously described pure sulfide are the same compound. The results of DFT band structure calculations (GGA+U, AFM spin ordering) are used to discuss and compare the chemical bonding in the new sulfido cobaltates(II) with that of the reference compound Na2[CoS2]. They also allow for obtaining insight into the superexchange path, which is responsible for the strong antiferromagnetic Co spin ordering along the chains.

Enhanced Photoresponse of FeS2 Films: The Role of Marcasite-Pyrite Phase Junctions

The beneficial role of marcasite in iron-sulfide-based photo-electrochemical applications is reported for the first time. A spectacular improvement of the photoresponse observed experimentally for mixed pyrite/marcasite-FeS₂ films can be ascribed to the presence of p/m phase junctions at the interface. Density functional theory calculations show that the band alignment at the phase boundary contributes to enhanced charge separation and transfer across the interface.

Structural Evolution of Iron Antimonides from Amorphous Precursors to Crystalline Products Studied by Total Scattering Techniques

Homogeneous reaction precursors may be used to form several solid-state compounds inaccessible by traditional synthetic routes, but there has been little development of techniques that allow for a priori prediction of what may crystallize in a given material system. Here, the local structures of FeSbx designed precursors are determined and compared with the structural motifs of their crystalline products. X-ray total scattering and atomic pair distribution function (PDF) analysis are used to show that precursors that first nucleate a metastable FeSb3 compound share similar local structure to the product. Interestingly, precursors that directly crystallize to thermodynamically stable FeSb2 products also contain local structural motifs of the metastable phase, despite their compositional disagreement. While both crystalline phases consist of distorted FeSb6 octahedra with Sb shared between either two or three octahedra as required for stoichiometry, a corner-sharing arrangement indicative of AX3-type structures is the only motif apparent in the PDF of either precursor. Prior speculation was that local composition controlled which compounds nucleate from amorphous intermediates, with different compositions favoring different local arrangements and hence different products. This data suggests that local environments in these amorphous intermediates may not be very sensitive to overall composition. This can provide insight into potential metastable phases which may form in a material system, even with a precursor that does not crystallize to the kinetically stabilized product. Determination of local structure in homogeneous amorphous reaction intermediates from techniques such as PDF can be a valuable asset in the development of systematic methods to prepare targeted solid-state compounds from designed precursors.

Charge density investigations on [2,2]-paracyclophane – in data we trust

Four datasets on [2,2]-paracyclophane were collected in-house and at the Advanced Photon Source at two different temperatures for charge density investigation. Global data quality indicators such as high resolution, high I/σ(I) values, low merging R values and high multiplicity were matched for all four datasets. The structural parameters did not show significant differences, but the synchrotron data depicted deficiencies in the topological analysis. In retrospect these deficiencies could be assigned to the low quality of the innermost data, which could have been identified by e.g. merging R values for only these reflections. In the multipole refinement these deficiencies could be monitored using DRK-plot and residual density analysis. In this particular example the differences in the topological parameters were relatively small but significant.

Electronic and structural properties of bulk arsenopyrite and its cleavage surfaces – a DFT study

We have investigated the structural and electronic properties of arsenopyrite and its cleavage surface formation using a density functional/plane waves method. QTAIM and ELF were applied for investigating the nature of the bonding in arsenopyrite.

Contemporary X-ray electron-density studies using synchrotron radiation

Synchrotron radiation has many compelling advantages over conventional radiation sources in the measurement of accurate Bragg diffraction data. The variable photon energy and much higher flux may help to minimize critical systematic effects such as absorption, extinction and anomalous scattering. Based on a survey of selected published results from the last decade, the benefits of using synchrotron radiation in the determination of X-ray electron densities are discussed, and possible future directions of this field are examined.