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.

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.

Multiscale characterizations of structural evolution in mesoporous CeO2.

In situ ultra-small-angle and wide-angle X-ray scattering enables simultaneous tracking of the structural parameters of mesoporous CeO2 from the atomic scale to the micron-size scale. This multiscale approach provides a path to better understand structure-property relationships in mesoporous polycrystalline materials under dynamic conditions such as high temperature cycling.

Low-Temperature Decomposition and Oxidation of the Nerve Agent Simulant on Mesoporous Nickel Oxide and Cu-Doped Nickel Oxide.

In an effort to develop the next frontier filtration material for chemical warfare agent (CWA) decomposition, we synthesized mesoporous NiO and CuxNi1-xO (x = 0.10 and 0.20) and studied the decomposition of CWA simulant diisopropyl fluorophosphate (DIFP) on their surfaces. Mesoporous NiO and CuxNi1-xO were fully characterized and found to be a solid solution with no phase separation up to 20% copper dopant. The synthesized materials were successfully templated producing ordered mesoporous metal oxides with high surface areas (67.89- 94.38 m2/g). Through Raman spectroscopy, we showed that pure NiO contained a high concentration of Ni2+ vacancies, while Cu2+ reduced these defects. Through in situ infrared spectroscopy, we determined the surface species formed, potential pathways, and driving factors for decomposition. Upon exposure of DIFP, all materials produced similar decomposition products CO, CO2, carbonyls, and carbonates. However, decomposition reactions were sustained longer on mesoporous NiO, facilitated by the higher Ni2+ vacancy concentration. NiO was further studied with DIFP, first at low dosing temperatures (-50 °C), which still resulted in the production of CO and carbonates, and then, second, with a higher pretreatment temperature, which showed the importance of terminal hydroxyls/water to fully oxidize decomposition products to CO2. Mesoporous NiO demonstrated high decomposition and oxidation capabilities at temperatures below room temperature, all without any external excitation or noble metals, making it a promising frontier filtration material for CWA decomposition.

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.

Room-Temperature CrI3 Magnets through Lithiation.

The pursuit of two-dimensional (2D) magnetism is promising for energy-efficient electronic devices, including magnetoelectric random access memory and radio frequency/microwave magnonics, and it is gaining fundamental insights into quantum sensing technology. The key challenge resides in overseeing magnetic exchange interactions through a precise chemical reduction process, wherein manipulation of the arrangement of atoms and electrons is essential for achieving room-temperature 2D magnetism tailoring in a manner compatible with device architectures. Here, we report an electrochemically crafted CrI3 layered magnet─a van der Waals material─with precisely tailored lithiation and delithiation degrees. The crystalline and packing structure within the intralayer are preserved during the lithium intercalation within the interlayer, owing to weak interlayer coupling. Intrinsic ferromagnetism featuring a Curie temperature reaching 420 K has been unequivocally demonstrated, showcasing a coercivity of 1120 Oe at room temperature. The degree of lithiation through the reduction from Cr3+ to Cr2+ plays a crucial role in determining a 28.5% change in magnetization and a 0.29 eV shift in the bandgap. Room temperature ferromagnetism and magnetoelectricity are critical for noncontact, specifically photon-driven, dynamic magnetism control of 2D magnet-based magnonics devices.

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.

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.

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.

High temperature magnetic ordering in the 4d perovskite SrTcO3.

We present evidence for possibly the highest magnetic ordering temperature in any compound without 3d transition elements. Neutron powder diffraction measurements, at both time-of-flight and constant wavelength sources, were performed on two independently prepared SrTcO3 powders. SrTcO3 adopts a distorted perovskite structure with G-type antiferromagnetic ordering and has a moment of 1.87(4)µB per Tc cation at room temperature with an extraordinarily high Néel point close to 750 °C. Electronic structure calculations reveal extensive mixing between the technetium 4d states and oxygen states proximal to the Fermi level. This hybridization leads to a close relationship between magnetic ordering temperature and moment formation in SrTcO3.

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.

Iodine as an oxidant in the topotactic deintercalation of interstitial iron in Fe(1+x)Te.

The layered telluride, Fe(1+x)Te, is a parent compound of the isostructural and superconducting phases, Fe(1+x)(Te, Se, S). Here we show that, through a simple reaction of I(2) vapor with both powder and single crystal samples, the interstitial iron can be removed from the FeTe framework topotactically. Neutron powder diffraction and X-ray single crystal diffraction confirm that the iron being extracted is the partially occupied site that lies between the 2-D blocks of edge-sharing FeTe(4) tetrahedra. The deintercalation process has consequences for both magnetic and crystallographic phase transitions in the compound at low temperatures. This technique could be of use for the tuning of stoichiometry of the superconducting phases and therefore enable more careful studies on how chemical composition affects magnetic and superconducting properties.

Phase separation and suppression of the structural and magnetic transitions in superconducting doped iron tellurides, Fe(1+x)Te(1-y)S(y).

Single crystal and powder samples of the series of iron chalcogenide superconductors with nominal composition, Fe((1.15))Te((1-)y)S(y), are found to form for 0 ≤ y ≤ 0.15. They crystallize in the tetragonal anti-PbO structure, which is composed of layers of edge-shared Fe(Te, S)(4) tetrahedra. For y = 0, Fe(1+x)Te (x ≈ 0.12(1)) is nonsuperconducting and undergoes a tetragonal (P4/nmm) to monoclinic (P2(1)/m) structural transition at ∼65 K, associated with the onset of commensurate antiferromagnetic order at q = (0.5 0 0.5). We show that on sulfur substitution, Fe(1+x)Te(1-y)S(y) becomes orthorhombic (Pmmn) at low temperature for 0 ≤ y ≤ 0.015, where the greatly suppressed magnetic scattering is now incommensurate at q = (0.5-δ 0 0.5) and possesses short ranged magnetic correlations that are well fitted with a two-dimensional Warren peak shape. At much higher concentrations of S (y ≥ 0.075), there is suppression of both the structural and magnetic transitions and a superconducting transition at 9 K is observed. Between these two composition regimes, there exists a region of phase separation (0.025 ≤ y ≤ 0.05), where the low temperature neutron diffraction data is best refined with a model containing both the tetragonal and orthorhombic phases. The increase in the amount of sulfur is found to be associated with a reduction in interstitial iron, x. Microprobe analysis of a single crystal of composition Fe((1.123(5)))Te((0.948(4)))S((0.052(4))) confirms the presence of compositional variation within the crystals, rationalizing the observed phase separation.

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.

Preparation of the binary technetium bromides: TcBr3 and TcBr4.

TcBr(3) (1) and TcBr(4) (2) were synthesized by reaction of Tc metal with elemental bromine at 400 degrees C. Single crystal XRD measurements indicate that TcBr(3) crystallizes in the orthorhombic space group Pmmn (a = 11.0656(2) A, b = 5.9717(1) A, c = 6.3870(1) A). The structure consists of infinite chains of face-sharing TcBr(6) octahedra with a regular alternation of short and long Tc-Tc distances (2.8283(4) A, 3.1434(4) A). TcBr(4) crystallizes in the orthorhombic space group Pbca (a = 6.3237(5) A, b = 12.1777(9) A, c = 14.7397(11) A). TcBr(4) contains infinite chains of edge-sharing TcBr(6) octahedra with no apparent metal-metal bond (Tc-Tc = 3.7914(4) A). Technetium tribromide is isomorphous with RuBr(3) and MoBr(3), while TcBr(4) is isomorphous with PtBr(4) and OsBr(4).

Technetium(IV) halides predicted from first-principles.

We report the crystal structures of the novel technetium tetrahalides TcX(4) [X = F, I], as predicted from first-principles calculations. Isomorphous with TcCl(4) and TcBr(4) crystals, TcF(4) is orthorhombic with the centro-symmetric space group Pbca, while TcI(4) crystallizes in the monoclinic space group P2(1)/c. The structures, [TcX(2)(mu-X)(4/2)](infinity), consist of distorted edge-sharing octahedral groups of composition TcX(6) linked into endless cis chains. A possible explanation for the differences between these structures is offered in terms of varying degrees of bonding within the chains.

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.