Interpenetrated Lattices of Quaternary Chalcogenides Displaying Magnetic Frustration, High Na-Ion Conductivity, and Cation Redox in Na-Ion Batteries.

A series of quaternary selenides, NaxMGaSe4 (M = Mn, Fe, and mixed Zn/Fe), have been synthesized for the first time employing a high-temperature solid-state synthesis route through stochiometric or polychalcogenide flux reactions. Along with the selenides, a previously reported sulfide analogue, NaxFeGaS4, is also revisited with new findings. These compounds form an interpenetrated structure made up of a supertetrahedral unit. The electrochemical evaluations exhibit a reversible (de)intercalation of ∼0.6 and ∼0.45 Na-ions, respectively, from Na2.87FeGaS4 (1a) and Na2.5FeGaSe4 (2) involving Fe2+/Fe3+ redox when cycled between 1.5 and 2.5 V. Mössbauer spectroscopy of 1a shows the existence of a mixed oxidation state of Fe2+/3+ in the pristine compound and reversible oxidation of Fe2+ to Fe3+ during the electrochemical cycles. Na2.79Zn0.6Fe0.4GaSe4 possesses a reasonably high room temperature ionic conductivity of 0.077 ms/cm with an activation energy of 0.30 eV. The preliminary magnetic measurements show a bifurcation of FC-ZFC at 4.5 and 2.5 K, respectively, for 1a and Na3MnGaSe4 (4) arising most likely from a spin-glass like transition. The high negative values of the Weiss constants -368.15 and -308.43 K for 1a and 4, respectively, indicate strong antiferromagnetic interactions between the magnetic ions and also emphasize the presence of a high degree of magnetic frustration in these compounds.

A Comparative Study of Cationic Copper(I) Reagents Supported by Bipodal Tetramethylguanidinyl-Containing Ligands as Nitrene-Transfer Catalysts.

The bipodal compounds [(TMG2biphenN-R)CuI-NCMe](PF6) (R = Me, Ar (4-CF3Ph-)) and [(TMG2biphenN-Me)CuI-I] have been synthesized with ligands that feature a diarylmethyl- and triaryl-amine framework and superbasic tetramethylguanidinyl residues (TMG). The cationic Cu(I) sites mediate catalytic nitrene-transfer reactions between the imidoiodinane PhI = NTs (Ts = tosyl) and a panel of styrenes in MeCN, to afford aziridines, demonstrating comparable reactivity profiles. The copper reagents have been further explored to execute C-H amination reactions with a variety of aliphatic and aromatic hydrocarbons and two distinct nitrene sources PhI = NTs and PhI = NTces (Tces = 2,2,2-trichloroethylsulfamate) in benzene/HFIP (10:2 v/v). Good yields have been obtained for sec-benzylic and tert-C-H bonds of various substrates, especially with the more electron-deficient catalyst [(TMG2biphenN-Ar)CuI-NCMe](PF6). In conjunction with earlier studies, the order of reactivity of these bipodal cationic reagents as a function of the metal employed is established as Cu > Fe > Co ≥ Mn. However, as opposed to the base-metal analogues, the bipodal Cu reagents are less reactive than a similar tripodal Cu catalyst. The observed fluorophilicity of the bipodal Cu compounds may provide a deactivation pathway.

On-Demand MXene-Coupled Pyroelectricity for Advanced Breathing Sensors and IR Data Receivers.

MXene-inspired two-dimensional (2D) materials like Ti3C2Tx are widely known for their versatile properties, including surface plasmon, higher electrical conductivity, exceptional in-plane tensile strength, EMI shielding, and IR thermal properties. The MXene nanosheets coupled poly(vinylidene fluoride) (PVDF) nanofibers with d33 ∼-26 pm V-1 are able to capture the smaller thermal fluctuation due to a superior pyroelectric coefficient of ∼130 nC m-2 K-1 with an improved (∼7 times with respect to neat PVDF nanofibers) pyroelectric current figure of merit (FOMi). The significant enhancement of the pyroelectric response is attributed to the confinement effect of 2D MXene (Ti3C2Tx) nanosheets within PVDF nanofibers, as evidenced from polarized Fourier transform infrared (FTIR) spectroscopy and scanning probe microscopy (SPM). In subsequent studies, the practical applications of self-powered pyroelectric sensors of MXene-PVDF have been demonstrated. The fabricated flexible, hydrophobic pyroelectric sensor could be utilized as an excellent pyroelectric breathing sensor, a proximity sensor, and an IR data transmission receiver. Further, supervised machine learning algorithms are proposed to distinguish different types of breathing signals with ∼98% accuracy for healthcare monitoring purposes.

Cationic Divalent Metal Sites (M = Mn, Fe, Co) Operating as Both Nitrene-Transfer Agents and Lewis Acids toward Mediating the Synthesis of Three- and Five-Membered N-Heterocycles.

The tripodal compounds [(TMG3trphen)MII-solv](PF6)2 (M = Mn, Fe, Co; solv = MeCN, DMF) and bipodal analogues [(TMG2biphen)MII(NCMe)x](PF6)2 (x = 3 for Mn, Fe; x = 2 for Co) and [(TMG2biphen)MIICl2] have been synthesized with ligands that feature a triaryl- or diarylmethyl-amine framework and superbasic tetramethylguanidinyl residues (TMG). The dicationic M(II) sites mediate catalytic nitrene-transfer reactions between the imidoiodinane PhI═NTs (Ts = tosyl) and a panel of styrenes in MeCN to afford aziridines and low yields of imidazolines (upon MeCN insertion) with an order of productivity that favors the bipodal over the tripodal reagents and a metal preference of Fe > Co ≥ Mn. In CH2Cl2, the more acidic Fe(II) sites favor formation of 2,4-diaryl-N-tosylpyrrolidines by means of an in situ (3 + 2) cycloaddition of the initially generated 2-aryl-N-tosylaziridine with residual styrene. In the presence of ketone, 1,3-oxazolidines can be formed in practicable yields, involving a single-pot cycloaddition reaction of alkene, nitrene, and ketone (2 + 1 + 2). Mechanistic studies indicate that the most productive bipodal Fe(II) site mediates stepwise addition of nitrene to olefins to generate aziridines with good retention of stereochemistry and further enables aziridine ring opening to unmask a 1,3-zwitterion that can undergo cycloaddition with dipolarophiles (MeCN, alkene, ketone) to afford five-membered N-heterocycles.

NaGaSe2: A Water-Loving Multifunctional Non-van der Waals Layered Selenogallate.

A missing member of well-known ternary chalcometallates, a sodium selenogallate, NaGaSe2, has been synthesized by employing a polyselenide flux and stoichiometric reaction. Crystal structure analysis using X-ray diffraction techniques reveals that it contains supertetrahedral adamantane-type Ga4Se10 secondary building units. These Ga4Se10 secondary building units are further connected via corners to form two-dimensional (2D) [GaSe2]∞- layers stacked along the c-axis of the unit cell, and the Na ions reside in the interlayer space. The compound has an unusual ability to absorb water molecules from the atmosphere or a nonanhydrous solvent to form distinct hydrated phases, NaGaSe2·xH2O (where x can be 1 and 2), with an expanded interlayer space, as verified by X-ray diffraction (XRD), thermogravimetric-differential scanning calorimetry (TG-DSC), desorption, and Fourier transform infrared spectroscopy (FT-IR) studies. The in situ thermodiffractogram indicates the emergence of an anhydrous phase before 300 °C with the decrease of interlayer spacings and reverting to the hydrated phase within a minute of re-exposure to the environment, supporting the reversibility of such a process. Structural transformation induced through water absorption results in an increase of Na ionic conductivity by 2 orders of magnitude compared to that of the pristine anhydrous phase, as verified by impedance spectroscopy. Na ions from NaGaSe2 can be exchanged in the solid-state route with other alkali and alkaline earth metals in a topotactic or nontopotactic way, leading to 2D isostructural and three-dimensional networks, respectively. Optical band gap measurements show a band gap of ∼3 eV for the hydrated phase, NaGaSe2·xH2O, which is in good agreement with the calculated band gap using a density functional theory (DFT)-based method. Sorption studies further confirm the selective absorption of water over MeOH, EtOH, and CH3CN with a maximum water uptake of 6 molecules/formula unit at a relative pressure, P/P0, of 0.9.

Lithium selenometallates of triel elements, Li5MSe4 (M = Al and Ga), aliovalent doping and their ionic conductivity.

Ternary selenometallates, Li5MSe4 (M = Al(I) and Ga(II)), have been synthesized for the first time through high temperature solid-state reactions combining elements and Li2Se in stoichiometric compositions. Li5MSe4 crystallizes in the P21/m space group, forming a pseudo-2D layer type structure with edge sharing LiSe4 and MSe4 tetrahedra along the a-axis. These layers are interleaved by octahedrally coordinated Li ions located in the interlayer space. AC impedance spectroscopy measurements yield room temperature ionic conductivities of 0.60 × 10-7 and 0.58 × 10-7 S cm-1 with calculated activation energies of 0.51 and 0.48 eV for I and II, respectively. An aliovalent substitution of Sn4+ in Li5MSe4 yields compositions of Li4.66Al0.82Sn0.22Se4 (III) and Li4.37Ga0.89Sn0.24Se4 (IV), which crystallize in the P21/m and P3̄m1 space groups, respectively. Sn-doped samples show an ∼5-fold increase in ionic conductivity, 3.37 × 10-7 S cm-1 and 2.4 × 10-7 S cm-1 with activation energies of 0.54 and 0.28 eV, respectively, for III and IV. The optical band gap values of the compounds are 3.65 and 3.2 eV for I and II, respectively, as measured by diffuse reflectance spectroscopy. Density functional theory (DFT) calculations predicted a major contribution from the Se 4p-states in forming the top of the valence band and strongly hybridized the Se 4p and ns orbitals of Al and Ga in forming the bottom of the conduction band with almost no contribution from the Li s-states near the Fermi level indicating their ionic interactions with the ligand.

Crystal structure, electronic structure, and optical properties of the novel Li4CdGe2S7, a wide-bandgap quaternary sulfide with a polar structure derived from lonsdaleite.

The novel quaternary thiogermanate Li4CdGe2S7 (tetralithium cadmium digermanium heptasulfide) was discovered from a solid-state reaction at 750 °C. Single-crystal X-ray diffraction data were collected and used to solve and refine the structure. Li4CdGe2S7 is a member of the small, but growing, class of I4-II-IV2-VI7 diamond-like materials. The compound adopts the Cu5Si2S7 structure type, which is a derivative of lonsdaleite. Crystallizing in the polar space group Cc, Li4CdGe2S7 contains 14 crystallographically unique ions, all residing on general positions. Like all diamond-like structures, the compound is built of corner-sharing tetrahedral units that create a relatively dense three-dimensional assembly. The title compound is the major phase of the reaction product, as evidenced by powder X-ray diffraction and optical diffuse reflectance spectroscopy. While the compound exhibits a second-harmonic generation (SHG) response comparable to that of the AgGaS2 (AGS) reference material in the IR region, its laser-induced damage threshold (LIDT) is over an order of magnitude greater than AGS for λ = 1.064 µm and τ = 30 ps. Bond valence sums, global instability index, minimum bounding ellipsoid (MBE) analysis, and electronic structure calculations using density functional theory (DFT) were used to further evaluate the crystal structure and electronic structure of the compound and provide a comparison with the analogous I2-II-IV-VI4 diamond-like compound Li2CdGeS4. Li4CdGe2S7 appears to be a better IR nonlinear optical (NLO) candidate than Li2CdGeS4 and one of the most promising contenders to date. The exceptional LIDT is likely due, at least in part, to the wider optical bandgap of ∼3.6 eV.

Discovery of an olivine-type lithium manganese thiophosphate, LiMnPS4, via a building block approach.

An olivine-type orthothiophospate LiMnPS4 has been synthesized for the first time through a building block approach by reacting preformed ternary lithium thiophospate with MnCl2. Diffuse reflectance measurements show an optical band gap of 2.36 eV, which is further confirmed by DFT calculations. Irreversible weak ferromagnetic ordering and metamagnetism are verified through preliminary magnetic measurements.

Sodium-Stuffed Open-Framework Quaternary Chalcogenide Built with (Cu2Ga6S18)16- Ribbons Cross-Linked by Unusual Linear Cu(I) Pillars.

A quaternary compound, Na15Cu3Ga6S18, the first member in the A-Cu-Ga-S (A = alkali metal) series, has been synthesized from a solid-state metathesis reaction between Na6Ga2S6 and CuCl as well as from a combination of Na2S, Ga, Cu, and S. The compound crystallizes in a monoclinic crystal system, space group C2/c, and represents a unique open-framework structure with channels filled with eight crystallographically distinct Na ions. The anionic framework is built up of infinite chains of corner-shared GaS4 tetrahedra fused together by an edge-shared dimer of CuS4 tetrahedra forming one-dimensional ribbons of (Cu2Ga6S18)16-, which are cross-linked by linearly coordinated S-Cu-S linkages resulting in a three-dimensional network with tunnels filled with Na atoms. Optical band gap measurements show that the compound has a direct band gap of 3.00 eV that is in good agreement with the theoretical band gap derived from density functional theory calculations. Band structure calculations further indicate that the states near the Fermi level are dominated by tetrahedral Cu+(d) and S(p) states resulting from the antibonding interactions, while s-d hybridization is prevalent in linear Cu+ coordination. Ionic conductivity measurements show that the compound has a room-temperature Na ion conductivity of 2.72 × 10-5 mS/cm with an activation energy of 0.68 eV, which corroborates well the nudged elastic band calculations.

Ternary alkali ion thiogallates, A5GaS4 (A = Li and Na), with isolated tetrahedral building units and their ionic conductivities.

Two new ternary thiogallates in the A5GaS4 (A = Li (i) and Na (ii)) series have been synthesized for the first time employing a gas passing route using oxide precursors and a high temperature solid state route using stoichiometric combinations of elements, respectively. Li5GaS4 crystallizes in the P21/m space group and the structure is built up of layers of corner sharing tetrahedra of LiS4 and GaS4 stacked along the a-axis and the octahedrally coordinated Li ions residing in the interlayer space. Na5GaS4 crystallizes in the Pbca space group and the structure consists of isolated (GaS4)5- tetrahedra held together by charge balancing sodium ions in distorted tetrahedral and octahedral coordination geometries. Measurements of ionic conductivity of the compounds showed room temperature ionic conductivities of 1.8 × 10-7 and 4.0 × 10-7 S cm-1 with activation energies of 0.54 and 0.28 eV, respectively, for I and II. Density functional theory calculations show close agreement in structural parameters with the measured data and predict band gaps of 2.75 eV (I) and 2.70 eV (II). Single point hybrid functional calculations result in band gaps of 3.95 and 3.65 eV correspondingly, in better agreement with the experimental value of ∼4.1 eV for both. Bond valence energy landscape maps suggest the absence of any suitable diffusion path for Li in Li5GaS4. On the other hand, BVEL maps of Na5GaS4 confirm that the tetrahedrally coordinated Na ions are responsible for ionic conduction, whereas the involvement of octahedrally coordinated Na ions in the conduction process could not be discerned.

Interplay between Oxo and Fluoro in Vanadium Oxyfluorides for Centrosymmetric and Non-Centrosymmetric Structure Formation.

Herein, we report the syntheses of two lithium-vanadium oxide-fluoride compounds crystallized from the same reaction mixture through a time variation experiment. A low temperature hydrothermal route employing a viscous paste of V2O5, oxalic acid, LiF, and HF allowed the crystallization of one metastable phase initially, Li2VO0.55(H2O)0.45F5⋅2H2O (I), which on prolonged heating transforms to a chemically similar yet structurally different phase, Li3VOF5 (II). Compound I crystallizes in centrosymmetric space group, I2/a with a = 6.052(3), b = 7.928(4), c = 12.461(6) Å, and ß = 103.99(2)°, while compound II crystallizes in a non-centrosymmetric (NCS) space group, Pna21 with a = 5.1173(2), b = 8.612(3), c = 9.346(3) Å. Synthesis of NCS crystals are highly sought after in solid-state chemistry for their second-harmonic-generation (SHG) response and compound II exhibits SHG activity albeit non-phase-matchable. In this article, we also describe their magnetic properties which helped in unambiguous assignment of mixed valency of V (+4/+5) for Li2VO0.55(H2O)0.45F5⋅2H2O (I) and +4 valency of V for Li3VOF5 (II).

Is the Electrophilicity of the Metal Nitrene the Sole Predictor of Metal-Mediated Nitrene Transfer to Olefins? Secondary Contributing Factors as Revealed by a Library of High-Spin Co(II) Reagents.

Recent research has highlighted the key role played by the electron affinity of the active metal-nitrene/imido oxidant as the driving force in nitrene additions to olefins to afford valuable aziridines. The present work showcases a library of Co(II) reagents that, unlike the previously examined Mn(II) and Fe(II) analogues, demonstrate reactivity trends in olefin aziridinations that cannot be solely explained by the electron affinity criterion. A family of Co(II) catalysts (17 members) has been synthesized with the assistance of a trisphenylamido-amine scaffold decorated by various alkyl, aryl, and acyl groups attached to the equatorial amidos. Single-crystal X-ray diffraction analysis, cyclic voltammetry and EPR data reveal that the high-spin Co(II) sites (S = 3/2) feature a minimal [N3N] coordination and span a range of 1.4 V in redox potentials. Surprisingly, the Co(II)-mediated aziridination of styrene demonstrates reactivity patterns that deviate from those anticipated by the relevant electrophilicities of the putative metal nitrenes. The representative L4Co catalyst (-COCMe3 arm) is operating faster than the L8Co analogue (-COCF3 arm), in spite of diminished metal-nitrene electrophilicity. Mechanistic data (Hammett plots, KIE, stereocontrol studies) reveal that although both reagents follow a two-step reactivity path (turnover-limiting metal-nitrene addition to the C b atom of styrene, followed by product-determining ring-closure), the L4Co catalyst is associated with lower energy barriers in both steps. DFT calculations indicate that the putative [L4Co]NTs and [L8Co]NTs species are electronically distinct, inasmuch as the former exhibits a single-electron oxidized ligand arm. In addition, DFT calculations suggest that including London dispersion corrections for L4Co (due to the polarizability of the tert-Bu substituent) can provide significant stabilization of the turnover-limiting transition state. This study highlights how small ligand modifications can generate stereoelectronic variants that in certain cases are even capable of overriding the preponderance of the metal-nitrene electrophilicity as a driving force.

Soft chemical routes to electrochemically active iron phosphates.

New iron phosphates with related structures have been synthesized using hydrothermal and ion-exchange routes, and their electrochemical properties were investigated. First, NaFe(HPO4)2 was synthesized employing a hydrothermal route and its structure was determined from single-crystal X-ray diffraction data. Subsequent Na+ and partial proton ion exchange with Li+ ion produced a known phase, Li2Fe(H0.5PO4)2, and complete deprotonation of Li2Fe(H0.5PO4)2 with Li+ by employing a solid-state ion-exchange route produced the new phase Li3Fe(PO4)2. The structure of the latter was solved from synchrotron powder X-ray data by employing ab initio methods. All of these phases are highly crystalline, built up of similar connectivities between FeO6 octahedra and PO4 tetrahedral units. Magnetic susceptibility measurements and room-temperature 57Fe Mössbauer spectroscopic studies confirm the 3+ oxidation state of the compounds and their antiferromagnetic ordering with Li2Fe(H0.5PO4)2 showing some interesting metamagnetic behavior. The compounds are stable up to 400 °C and undergo facile electrochemical lithium/sodium insertion through the reduction of Fe3+ to Fe2+. Galvanostatic charge-discharge studies indicate that up to 0.6 lithium ion and 0.5 sodium ion per formula unit can be inserted at average voltages of 3.0 and 2.75 V for lithium and sodium ion batteries, respectively, for NaFe(HPO4)2. The partially Li ion exchanged compound Li2Fe(H0.5PO4)2 showed better cycle life and experimentally achievable capacities up to 0.9 Li insertion with strong dependence on particle size. The electrochemical Li insertion in Li3Fe(PO4)2 was also investigated. The electrochemistry of these three related phases were compared with each other, and their mechanism of Li insertion was investigated by ex situ PXRD.

A highly fluorinated lithium iron phosphate with interpenetrating lattices: electrochemistry and ionic conductivity.

Li5Fe2PO4F8, a new member of the family of alkali transition metal fluorophosphates, has been synthesized and characterized using single-crystal X-ray diffraction, 57Fe Mössbauer spectroscopy and magnetic susceptibility measurements. The existence of an infinite {-[PO4(FeF4)2]-}∞ tetrahedral network in an inter-penetrated diamond lattice, along with the presence of seven unique Li sites, presents interesting structural features of this structure-type for energy storage applications. The initial results of (de)lithiation reveal that a relatively low fraction of theoretical capacity may be utilized reversibly (0.2 Li+ ion per formula unit), possibly due to the lack of available free volume for Li+ insertion. The high Li content and the existence of large channels in all 3-dimensions of space also offer opportunities to study this material as a candidate for solid-state electrolytes. The results from electro-impedance measurements reveal the reasonable activation energy of Li diffusion (0.70 eV), which is also supported by theoretical calculations.

Magnetically Frustrated Quaternary Chalcogenides with Interpenetrating Diamond Lattices.

A series of quaternary sulfides of the composition Na3MGaS4 (M = Mn (1), Fe (2), and Co (3)) have been synthesized in sealed quartz ampules. In these compounds, divalent transition metal and Ga occupy the same crystallographic site in the Ga-S network, forming a supertetrahedral, T2 (adamantane) unit, through the corner-sharing of four M/GaS4 tetrahedra. The corner sulfur atoms of the T2 clusters are further connected to similar T2 units to form an open continuous three-dimensional (3D) anionic framework of composition {[Ga2M2S8]n}6-. The framework resembles a zinc blende structure type if each T2 cluster is considered as a single tetrahedron and two such frameworks are intertwined to generate channels wherein reside the extra-framework Na+ ions. Placement of transition metals (Mn or Fe or Co) in the corner of a perfect supertetrahedron, adamantane building unit, generates an ideal lattice for geometrical magnetic frustration, which, on dilution with nonmagnetic metal (Ga), creates an ideal case for random frustration. Preliminary magnetic measurements indicate high negative values of the Weiss constant (-200 to -400 K) and the absence of any magnetic ordering, reinforcing the presence of magnetic frustration in all of these compounds.

Tetragonal versus Hexagonal: Structure-Dependent Catalytic Activity of Co/Zn Bimetallic Metal-Organic Frameworks.

Tetragonal and hexagonal phases of monometallic Zn and bimetallic Co/Zn metal-organic frameworks (MOFs), with secondary building units (SBUs) containing a M3O (M = metal) cluster, were synthesized from identical constituents using a benzenetricarboxylate (BTC(3-)) linker that forms decorated 3,6- and 3,5-connected networks, respectively. There exist subtle differences between the SBUs; one of the metal atoms in the M3O cluster in the tetragonal phase has one dissociable DMF solvent molecule while that in the hexagonal phase has three. Connectivities between the SBUs form one-dimensional channels in both MOFs. These MOFs catalyze the chemoselective addition of amines to epoxides, giving exclusively ß-hydroxyamine under heterogeneous conditions. A ring-opening reaction of a symmetrical epoxide showed that the hexagonal phase diastereoselectively yields trans-alcohol, exhibiting an exquisite model for structure-dependent activity.

Employing Synergetic Effect of Doping and Thin Film Coating to Boost the Performance of Lithium-Ion Battery Cathode Particles.

Atomic layer deposition (ALD) has evolved as an important technique to coat conformal protective thin films on cathode and anode particles of lithium ion batteries to enhance their electrochemical performance. Coating a conformal, conductive and optimal ultrathin film on cathode particles has significantly increased the capacity retention and cycle life as demonstrated in our previous work. In this work, we have unearthed the synergetic effect of electrochemically active iron oxide films coating and partial doping of iron on LiMn1.5Ni0.5O4 (LMNO) particles. The ionic Fe penetrates into the lattice structure of LMNO during the ALD process. After the structural defects were saturated, the iron started participating in formation of ultrathin oxide films on LMNO particle surface. Owing to the conductive nature of iron oxide films, with an optimal film thickness of ~0.6 nm, the initial capacity improved by ~25% at room temperature and by ~26% at an elevated temperature of 55 °C at a 1C cycling rate. The synergy of doping of LMNO with iron combined with the conductive and protective nature of the optimal iron oxide film led to a high capacity retention (~93% at room temperature and ~91% at 55 °C) even after 1,000 cycles at a 1C cycling rate.

Metallic Ternary Telluride with Sphalerite Superstructure.

A new ternary compound with composition Cu5Sn2Te7 has been synthesized using the stoichiometric reaction of Cu, Sn, and Te. The compound crystallizes in C2 space group with unit cell parameters of a = 13.549(2) Å, b = 6.0521(11) Å, c = 9.568(2) Å, and ß = 98.121(2)°. Cu5Sn2Te7 is a superstructure of sphalerite and exhibits tetrahedral coordination of Cu, Sn, and Te atoms, containing a unique adamantane-like arrangement. The compound is formally mixed valent with a high electrical conductivity of 9.8 × 10(5) S m(-1) at 300 K and exhibits metallic behavior having p-type charge carriers as indicated from the positive Seebeck coefficient. Hall effect measurements further confirm holes as charge carriers with a carrier density of 1.39 × 10(21) cm(-3) and Hall mobility of 4.5 cm(2) V(-1) s(-1) at 300 K. The electronic band structure calculations indicate the presence of a finite density of states around the Fermi level and agree well with the p-type metallic conductivity. Band structure analysis suggests that the effective mass of the hole state is small and could be responsible for high electronic conductivity and Hall mobility. The high thermal conductivity of 15.1 W m(-1) K(-1) at 300 K coupled with the low Seebeck coefficient results in a poor thermoelectric figure of merit (ZT) for this compound. Theoretical calculations indicate that if Cu5Sn2Te7 is turned into a valence precise compound by substituting one Cu by a Zn, a semiconducting material, Cu4ZnSn2Te7, with a direct band gap of ∼ 0.5 eV can be obtained.

Phosphite as Polyanion-Based Cathode for Li-Ion Battery: Synthesis, Structure, and Electrochemistry of LiFe(HPO3)2.

A new lithium containing iron(III) phosphite, LiFe(HPO3)2, has been synthesized via a solvent-free, low temperature, solid-state synthesis route. The crystal structure of this material has been determined employing single-crystal X-ray diffraction, which indicates that the compound has a three-dimensional structure formed by isolated FeO6 octahedral units joined together via bridging HPO3 pseudopyramidal moieties. This arrangement leads to the formation of channels along all the three crystallographic directions, where channels along the a- and b-axes host Li(+) ions. The compound was further characterized by TGA, IR, and Mössbauer spectroscopic techniques. Additionally, it has been demonstrated that this phase is electrochemically active toward reversible intercalation of Li(+) ions and thus can be used as a cathode material in Li-ion cells. An average discharge potential of 2.8 V and a practical capacity of 70 mAh·g(-1) has been achieved as indicated by the results of cyclic voltammetry and galvanostatic charge-discharge tests.

Alkali-metal thiogermanates: sodium channels and variations on the La3CuSiS7 structure type.

Five new isotypic quaternary chalcogenides containing rare-earth metal atoms crystallizing in the hexagonal noncentrosymmetric space group P6(3) (No. 173) with the La(3)CuSiS(7) structure type have been synthesized by reacting the appropriate anhydrous rare-earth trichloride with sodium thiogermanate, Na(2)GeS(3). The reaction between LnCl(3) and Na(2)GeS(3) in an evacuated fused-silica ampule produced high yields of good-quality crystals of NaLn(3)GeS(7) [Ln = Ce (I), Nd (II), Sm (III), Gd (IV), and Yb (V)], while a similar reaction between EuCl(3) and Na(2)GeS(3) yielded a quinary chloride thiogermanate, Na(1.2)Eu(3.4)Cl(2)Ge(3)S(9) (VI), incorporating a cyclic trimeric Ge(3)S(9) building unit and adopting a structure related to La(3)CuSiS(7). The crystal structure of the compounds comprises a complex network of bicapped trigonal-prismatic LnS(8) and GeS(4) tetrahedra, which creates channels along the [001] direction. The Na(+) cations reside in these channels within trigonally distorted octahedral coordination environments, surrounded by six S atoms. For compounds III-V, the temperature dependence of the magnetic susceptibility indicates that these compounds are paramagnetic with µ(eff). = 1.86, 8.01, and 3.87 µ(B), for III-V, respectively. The experimental µ(eff) for IV is close to the theoretical value of 7.94 for free Gd(3+) ions, while µ(eff) values for III and V deviate from their theoretical values of 0.86 and 4.54 µ(B) for Sm(3+) and Yb(3+) ions, respectively. These compounds are semiconductors with optical band gaps of around 1.3 eV for III and V. Extended Hückel calculations suggest that the valence band comprises primarily S 3p and the bottom of the conduction band is dominated by empty rare-earth 5d orbitals. Compound VI exhibits a sharp optical absorption of around 2.18 eV, which is attributed to the f → d transition of Eu(II). The effective magnetic moment of 7.94 µ(B)/Eu is in excellent agreement with the theoretical value of 7.94 µ(B) for the free Eu(2+) ion.