Twisting two-dimensional iron sulfide layers into coincident site superlattices via intercalation chemistry.

Layered van der Waals (vdW) materials are susceptible not only to various stacking polymorphs through translations but also twisted structures due to rotations between layers. Here, we study the influence of such layer-to-layer twisting through the intercalation of ethylenediamine (EDA) molecules into tetragonal iron sulfide (Mackinawite FeS). Selected area electron diffraction patterns of intercalated FeS display reflections corresponding to multiple square lattices with a fixed angle between them, contrary to a single square lattice seen in the unintercalated phase. The observed twist angles of 49.13° and 22.98° result from a superstructure formation well described by the coincident site lattice (CSL) theory. According to the CSL theory, these measured twist angles lead to the formation of larger coincident site supercells. We build these CSL models for FeS using crystallographic group-subgroup transformations and find simulated electron diffraction patterns from the model to agree with the experimentally measured data.

Experimental Study of the Adsorption and Decomposition of Sarin on Dry Copper(II) Oxide.

Organophosphonates were originally developed as insecticides but were quickly identified as highly toxic acetylcholinesterase inhibitors, leading to their exploitation as chemical warfare agents (CWA). To develop next generation filtration technologies, there must be a fundamental understanding of the molecular interactions occurring with toxic chemicals, such as CWAs. In this paper, we investigate the interaction between dry CuO nanoparticles and sarin (GB), using infrared (IR) spectroscopy in an effort to build an atomic understanding. We show sarin strongly interacts with CuO and then quickly degrades, primarily through the cleavage of the P-F bond, creating a bridging species on the CuO surface with the assistance of lattice oxygen. Upon heating, the decomposition product isopropyl methyl phosphonic acid (IMPA) does not continue to decompose but desorbs from the surface. These observations are further elaborated through theoretical models of sarin on dry CuO (111).

Spectroscopic studies of methyl paraoxon decomposition over mesoporous Ce-doped titanias for toxic chemical filtration.

The ever-constant threat of chemical warfare agents (CWA) motivates the design of materials to provide better protection to warfighters and civilians. Cerium and titanium oxide are known to react with organophosphorus compounds such Sarin and Soman. To study the decomposition of methyl paraoxon (CWA simulant) on such materials, we synthesized ordered mesoporous metal oxides (MMO) TiO2, CexTi1-xO2 (x = 0.005, 0.5, 0.10, 0.15) and CeO2. We fully characterized TiO2 and Ce-doped TiO2 and found phase-pure oxides with cylindrical hexagonally packed pores and high surface areas (176-252 m2/g). Methyl paraoxon decomposition was tracked through UV/Vis and found Ce0.15Ti0.85O2 to decompose the most methyl paraoxon, but CeO2 to be the most reactive when normalized to surface area. The surface area normalized rate constant (kSA) for CeO2 was 3-4.6 times larger than that of TiO2 and the CexTi1-xO2 series. While TiO2 and CexTi1-xO2 for 0.05 ≤ x ≤ 0.10 displayed no significant differences in the kinetics, the mostly amorphous Ce0.15Ti0.85O2 displayed a slight increase in reactivity. Our findings indicate that the nature of the cation, Ce4+ vs Ti4+, is less important to methyl paraoxon reactivity on these MMOs compared to other factors such as crystal structure type.

Mesoporous perovskite titanates via hydrothermal conversion.

We demonstrate the successful hydrothermal conversion of mesoporous TiO2 to mesoporous perovskite SrTiO3. This method allows for control of pore size distribution and can be readily applied for the preparation of other mesoporous titanates such as BaTiO3 and Li2TiO3. Such high-surface perovskites have potential in high-temperature applications due to their thermal stability.

Understanding Dimethyl Methylphosphonate Adsorption and Decomposition on Mesoporous CeO2.

The increased risk of chemical warfare agent usage around the world has intensified the search for high-surface-area materials that can strongly adsorb and actively decompose chemical warfare agents. Dimethyl methylphosphonate (DMMP) is a widely used simulant molecule in laboratory studies for the investigation of the adsorption and decomposition behavior of sarin (GB) gas. In this paper, we explore how DMMP interacts with the as-synthesized mesoporous CeO2. Our mass spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy measurements indicate that DMMP can dissociate on mesoporous CeO2 at room temperature. Two DMMP dissociation pathways are observed. Based on our characterization of the as-synthesized material, we built the pristine and hydroxylated (110) and (111) CeO2 surfaces and simulated the DMMP interaction on these surfaces with density functional theory modeling. Our calculations reveal an extremely low activation energy barrier for DMMP dissociation on the (111) pristine CeO2 surface, which very likely leads to the high activity of mesoporous CeO2 for DMMP decomposition at room temperature. The two reaction pathways are possibly due to the DMMP dissociation on the pristine and hydroxylated CeO2 surfaces. The significantly higher activation energy barrier for DMMP to decompose on the hydroxylated CeO2 surface implies that such a reaction on the hydroxylated CeO2 surface may occur at higher temperatures or proceed after the pristine CeO2 surfaces are saturated.

Orbital Contribution to Paramagnetism and Noninnocent Thiophosphate Anions in Layered Li2MP2S6 Where M = Fe and Co.

Transition-metal thiophosphates and selenophosphates are layered systems with the potential for displaying two-dimensional (2D) magnetic phenomena. We present the crystal structures and magnetic properties of two lithium transition-metal thiophosphates, Li1.56Co0.71P2S6 and Li2.26Fe0.94P2S6. The previously unreported Li1.56Co0.71P2S6 crystallizes in the trigonal space group P31m with lattice parameters a = 6.0193(6) Å and c = 6.5675(9) Å. The CoS6 octahedra are arranged in a honeycomb lattice and form 2D layers separated by lithium cations. The previously solved Li2.26Fe0.94P2S6 is isostructural to Li1.56Co0.71P2S6 but displays site mixing between the Li+ and Fe2+ cations within the thiophosphate layer. Unusually, Li1.56Co0.71P2S6 appears to have P2S63- and not P2S64- anions. We therefore term it a "noninnocent" anion because of the ambiguous nature of its oxidation state. Combined neutron diffraction and magnetization measurements reveal that both Li1.56Co0.71P2S6 and Li2.26Fe0.94P2S6 display magnetic anisotropy as well as no long-range magnetic order down to 5 K. In the iron thiophosphate, susceptibility indicates an effective moment of 5.44(3) µB, which may be best described by an S + L model, where S = 2 and L = 2, or close to the free ion limit. In the cobalt thiophosphate, we found the effective moment to be 4.35(2) µB, which would point to an S = 3/2 and L = 1 model due to octahedral crystal-field splitting.

On the Electrochemical Phase Evolution of Anti-PbO-Type CoSe in Alkali Ion Batteries.

The reaction mechanism of anti-PbO type CoSe in Li, Na, and K ion half cells is studied. Ex situ X-ray diffraction data is analyzed with the Rietveld method, in conjunction with discharge profiles and extended cycling data. These indicate that intercalation followed by a conversion reaction occur in all systems. For the case of Na, the intercalation reaction was associated with a contraction in the stacking axis lattice parameter, whereas Li and K exhibited expansion. Magnetic susceptibility versus temperature measurements of Li- and Na-intercalated CoSe samples produce unusual results, and several explanations are proposed, including the formation of a superconductive phase. Extended cycling experiments are also performed, and high initial capacities of 937, 657, and 972 mAh/g are observed for Li, Na, and K, respectively. However, all systems exhibit significantly lower second discharge capacities of 796, 530, and 515 mAh/g. The capacities continue to decline during extended cycling, with the systems exhibiting tenth cycle capacity fades of 52, 85, and 95% and Li half cells exhibit capacities over 150 mAh/g at 15 mA/g after 50 cycles. The capacity fade is likely attributable to volume changes and irreversibility associated with conversion and intercalation reactions. This work correlates electrochemical features to the structural evolution, magnetic properties, and reaction mechanisms.

Distinguishing the Intrinsic Antiferromagnetism in Polycrystalline LiCoPO4 and LiMnPO4 Olivines.

We report a detailed investigation of the long-range magnetic ordering in polycrystalline samples of LiCoPO4 and LiMnPO4, which belong to a series of well-known olivine cathode materials LiMPO4 (M = Mn, Fe, Co, Ni). Samples were prepared by hydrothermal and solid state methods. The magnetic susceptibility is found to be strongly field-dependent, impacting the antiferromagnetic transition temperature and the bifurcation of the FC and ZFC curves. We discuss the role synthesis conditions have on impurity formation and particle size. We report neutron powder diffraction data for the samples prepared by solid state methods. Based upon representational analysis of the observed reflections, we affirm the magnetic structure Pnma' for LiCoPO4 and Pn'm'a' for LiMnPO4. The refined magnetic moments from these models are 3.28(4) µB for LiCoPO4 and 4.28(3) µB for LiMnPO4. We also study the onset of magnetic ordering in each sample and affirm that the ordering temperature is 22 K for LiCoPO4 and 34 K for LiMnPO4. The critical parameters describing those transitions are ßc = 0.21(4) (LiCoPO4) and ßc = 0.31(3) (LiMnPO4). These values are characteristic of a 3D Ising system for LiMnPO4 and intermediate behavior between a 2D and 3D Ising system for LiCoPO4. We compare these observations with other reports proposing lower magnetic symmetry.

Long-range magnetic order in hydroxide-layer-doped (Li1-x-y Fe x Mn y OD)FeSe.

The (Li1-x Fe x OH)FeSe superconductor has been suspected of exhibiting long-range magnetic ordering due to Fe substitution in the LiOH layer. However, no direct observation such as magnetic reflection from neutron diffraction has been reported. Here, we use a chemical design strategy to manipulate the doping level of transition metals in the LiOH layer to tune the magnetic properties of the (Li1-x-y Fe x Mn y OD)FeSe system. We find Mn doping exclusively replaces Li in the hydroxide layer resulting in enhanced magnetization in the (Li0.876Fe0.062Mn0.062OD)FeSe superconductor without significantly altering the superconducting behavior as resolved by magnetic susceptibility and electrical/thermal transport measurements. As a result, long-range magnetic ordering was observed below 12 K with neutron diffraction measurements. This work has implications for the design of magnetic superconductors for the fundamental understanding of superconductivity and magnetism in the iron chalcogenide system as well as exploitation as functional materials for next-generation devices.

Structural studies of the perovskite series La1-xSrxCoO3-δ during chemical looping with methane.

Perovskite oxides are promising materials as oxygen carriers in chemical looping applications. We analyze in situ X-ray diffraction data on the perovskite phases La1-xSrxCoO3-δ for x = 0, 0.25, 0.5, and 0.75 under chemical looping conditions. We report and discuss their structural evolution, cycling stability, and suitability as oxygen storage materials.

Manipulating surface magnetic order in iron telluride.

Control of emergent magnetic orders in correlated electron materials promises new opportunities for applications in spintronics. For their technological exploitation, it is important to understand the role of surfaces and interfaces to other materials and their impact on the emergent magnetic orders. Here, we demonstrate for iron telluride, the nonsuperconducting parent compound of the iron chalcogenide superconductors, determination and manipulation of the surface magnetic structure by low-temperature spin-polarized scanning tunneling microscopy. Iron telluride exhibits a complex structural and magnetic phase diagram as a function of interstitial iron concentration. Several theories have been put forward to explain the different magnetic orders observed in the phase diagram, which ascribe a dominant role either to interactions mediated by itinerant electrons or to local moment interactions. Through the controlled removal of surface excess iron, we can separate the influence of the excess iron from that of the change in the lattice structure.

Proton and ammonia intercalation into layered iron chalcogenides.

Structurally related to the iron-based superconductors, two new intercalated iron chalcogenides (H0.5NH3)Fe2Ch2 where Ch = S, Se have been prepared. By topochemical conversion, the protons were exchanged by lithium to form (Li0.5NH3)Fe2Ch2. Hydrogen bonding plays a significant role in the guest-host interactions of these intercalated phases.

Superconductivity and magnetism in iron sulfides intercalated by metal hydroxides.

Inspired by naturally occurring sulfide minerals, we present a new family of iron-based superconductors. A metastable form of FeS known as the mineral mackinawite forms two-dimensional sheets that can be readily intercalated by various cationic guest species. Under hydrothermal conditions using alkali metal hydroxides, we prepare three different cation and metal hydroxide-intercalated FeS phases including (Li1-x Fe x OH)FeS, [(Na1-x Fe x )(OH)2]FeS, and K x Fe2-y S2. Upon successful intercalation of the FeS layer, the superconducting critical temperature Tc of mackinawite is enhanced from 5 K to 8 K for the (Li1-x Fe x OH) δ+ intercalate. Layered heterostructures of [(Na1-x Fe x )(OH)2]FeS resemble the natural mineral tochilinite, which contains an iron square lattice interleaved with a hexagonal hydroxide lattice. Whilst heterostructured [(Na1-x Fe x )(OH)2]FeS displays long-range magnetic ordering near 15 K, K x Fe2-y S2 displays short range antiferromagnetism.

Metastable Layered Cobalt Chalcogenides from Topochemical Deintercalation.

We present a general strategy to synthesize metastable layered materials via topochemical deintercalation of thermodynamically stable phases. Through kinetic control of the deintercalation reaction, we have prepared two hypothesized metastable compounds, CoSe and CoS, with the anti-PbO type structure from the starting compounds KCo2Se2 and KCo2S2, respectively. Thermal stability, crystal structure from X-ray and neutron diffraction, magnetic susceptibility, magnetization, and electrical resistivity are studied for these new layered chalcogenides; both CoSe and CoS are found to be weak itinerant ferromagnets with Curie temperatures close to 10 K. Due to the weak van der Waals forces between the layers, CoSe is found to be a suitable host for further intercalation of guest species such as Li-ethylenediamine. From first-principles calculations, we explain why the Co chalcogenides are ferromagnets instead of superconductors as in their iron analogues. Bonding analysis of the calculated electronic density of states both explains their phase stability and predicts the limits of our deintercalation technique. Our results have broad implications for the rational design of new two-dimensional building blocks for functional materials.

Influence of transition metal electronegativity on the oxygen storage capacity of perovskite oxides.

The selection of highly efficient oxygen carriers (OCs) is a key step necessary for the practical development of chemical looping combustion (CLC). In this study, a series of ABO3 perovskites, where A = La, Ba, Sr, Ca and B = Cr, Mn, Fe, Co, Ni, Cu, are synthesized and tested in a fixed bed reactor for reactivity and stability as OCs with CH4 as the fuel. We find that the electronegativity of the transition metal on the B-site (λB), is a convenient descriptor for oxygen storage capacity (OSC) of our perovskite samples. By plotting OSC for total methane oxidation against λB, we observe an inverted volcano plot relationship. These results could provide useful guidelines for perovskite OC design and their other energy related applications.

Stabilization of cubic Sr2FeMoO6 through topochemical reduction.

Sr2FeMoO6 has been extensively studied for application in spintronic devices. Through the topochemical de-intercalation of oxygen anions with metal hydride reduction, we demonstrate that the high temperature cubic phase is stabilized, at room temperature, whilst leaving the magnetic ordering intact. Synchrotron X-ray and neutron powder diffraction were used to characterize the structure and stoichiometry of the reduced oxide.

PZT-like structural phase transitions in the BiFeO3-KNbO3 solid solution.

Despite the high prominence of the perovskites BiFeO(3) and KNbO(3) the solid solution between the two has received little attention. We report a detailed neutron and synchrotron X-ray powder diffraction, and Raman spectroscopy study which demonstrates an R3c→P4mm→Amm2 series of structural phase transitions similar to that exhibited by the PbZrO(3)-PbTiO(3) solid solution.

Magnetically stabilized Fe8(µ4-S)6S8 clusters in Ba6Fe25S27.

We have prepared Ba6Fe25S27, and studied its magnetic properties and electronic structure. Single crystal diffraction revealed a cubic phase (Pm3[combining macron]m) with a = 10.2057(9) Å and Z = 1. Within the large cubic cell, tetrahedrally coordinated Fe atoms arrange into octonuclear Fe8(µ4-S)6(S8) clusters, which can be described as a cube of Fe atoms with six face-capping and eight terminal S atoms. SQUID magnetometry measurements reveal an antiferromagnetic transition at 25 K and anomalous high-temperature dependence of magnetic susceptibility that is non-Curie like-two magnetic signatures which mimic behavior seen in the parent phases of Fe-based superconductors. Using a combined DFT and molecular orbital based approach, we provide an interpretation of the bonding and stability within Ba6M25S27 (M = Fe, Co, Ni) and related M9S8 phases. Through a σ-bonding molecular orbital model of the transition metal coordination environments, we illustrate how the local stability can be enhanced through addition of Ba. In addition, we perform spin-polarized DFT calculations on Ba6Fe25S27 to determine the effect of adopting an antiferromagnetic spin state on its electronic structure. By studying the magnetic properties from an empirical and computational perspective, we hope to elucidate what aspects of the magnetic structure are significant to bonding.

Synthetic and coordination chemistry of the heavier trivalent technetium binary halides: uncovering technetium triiodide.

Technetium tribromide and triiodide were obtained from the reaction of the quadruply Tc-Tc-bonded dimer Tc2(O2CCH3)4Cl2 with flowing HX(g) (X = Br, I) at elevated temperatures. At 150 and 300 °C, the reaction with HBr(g) yields TcBr3 crystallizing with the TiI3 structure type. The analogous reactions with flowing HI(g) yield TcI3, the first technetium binary iodide to be reported. Powder X-ray diffraction (PXRD) measurements show the compound to be amorphous at 150 °C and semicrystalline at 300 °C. X-ray absorption fine structure spectroscopy indicates TcI3 to consist of face-sharing TcI6 octahedra. Reactions of technetium metal with elemental iodine in a sealed Pyrex ampules in the temperature range 250-400 °C were performed. At 250 °C, no reaction occurred, while the reaction at 400 °C yielded a product whose PXRD pattern matches the one of TcI3 obtained from the reaction of Tc2(O2CCH3)4Cl2 and flowing HI(g). The thermal stability of TcBr3 and TcI3 was investigated in Pyrex and/or quartz ampules at 450 °C under vacuum. Technetium tribromide decomposes to Na{[Tc6Br12]2Br} in a Pyrex ampule and to technetium metal in a quartz ampule; technetium triiodide decomposes to technetium metal in a Pyrex ampule.

Structural disorder, magnetism, and electrical and thermoelectric properties of pyrochlore Nd2Ru2O7.

Polycrystalline Nd2Ru2O7 samples have been prepared and examined using a combination of structural, magnetic, and electrical and thermal transport studies. Analysis of synchrotron x-ray and neutron diffraction patterns suggests some site disorder on the A-site in the pyrochlore sublattice: Ru substitutes on the Nd-site up to 7.0(3)%, regardless of the different preparative conditions explored. Intrinsic magnetic and electrical transport properties have been measured. Ru 4d spins order antiferromagnetically at 143 K, as seen both in the susceptibility and in the specific heat, and there is a corresponding change in the electrical resistivity. The onset of a second antiferromagnetic ordering transition seen below 5 K is attributed to ordering of Nd 4f spins. Nd2Ru2O7 is an electrical insulator, and this behaviour is believed to be independent of the Ru-antisite disorder on the Nd-site. The electrical properties of Nd2Ru2O7 are presented in the light of data published on all A2Ru2O7 pyrochlores, and we emphasize the special structural role that Bi(3+) ions on the A-site play in driving metallic behaviour. High-temperature thermoelectric properties have also been measured. When considered in the context of known thermoelectric materials with useful figures-of-merit, it is clear that Nd2Ru2O7 has excessively high electrical resistivity which prevents it from being an effective thermoelectric. A method for screening candidate thermoelectrics is suggested.