Synthesis and Properties of BaMTeS (M = Fe, Mn, Zn) and the Disordered Structural Analog BaGe0.5TeS.

A series of tertiary sulfide-tellurides, BaMxTeS (M = Fe, Mn, Zn, Ge), has been synthesized by solid-state synthesis. The compounds assume an orthorhombic crystal structure, described by the Cmcm (No. 63) space group, and are structural analogs of the BaMSO (M = Co, Zn) phases. The properties of all four analogs are investigated by DFT analysis. As only the BaFeTeS analog was prepared as a relatively pure phase, this homologue was subject to further experimental investigations, including heat capacity, magnetometry, and Mössbauer spectroscopy. BaFeTeS exhibits no obvious phase transition between 2 and 300 K, has no paramagnetic behavior, and lacks long-range magnetic ordering. However, the Mössbauer spectra, as well as electrical resistance data, indicate a hidden transition near 200 K that is tentatively explained by a dynamic charge-density-wave mechanism, based on a resonating valence bond (RVB) model.

Order-to-Disorder Transition and Hydrogen Bonding in the Jahn-Teller Active NH4CrF3 Fluoroperovskite.

Large quantities of high-purity NH4CrF3 have been synthesized using a wet-chemical method, and its structural chemistry and magnetic properties are investigated in detail for the first time. NH4CrF3 is a tetragonal fluoroperovskite that displays an ordering of the ammonium (NH4+) groups at room temperature and C-type orbital ordering. The ammonium groups order and display distinct signs of hydrogen bonds to nearby fluoride anions by buckling the Cr-F-Cr angle away from 180°. The ammonium ordering remains up to 405 K, much higher than in other ammonium fluoroperovskites, indicating a correlation between the flexibility of the Jahn-Teller ion, the hydrogen bond formation, and the ammonium ordering. At 405 K, an order-to-disorder transition occurs, where the ammonium groups disorder, corresponding to a transition to higher symmetry. This is accompanied by a contraction of the unit cell from breaking hydrogen bonds, similar to the phenomenon observed in water ice melting. The compound orders antiferromagnetically with a Neél temperature of 60 K, an effective paramagnetic moment of 4.3 µB, and a Weiss temperature of -33 K. An A-type antiferromagnetic structure is identified by neutron diffraction, with an ordered moment of 3.72(2) µB.

Synthesis and Properties of Ba6Fe2Te3S7, with an Fe Dimer in a Magnetic Singlet State.

A new quaternary sulfide telluride, Ba6Fe2Te3S7, was synthesized by a solid-state reaction, and its crystal structure is novel. X-ray diffraction data on powder and single crystals reveal an orthorhombic lattice with a = 9.7543(3) Å, b = 18.2766(6) Å, and c = 12.0549(4) Å, and the noncentrosymmetric space group Cmc21 (No. 36). The properties of the compound were studied by magnetic susceptibility investigations, specific heat measurements, Mössbauer spectroscopy, and density functional theory calculations. Assuming Ba2+ and, as verified by the Mössbauer spectra, Fe3+, the charge balance requires the presence of a polytelluride, suggested to be a straight-chain [Te34-] polyanion. Further, the crystal structure contains [Fe2S7]8- dimers of two vertex-sharing tetrahedra, with a nearly linear Fe-S-Fe atom arrangement. The dimer exhibits antiferromagnetic coupling, with a coupling constant J = -10.5 meV (H = -2JS1S2) and S = 5/2, resulting in a spin singlet ground state. The interdimer magnetic interaction is so weak that the magnetic dimers can be treated as individuals.

X-ray and Neutron Diffraction Studies of SrTe2FeO6Cl, an Oxide Chloride with Rare Anion Ordering.

The oxychloride SrTe2FeO6Cl is obtained by high-temperature solid-state synthesis under inert conditions in closed reaction vessels. The compound crystallizes in a novel monoclinic crystal structure that is described in the space group P121/n1 (No. 14). The unit cell parameters, a = 10.2604(1) Å, b = 5.34556(5) Å, c = 26.6851(3) Å, and ß = 93.6853(4)°, and atomic parameters were determined from synchrotron diffraction data, starting from a model that was obtained from single-crystal X-ray diffraction data. The anion lattice exhibits a rare ordering of oxide and chloride ions: one-dimensional zig-zag ladders of chlorine (squarelike motif) are surrounded by an oxygen matrix. Two different iron sites coordinated solely to oxygen are present in the structure, one octahedral and one square pyramidal, both distorted. Similarly, two different strontium coordinations are present; the first homoleptic coordinated to eight oxygen atoms and the second heteroleptic coordinated to four oxygen and four chlorine atoms in a fac-like manner. The lone pair of Te(IV) is directed toward the larger chlorine atoms. Magnetic susceptibility measurements confirm that Fe is +3 (d5) in the high-spin electronic configuration, exhibiting an almost ideal spin-only moment, µeff = 5.65 µB Fe-1. The slightly negative Weiss constant (θCW = -39 K) suggests dominating antiparallel spin-to-spin coupling in the paramagnetic temperature range, agreeing with an observed long-range antiferromagnetic spin ordering below Néel temperature, TN ∼ 13 K, and a broad second order-like anomaly in the specific heat measurement data. Low-temperature neutron diffraction data reveal that the antiferromagnetic ordered phase is C-type, with a k-vector (1/2, 1/2, 0) and ordered moment of 4.14(7) µB. The spin structure can be described as antiferromagnetic ordered layers stacked along the a-axis, forming layers of squares that alternate along the c-axis.

Synthesis of (Li2Fe1-yMny)SO Antiperovskites with Comprehensive Investigations of (Li2Fe0.5Mn0.5)SO as Cathode in Li-ion Batteries.

A series of solid solutions (Li2Fe1-yMny)SO with a cubic antiperovskite structure was successfully synthesized. The composition (Li2Fe0.5Mn0.5)SO was intensively studied as a cathode in Li-ion batteries showing a reversible specific capacity of 120 mA h g-1 and almost a 100% Coulombic efficiency after 50 cycles at 0.1C meaning extraction/insertion of 1 Li per formula unit during 10 h. Operando X-ray absorption spectroscopy confirmed the redox activity of both Fe2+ and Mn2+ cations during battery charge and discharge, while operando synchrotron X-ray diffraction studies revealed a reversible formation of a second isostructural phase upon Li-removal and insertion at least for the first several cycles. In comparison to (Li2Fe)SO, the presence of Mn stabilizes the crystal structure of (Li2Fe0.5Mn0.5)SO during battery operation, although post mortem TEM studies confirmed a gradual amorphization after 50 cycles. A lower specific capacity of (Li2Fe0.5Mn0.5)SO in comparison to (Li2Fe)SO is probably caused by slower kinetics, especially in the two-phase region, as confirmed by Li-diffusion coefficient measurements.

A soft chemistry approach to the synthesis of single crystalline and highly pure (NH4)CoF3 for optical and magnetic investigations.

A new ionothermal synthesis utilizing 1-alkyl-pyridinium hexafluorophosphates [CxPy][PF6] (x = 2, 4, 6) led to the formation of highly crystalline single-phase ammonium cobalt trifluoride, (NH4)CoF3. Although ammonium transition-metal fluorides have been extensively studied with respect to their structural and magnetic properties, multiple aspects remain unclear. For that reason, the obtained (NH4)CoF3 has been investigated over a broad temperature range by means of single-crystal and powder x-ray diffraction as well as magnetization and specific heat measurements. In addition, energy-dispersive x-ray and vibrational spectroscopy as well as thermal analysis measurements were undertaken. (NH4)CoF3 crystallizes in the cubic perovskite structure and undergoes a structural distortion to a tetragonal phase at 127.7 K, which also is observable in the magnetic susceptibility measurements, which has not been observed before. A second magnetic phase transition occurring at 116.9 K is of second-order character. The bifurcation of the susceptibility curves indicates a canted antiferromagnetic ordering. At 2.5 K, susceptibility measurements point to a third phase change for (NH4)CoF3.

Bichalcogenide Model Systems for Magnetic Chains with Variable Spin Sizes and Optional Crystallographic Inversion Symmetry.

To develop an understanding of the magnetism on one-dimensional lattices, one of the great challenges is to identify novel model systems with enough chemical flexibility to design the magnetic interactions in measurable samples. To contribute to this endeavor, we present a number of bichalcogenides with similar trigonal packing of magnetic chains. These chains consist of 3d transition-metal (TM) ions that are 6-fold-coordinated by S or Se. Each TM coordination can be described as a trigonally distorted octahedron that shares faces with two neighboring octahedra. A unique ability with these model systems is that an entity with electric polarity can be introduced between the chains that causes the TM ions in the chains to shift to polar positions, thereby allowing for magnetoelectric coupling. By a comparison of the macroscopic data of polar and nonpolar chains with either S = 1 or S = 3/2, it is obvious that the magnetic properties are affected by the indirect electric polarity from the entity between the chains.

Synthesis, Crystal Structure, and Optical Characterization of the Sulfide Chloride Oxide CsBa6V4S12ClO4 with a Near-Infrared Fluorescence.

When CsCl, BaS, BaO, V, and S are reacted in a solid-state reaction under inert conditions, pure powders and single crystals of senary CsBa6V4S12ClO4 can be obtained. Its unique crystal structure has the symmetry R3̅H (no. 148) and unit cell parameters a = 9.0575(2) and c = 28.339(1) Å. The crystal structure contains polar units [VS3O]3- and a complex BaS7ClO2 coordination. The compound gets its deep-red color from a low-energy charge transfer, which can be explained by an electron transfer from S2- to V5+. In the near-infrared range, down-converted fluorescence occurs at 1.06 and 0.90 eV, and both emissions appear <450 ps after excitation at about 1.27 eV.

Ionothermal Synthesis, Structures, and Magnetism of Three New Open Framework Iron Halide-Phosphates.

A set of different open framework iron phosphates have been synthesized ionothermally using a task-specific ionic liquid, 1-butyl-4-methylpyridinium hexafluorophosphate, that acts in the synthesis as the reaction medium and mineralizer: (NH4)2Fe2(HPO4)(PO4)Cl2F (1) and K2Fe2(HPO4)(PO4)Cl2F (2) exhibit similar composition and closely related structural features. Both structures consist of {Fe2(HPO4)(PO4)Cl2F}2- macroanions and charge balancing ammonium or potassium cations. Their open framework structure contains layers and chains of corner-linked {Fe(1)O2Cl4} and {Fe(2)F2O4} octahedra, respectively, interconnected by PO4 tetrahedra forming 10-ring channels. KFe(PO3F)F2 (3) is built up by {Fe[(PO3F)4/3F2/2]}{Fe(PO3F)2/3F2/2F2} layers separated by K+ cations. Chains of alternating {FeF2O4} and {FeO2F4} octahedra, which are linear for 1 but undulated for 2, are linked to each other via corner-sharing {PO3F} tetrahedra with the fluorine pointing into the interlayer space. The compounds were characterized by means of single crystal and powder X-ray diffraction, infrared spectroscopy, and magnetic measurements. 1 reveals a strong ground state spin anisotropy with a spin 5/2 state and a magnetic moment of 5.3 µB/Fe3+. Specific heat and magnetic data unveil three magnetic transitions at 95, 50, and 3.6 K. Compound 2 has a very similar crystal structure as compared to 1 but exhibits a different magnetic behavior: a slightly lower magnetic moment of 4.7 µB/Fe3+ and a magnetic transition to a canted antiferromagnetic state below 90 K. Compound 3 exhibits typical paramagnetic behavior close to room-temperature (5.71 µB/Fe3+). There are no clear indications for a phase transition down to 2 K despite strong antiferromagnetic spin-spin interactions; only a magnetic anomaly appears at 50 K in the zero-field cooled data.

Extended Chemical Flexibility of Cubic Anti-Perovskite Lithium Battery Cathode Materials.

Novel bichalcogenides with the general composition (Li2TM)ChO (TM = Mn, Co; Ch = S, Se) were synthesized by single-step solid-state reactions. These compounds possess cubic anti-perovskite crystal structure with Pm3̅ m symmetry; TM and Li are disordered on the crystallographic site 3c. According to Goldschmidt tolerance factor calculations, the available space at the 3c site is too large for Li+ and TM2+ ions. As cathode materials, all title compounds perform less prominent in lithium-ion battery setups in comparison to the already known TM = Fe homologue; e.g., (Li2Co)SO has a charge density of about 70 mAh g-1 at a low charge rate. Nevertheless, the title compounds extend the chemical flexibility of the anti-perovskites, revealing their outstanding chemical optimization potential as lithium battery cathode material.

Synthesis, Characterization, and Electrochemistry of Layered Chalcogenides LiCu Ch ( Ch = Se, Te).

Two novel compounds, LiCu Ch ( Ch = Se or Te), were synthesized by direct reaction between elements in closed ampules inside corundum crucibles. Both compounds are highly air-sensitive and possess an anti-PbClF crystal structure, which contains Cu Ch layer analogues to the Fe[As/Se] layers in Fe-based superconductors. In electrochemical battery cells, Li can be almost completely extracted from LiCuSe, but the reverse reaction is only partly successful and Li2Se and Cu2- xSe are formed instead. LiCuSe exhibits a temperature independent and slightly positive magnetic susceptibility. From 7Li NMR measurements, the activation energy of the Li ion diffusion process is about 0.5 eV but is slightly lower for LiCuTe as compared to LiCuSe. Also, the small and almost temperature independent NMR shifts of the 7Li nucleus indicate the absence of Pauli paramagnetism in these compounds, consistent with a 3 d10 full valence state of the Cu ions.

Anti-Perovskite Li-Battery Cathode Materials.

Through single-step solid-state reactions, a series of novel bichalcogenides with the general composition (Li2Fe)ChO (Ch = S, Se, Te) are successfully synthesized. (Li2Fe)ChO (Ch = S, Se) possess cubic anti-perovskite crystal structures, where Fe and Li are completely disordered on a common crystallographic site (3c). According to Goldschmidt calculations, Li+ and Fe2+ are too small for their common atomic position and exhibit large thermal displacements in the crystal structure models, implying high cation mobility. Both compounds (Li2Fe)ChO (Ch = S, Se) were tested as cathode materials against graphite anodes (single cells); They perform outstandingly at very high charge rates (270 mA g-1, 80 cycles) and, at a charge rate of 30 mA g-1, exhibit charge capacities of about 120 mA h g-1. Compared to highly optimized Li1-xCoO2 cathode materials, these novel anti-perovskites are easily produced at cost reductions by up to 95% and, yet, possess a relative specific charge capacity of 75%. Moreover, these iron-based anti-perovskites are comparatively friendly to the environment and (Li2Fe)ChO (Ch = S, Se) melt congruently; the latter is advantageous for manufacturing pure materials in large amounts.

Successive Phase Transitions in Fe2+ Ladder Compounds Sr2Fe3Ch2O3 (Ch = S, Se).

Small single crystals of Sr2Fe3Ch2O3 (Ch = S, Se) have been synthesized by flux methods, and bulk materials have been obtained by solid state reactions. Both compounds are isostructural to the compound Sr2Co3S2O3 (space group Pbam), which contains a novel hybrid spin ladder: a combination of a 2-leg rectangular ladder and a necklace ladder. The 2-leg ladder acts as a well-defined magnetic entity, while intimate magnetic coupling to the necklace ladder induces three successive phase transitions in the range of 40-120 K in each composition (Ch = S or Se), as revealed by Mössbauer spectroscopy, thermodynamics, and magnetometry. The complex magnetic behaviors can be explained by the unique spin-lattice topology.

Three Oxidation States of Manganese in the Barium Hexaferrite BaFe12-xMnxO19.

The coexistence of three valence states of Mn ions, namely, +2, +3, and +4, in substituted magnetoplumbite-type BaFe12-xMnxO19 was observed by soft X-ray absorption spectroscopy at the Mn-L2,3 edge. We infer that the occurrence of multiple valence states of Mn situated in the pristine purely iron(III) compound BaFe12O19 is made possible by the fact that the charge disproportionation of Mn3+ into Mn2+ and Mn4+ requires less energy than that of Fe3+ into Fe2+ and Fe4+, related to the smaller effective Coulomb interaction of Mn3+ (d4) compared to Fe3+ (d5). The different chemical environments determine the location of the differently charged ions: with Mn3+ occupying positions with (distorted) octahedral local symmetry, Mn4+ ions prefer octahedrally coordinated sites in order to optimize their covalent bonding. Larger and more ionic bonded Mn2+ ions with a spherical charge distribution accumulate at tetrahedrally coordinated sites. Simulations of the experimental Mn-L2,3 XAS spectra of two different samples with x = 1.5 and x = 1.7 led to Mn2+:Mn3+:Mn4+ atomic ratios of 0.16:0.51:0.33 and 0.19:0.57:0.24.

Canted Antiferromagnetism on Rectangular Layers of Fe2+ in Polymorphic CaFeSeO.

From stoichiometric amounts of CaO, Fe, and Se, pure powders and single crystals of quaternary [Formula: see text] can be obtained by solid-state reaction and self-flux growth, respectively. The as-synthesized compound exhibits a polymorphic crystal structure, where the two modifications have different stacking sequences of [Formula: see text] layers. The two polymorphs have similar unit cells but different crystal symmetries (Cmc21 and Pnma), of which the former is non-centrosymmetric. Fe is divalent (d6) and high-spin, as proven by X-ray spectroscopy, Mössbauer spectroscopy, and powder neutron diffraction data. The latter two, in combination with magnetic susceptibility and specific heat data, reveal a long-range antiferromagnetic spin order (TN = 160 K) with a minor spin canting. CaFeSeO is an electronic insulator, as confirmed by resistivity measurements and density functional theory calculations. The latter also suggest a relatively small energy difference between the two polymorphs, explaining their intimate intergrowth.

Fused Perovskite Tunnel Structures in Ba5 Fe6+x S4+x O8 (0.44≤x≤0.55) with x-Dependent Two-Stage Magnetizations.

Ba5 Fe6+x S4+x O8 was synthesized through a solid-state reaction, and pure powders of nominal compositions x=0.44-0.55 were obtained after being rinsed with water. The crystal structures (P4/mmm, a=10.13, c=4.03 Å) and sample purities were investigated by powder synchrotron X-ray diffraction and were found to be composed of a tunnel lattice (Ba5 Fe6 S4 O8 ), built from fused perovskite units and the tunnel filling (Fex Sx ). The variable composition, that is, the tunnel filling (x), causes partially occupied sites as well as crystallographic split positions. Ba5 Fe6+x S4+x O8 (x=0.525) is semiconducting and all investigated compositions exhibit magnetic ground states that could be described as either semi-spin-glass-like (x>0.5) or canted antiferromagnetic (x<0.5). The spin-glass in x=0.525 exhibits magnetic relaxations that are affected by ageing.

[Cs6 Cl][Fe24 Se26 ]: A Host-Guest Compound with Unique Fe-Se Topology.

The novel host-guest compound [Cs6Cl][Fe24Se26] (I4/mmm; a=11.0991(9), c=22.143(2) Å) was obtained by reacting Cs2Se,CsCl, Fe, and Se in closed ampoules. This is the first member of a family of compounds with unique Fe-Se topology, which consists of edge-sharing, extended fused cubane [Fe8Se6Se8/3] blocks that host a guest complex ion, [Cs6Cl](5+). Thus Fe is tetrahedrally coordinated and divalent with strong exchange couplings, which results in an ordered antiferromagnetic state below TN =221 K. At low temperatures, a distribution of hyperfine fields in the Mössbauer spectra suggests a structural distortion or a complex spin structure. With its strong Fe-Se covalency, the compound is close to electronic itinerancy and is, therefore, prone to exhibit tunable properties.

Synthesis and Characterization of Cs1-xTi2Te2O (x ≈ 0.2): Electron Doping by Te Resulting in a Layered Metal.

Reacting Cs2O1.3, TiTe, TiO2, and Te under inert conditions gives powders of Cs1-xTi2Te2O (x ≈ 0.2). Small single crystals of the same phase were obtained from a CsCl salt melt in closed ampoules. This cesium dititanium ditelluride oxide (P4/mmm, a = 4.0934(3) Å, c = 8.9504(9) Å) is isostructural to CeCr2Si2C and contains layers of face-sharing trans-TiTe4O2 octahedra that are separated by Cs. As Ti occupies only one crystallographic site, its average oxidation state is +2.6, for the Cs deficit x = 0.2. The formally intermediate Ti valence state agrees well with the metallic conductivity and temperature-independent paramagnetic behavior. No superconductivity is observed down to 0.1 K in Cs0.8Ti2Te2O, but the fact that this structure type can accommodate Te2- suggests that electron doping of structurally closely related pnictide oxide superconductors, for example, BaTi2Bi2O, might be possible.

Metal Vacancy Ordering in an Antiperovskite Resulting in Two Modifications of Fe2 SeO.

Small, red Fe2 SeO single crystals in two modifications were obtained from a CsCl flux. The metastable α-phase is pseudo-tetragonal (Cmce, a=16.4492(8) Å, b=11.1392(4) Å, c=11.1392(4) Å), whereas the ß-phase is trigonal (P31 , a=9.8349(4) Å, c=6.9591(4) Å)) and thermodynamically stable within a narrow temperature range. Both crystal structures were solved from twinned specimens. The enantiomers of the ß-phase appear as racemic mixtures. Selenium and oxygen form two individual interpenetrating primitive cubic lattices, giving a bcc packing. A quasi-octahedrally coordinated iron atom is found close to the center of each surface of the selenium sublattice. The difference between the α- and ß-phases is the distribution of iron at 2/3 of the surfaces. α- and ß-Fe2 SeO are comparable with metal-vacancy-ordered antiperovskites. Each Fe/O lattice can also be described in terms of vertex-sharing OFe4 tetrahedra, with a crystal structure similar to that of an antisilicate. Iron is divalent and has a high-spin d(6) (S=2) configuration. The ß-phase exhibits magnetoelectric coupling.

Synthesis and Characterization of Ba[CoSO]: Magnetic Complexity in the Presence of Chalcogen Ordering.

Barium thio-oxocobaltate(II), Ba[CoS2/2 O2/2 ], was synthesized by the reaction of equimolar amounts of BaO, Co, and S in closed silica ampoules. The title compound (Cmcm, a=3.98808(3), b=12.75518(9), c=6.10697(4) Å) is isostructural to Ba[ZnSO]. The use of soft X-ray absorption spectroscopy confirmed that cobalt is in the oxidation state +2 and tetrahedrally coordinated. Its coordination consists of two sulfur and two oxygen atoms in an ordered fashion. High-temperature magnetic susceptibility data indicate strong low-dimensional spin-spin interactions, which are suggested to be closely related to the layer-type crystal structure and perhaps the ordered distribution of sulfur and oxygen. Antiferromagnetic ordering below TN =222 K is observed as an anomaly in the specific heat, coinciding with a significant lowering of the magnetic susceptibility. Density functional theory calculations within a generalized-gradient approximation (GGA)+U approach identify an antiferromagnetic ground state within the square-like two-dimensional layers of Co, and antiferromagnetic correlations for nearest and next nearest neighbors along bonds mediated by oxygen or sulfur. However, this magnetic state is subject to frustration by relatively strong interlayer couplings.