High Lithium Capacity MxV2O5Ay·nH2O for Rechargeable Batteries

The aqueous synthesis and electrochemical properties of nanocrystalline M_xV₂O₅A_y·nH₂O are described. It is easily and quickly prepared by precipitation from acidified vanadate solutions. M_xV₂O₅A_y·nH₂O has been characterized by X-ray powder diffraction, electron microscopy, TGA, chemical analyses, and electrochemical studies. The atomic structure is related to that of xerogel-derived V₂O₅·nH₂O. In M_xV₂O₅A_y·nH₂O, M is a cation from the starting vanadate salt and A is an anion from the mineral acid. This material exhibits high, reversible Li capacity and may be considered for use in a cathode in primary and secondary batteries. The lithium capacity of an electrode composed of M_xV₂O₅A_y·nH₂O/EPDM/carbon (88/4/8) is ∼380(mA h)/g (C/80 rate) and the energy density is ∼1000(W h)/kg (120-μm-thick cathode, 4-1.5 V, versus Li metal anode). Critical parameters identified in the synthesis of M_xV₂O₅A_y·nH₂O, with respect to achieving high Li-ion insertion capacity, are acid/vanadium ratio, starting vanadate salt, and temperature. Inclusion of carbon black in the synthesis yields a composite that maintains the high Li capacity, lowers the electrochemical-cell polarization, and preserves the lithium capacity at higher discharge rates. Li-ion coin cells, using pre-lithiated graphite anodes, exhibit electrochemical performance comparable to that of Li-metal coin cells.     

Long-range ordering in the Bi1−xAexFeO3−x/2 perovskites: Bi1/3Sr2/3FeO2.67 and Bi1/2Ca1/2FeO2.75

Two-ordered perovskites, Bi_1/3Sr_2/3FeO_2.67 and Bi_1/2Ca_1/2FeO_2.75, have been stabilized and characterized by transmission electron microscopy, Mössbauer spectroscopy and X-ray powder diffraction techniques. They both exhibit orthorhombic superstructures, one with a≈b≈2a_p and c≈3a_p (S.G.: Pb2n or Pbmn) for the Sr-based compound and one with a≈b≈2a_p and c≈8a_p (S.G.: B222, Bmm2, B2mm or Bmmm) for the Ca-based one. The high-resolution transmission electron microscopy (HRTEM) images evidence the existence of one deficient [FeO_x]_∞ layer, suggesting that Bi_1/3Sr_2/3FeO_2.67 and Bi_1/2Ca_1/2FeO_2.75 behave differently compared to their Ln-based homolog. The HAADF-STEM images allow to propose a model of cation ordering on the A sites of the perovskite. The Mössbauer analyses confirm the trivalent state of iron and its complex environment with three types of coordination. Both compounds exhibit a high value of resistivity and the inverse molar susceptibility versus temperature curves evidence a magnetic transition at about 730 K for the Bi_1/3Sr_2/3FeO_2.67 and a smooth reversible transition between 590 and 650 K for Bi_1/2Ca_1/2FeO_2.75.     

Charge compensation and oxidation in NaxCoO2−δ and LixCoO2−δ studied by XANES

A comparative study on the oxidation and charge compensation in the A_xCoO_2−δ systems, A=Na (x=0.75, 0.47, 0.36, 0.12) and Li (x=1, 0.49, 0.05), using X-ray absorption spectroscopy at O 1s and Co 2p edges is reported. Both the O 1s and Co 2p XANES results show that upon removal of alkali metal from A_xCoO_2−δ the valence of cobalt increases more in Li_xCoO_2−δ than in Na_xCoO_2−δ. In addition, the data of O 1s XANES indicate that charge compensation by oxygen is more pronounced in Na_xCoO_2−δ than in Li_xCoO_2−δ.     

Lanthanum Gallium Tin Antimonides LaGaxSnySb2

A series of quaternary lanthanum gallium tin antimonides LaGa_xSn_ySb₂ was elaborated to trace the structural evolution between the known end members LaGaSb₂ (SmGaSb₂-type) and LaSn_ySb₂ (LaSn_0.75Sb₂-type). Five members of this series were characterized by single-crystal X-ray diffraction. For low Sn content, the Sn atoms disorder with Ga atoms in zigzag chains to form solid solutions LaGa_1-ySn_ySb₂ (0≤y≤0.2) adopting the SmGaSb₂-type structure, as exemplified by LaGa_0.92(3)Sn_0.08Sb₂ and LaGa_0.80(3)Sn_0.20Sb₂ (orthorhombic, space group D⁵₂−C222₁,Z=4). For higher Sn and lower Ga content, there is a segregation in which the Sn atoms appear in chains of closely spaced partially occupied sites as in the parent LaSn_0.75Sb₂-type structure whereas the Ga atoms remain in zigzag chains as in the parent SmGaSb₂-type structure. This feature is observed in the structures of LaGa_0.68(4)Sn_0.31(3)Sb₂, LaGa_0.62(3)Sn_0.32(3)Sb₂, and LaGa_0.43(3)Sn_0.39(3)Sb₂ (orthorhombic, space group D¹⁷_2h−Cmcm,Z=4). The last example illustrates that the combined Ga/Sn content can be substoichiometric (x+y<1). These compounds have a layered nature, with the chains of Ga or Sn atoms residing between ²_∞[LaSb₂] slabs.     

Far-infrared and X-ray investigations of the mixed platinum dichalcogenides PtS2−xSex, PtSe2−xTex, and PtS2−xTex

Polycrystalline samples of the CdI₂-type mixed crystals PtS_2?xSe_x, PtSe_2?xTe_x, and PtS_2?xTe_x were prepared and characterized by far-infrared (4000-40 cm^?1) and X-ray techniques. A change in the behavior of the c/a-ratio and in the x dependence of the plasma resonance frequency of the free carriers present is observed in the system PtS_2?xSe_x near PtS_0.6Se_1.4, which is interpreted as a critical composition for (p,d)-band overlap in the platinum dichalcogenides. Optical phonon frequencies are given for the two-mode system PtS_2?xSe_x.     

Relationship between interlayer hydration and photocatalytic water splitting of A′1−xNaxCa2Ta3O10·nH2O (A′=K and Li)

Partial replacement of alkaline metals in anhydrous KCa₂Ta₃O₁₀ and LiCa₂Ta₃O₁₀ was studied to control interlayer hydration and photocatalytic activity for water splitting under UV irradiation. A′_1−xNa_xCa₂Ta₃O₁₀·nH₂O (A′=K and Li) samples were synthesized by ion exchange of CsCa₂Ta₃O₁₀ in mixed molten nitrates at 400 °C. In K_1−xNa_xCa₂Ta₃O₁₀·nH₂O, two phases with the orthorhombic (C222) and tetragonal (I4/mmm) structures were formed at x?0.7 and x?0.5, respectively. Upon replacement by Na⁺ having a larger enthalpy of hydration (ΔH_h⁰), the interlayer hydration occurred at x?0.3 and the hydration number (n) was increased monotonically with an increase of x. Li_1−xNa_xCa₂Ta₃O₁₀·nH₂O showed a similar hydration behavior, but the phase was changed from I4/mmm (x<0.5, n∼0) via P4/mmm (x∼0.5, n∼1) to I4/mmm (x∼1.0, n∼2). The photocatalytic activities of these systems after loading 0.5 wt% Ni were quite different each other. K_1−xNa_xCa₂Ta₃O₁₀·nH₂O exhibited the activity increasing in consistent with n, whereas Li_1−xNa_xCa₂Ta₃O₁₀·nH₂O exhibited the activity maximum at x=0.77, where the rates of H₂/O₂ evolution were nearly doubled compared with those for end-member compositions (x=0 and 1).     

Water-containing derivative phases of the Srn+1FenO3n+1 series

The n=1, 2, 3 and ∞ members of the homologous series Sr_n+1Fe_nO_3n+1 of layered iron oxides are investigated for their tendency to accept additional layers of water in their crystals. The phases possess a Ruddlesden-Popper-type SrO-(SrO-FeO₂)_n crystal structure, where the n=∞ limit is nothing but the perovskite structure. It is revealed that the n=1, 2 and 3 phases readily accommodate one or two layers of water between adjacent SrO layers, whereas the n=∞ member which lacks the SrO-SrO double-layer unit remains intact in the presence of water. The speed of the water intercalation process is found to decrease with increasing n. Among the layered water derivatives, the n=2 phase with two water molecules per formula unit, i.e. Sr₃Fe₂O₇·2H₂O, was found to be most stable.     

Preparation and properties of the oxyfluoride systems V2O5−xFx and VO2−xFx

Partial substitution of fluorine for oxygen in VO₂ and V₂O₅ was achieved by reacting V and V₂O₅ under 1.33 kb pressure in the presence of concentrated or dilute solutions of HF. Two new phases having the composition V₂O_5−xF_x (0 < x < 0.025) and VO_2−xF_x (0 < x < 0.2) were prepared. X-Ray diffraction studies have been carried out on both phases and show the structure of V₂O_5−xF_x to be orthorhombic and isostructural to V₂O₅, while VO_2−xF_x has a tetragonal structure of the rutile type (for x ? 0.03). Single-crystal-resistivity data show V₂O_5−xF_x to be a semiconductor, whereas VO_2−xF_x undergoes a metallic to semiconductor transition at a temperature solely dependent upon the value of x.     

Visible light-sensitive yellow TiO2−xNx and Fe-N co-doped Ti1−yFeyO2−xNx anatase photocatalysts

Nitrogen substituted yellow colored anatase TiO_2−xN_x and Fe-N co-doped Ti_1−yFe_yO_2−xN_x have been easily synthesized by novel hydrazine method. White anatase TiO_2−δ and N/Fe-N-doped samples are semiconducting and the presence of ESR signals at g ∼1.994-2.0025 supports the oxygen vacancy and g∼4.3 indicates Fe³⁺ in the lattice. TiO_2−xN_x has higher conductivity than TiO_2−x and Fe/Fe-N-doped anatase and the UV absorption edge of white TiO_2−x extends in the visible region in N, Fe and Fe-N co-doped TiO₂, which show, respectively, two band gaps at ∼3.25/2.63, ∼3.31/2.44 and 2.8/2.44 eV. An activation energy of ∼1.8 eV is observed in Arrhenius log resistivity vs. 1/T plots for all samples. All TiO₂ and Fe-doped TiO₂ show low 2-propanol photodegradation activity but have significant NO photodestruction capability, both in UV and visible regions, while standard Degussa P-25 is incapable in destroying NO in the visible region The mid-gap levels that these N and Fe-N-doped TiO₂ consist may cause this discrepancy in their photocatalytic activities.     

Synthesis of Li(x)Na(2−x)Mn2S3 and LiNaMnS2 through redox-induced ion exchange reactions

Na₂Mn₂S₃ was oxidatively deintercalated using iodine in acetonitrile to yield Na_1.3Mn₂S₃, with lattice constants nearly identical to that of the reactant. Lithium was then reductively intercalated into the oxidized product to yield Li_0.7Na_1.3Mn₂S₃. When heated, this metastable compound decomposed to form a new crystalline compound, LiNaMnS₂, along with MnS and residual Na₂Mn₂S₃. Single crystal X-ray diffraction structural analysis of LiNaMnS₂ revealed that this compound crystallizes in P-3m1 with cell parameters a=4.0479(6) Å, c=6.7759(14) Å, V=96.15(3) Å³ (Z=1, wR2=0.0367) in the NaLiCdS₂ structure-type.     

[R-C7H16N2][V2Te2O10] and [S-C7H16N2][V2Te2O10]; new polar templated vanadium tellurite enantiomers

New polar vanadium tellurite enantiomers have been synthesized under mild hydrothermal conditions through the use of sodium metavanadate, sodium tellurite and enantiomerically pure sources of either R-3-aminioquinuclidine or S-3-aminioquinuclidine. [R-C₇H₁₆N₂][V₂Te₂O₁₀] and [S-C₇H₁₆N₂][V₂Te₂O₁₀] contain [V₂Te₂O₁₀]_n²ⁿ⁻ layers constructed from [(VO₂)₂O(TeO₄)₂] monomers. Steric effects associated with the hydrogen-bonding network between the [V₂Te₂O₁₀]_n²ⁿ⁻ layers and [C₇H₁₆N₂]²⁺ result in polar structures and crystallization in the space group P2₁ (no. 4). Electron localization functions were calculated to visualize the tellurite stereoactive lone pairs. Both iterative and non-iterative Hirshfeld techniques were evaluated as means to determine atomic partial charges, with iterative Hirshfeld charges more accurately representing charge distributions in the reported enantiomers. These charges were used to calculate both component and net dipole moments. [R-C₇H₁₆N₂][V₂Te₂O₁₀] and [S-C₇H₁₆N₂][V₂Te₂O₁₀] exhibit dipole moments of 17.37 and 16.62D, respectively. [R-C₇H₁₆N₂][V₂Te₂O₁₀] and [S-C₇H₁₆N₂][V₂Te₂O₁₀] both display type 1 phase-matching capabilities and exhibit second harmonic generation activities of ∼50×α-SiO₂.     

Polymorphism in the Sc2Si2O7-Y2Si2O7 system

This paper examines the structural changes with temperature and composition in the Sc₂Si₂O₇-Y₂Si₂O₇ system; members of this system are expected to form in the intergranular region of Si₃N₄ and SiC structural ceramics when sintered with the aid of Y₂O₃ and Sc₂O₃ mixtures. A set of different compositions have been synthesized using the sol-gel method to obtain a xerogel, which has been calcined at temperatures between 1300 and 1750 °C during different times. The temperature-composition diagram of the system, obtained from powder XRD data, is dominated by the β-RE₂Si₂O₇ polymorph, with γ-RE₂Si₂O₇ and δ-RE₂Si₂O₇ showing very reduced stability fields. Isotherms at 1300 and 1600 °C have been analysed in detail to evaluate the solid solubility of the components. Although, the XRD data show a complete solid solubility of β-Sc₂Si₂O₇ in β-Y₂Si₂O₇ at 1300 °C, the ²⁹Si MAS-NMR spectra indicate a local structural change at x ca. 1.15 (Sc_2−xY_xSi₂O₇) related to the configuration of the Si tetrahedron, which does not affect the long-range order of the β-RE₂Si₂O₇ structure. Finally, it is interesting to note that, although Sc₂Si₂O₇ shows a unique stable polymorph (β), Sc³⁺ is able to replace Y³⁺ in γ-Y₂Si₂O₇ in the compositional range 1.86?x?2 (where x is Sc_2−xY_xSi₂O₇) as well as in δ-Y₂Si₂O₇ for compositions much closer to the pure Y₂Si₂O₇.     

Synthesis of the first difunctional N-fluoro perfluoroalkylsulfonylimides, CF3SO2NFSO2(CF2)nSO2NFSO2CF3

Synthesis of difunctional N,N′-difluoro perfluoroalkylsulfonamides, CF₃SO₂NFSO₂(CF₂)_nSO₂NFSO₂CF₃, where n=4, 6 is reported. A related compound with an oxygen linkage CF₃SO₂NFSO₂(CF₂)₂O(CF₂)₂SO₂NFSO₂CF₃ has also been prepared. These reagents showed good activity for electrophilic fluorination.     

Syntheses, structures, optical properties, and theoretical calculations of Cs2Bi2ZnS5, Cs2Bi2CdS5, and Cs2Bi2MnS5

Three new compounds, Cs₂Bi₂ZnS₅, Cs₂Bi₂CdS₅, and Cs₂Bi₂MnS₅, have been synthesized from the respective elements and a reactive flux Cs₂S₃ at 973 K. The compounds are isostructural and crystallize in a new structure type in space group Pnma of the orthorhombic system with four formula units in cells of dimensions at 153 K of a=15.763(3), b=4.0965(9), c=18.197(4) Å, V=1175.0(4) Å³ for Cs₂Bi₂ZnS₅; a=15.817(2), b=4.1782(6), c=18.473(3)  Å, V=1220.8(3)  ų for Cs₂Bi₂CdS₅; and a=15.830(2), b=4.1515(5), c=18.372(2) Å, V=1207.4(2) Å³ for Cs₂Bi₂MnS₅. The structure is composed of two-dimensional _∞²[Bi₂MS₅²⁻] (M=Zn, Cd, Mn) layers that stack perpendicular to the [100] axis and are separated by Cs⁺ cations. The layers consist of edge-sharing _∞¹[Bi₂S₆⁶⁻] and _∞¹[MS₃⁴⁻] chains built from BiS₆ octahedral and MS₄ tetrahedral units. Two crystallographically unique Cs atoms are coordinated to S atoms in octahedral and monocapped trigonal prismatic environments. The structure of Cs₂Bi₂MS₅, is related to that of Na₂ZrCu₂S₄ and those of the AMM′Q₃ materials (A=alkali metal, M=rare-earth or Group 4 element, M′= Group 11 or 12 element, Q=chalcogen). First-principles theoretical calculations indicate that Cs₂Bi₂ZnS₅ and Cs₂Bi₂CdS₅ are semiconductors with indirect band gaps of 1.85 and 1.75 eV, respectively. The experimental band gap for Cs₂Bi₂CdS₅ is ≈1.7 eV, as derived from its optical absorption spectrum.     

Structural and physical properties of Mg3−xZnxSb2 (x=0-1.34)

The Mg_3−xZn_xSb₂ phases with x=0-1.34 were prepared by direct reactions of the elements in tantalum tubes. According to the X-ray single crystal and powder diffraction, the Mg_3−xZn_xSb₂ phases crystallize in the same P3¯m1 space group as the parent Mg₃Sb₂ phase. The Mg_3−xZn_xSb₂ structure is different from the other substituted structures of Mg₃Sb₂, such as (Ca, Sr, Ba) Mg₂Sb₂ or Mg_5.23Sm_0.77Sb₄, in a way that in Mg_3−xZn_xSb₂ the Mg atoms on the tetrahedral sites are replaced, while in the other structures Mg on the octahedral sites is replaced. Thermoelectric performance for the two members of the series, Mg₃Sb₂ and Mg_2.36Zn_0.64Sb₂, was evaluated from low to room temperatures through resistivity, Seebeck coefficient and thermal conductivity measurements. In contrast to Mg₃Sb₂ which is a semiconductor, Mg_2.36Zn_0.64Sb₂ is metallic and exhibits an 18-times larger dimensionless figure-of-merit, ZT, at room temperature. However, thermoelectric performance of Mg_2.36Zn_0.64Sb₂ is still poor and it is mostly due to its large electrical resistivity.     

Three-dimensional five-connected coordination polymer [M2(C3H2O4)2(H2O)2(μ2-hmt)]n with 46 topologies (M=Zn, Cu; hmt=hexamethylenetetramine)

Two novel three-dimensional five-connected coordination polymers [M₂(C₃H₂O₄)₂(H₂O)₂(μ₂-hmt)]_n with 4⁴6⁶ topologies (M=Zn, Cu; hmt=hexamethylenetetramine) were synthesized and characterized by elemental analysis, crystal structure, IR, thermal gravimetric analyses. Both [Zn₂(C₃H₂O₄)₂(H₂O)₂(μ₂-hmt)]_n and [Cu₂(C₃H₂O₄)₂(H₂O)₂(μ₂-hmt)]_n all crystallize in the orthorhombic system, space group Imm2, and with Z=2. Metal ions have all octahedral geometry coordinated by four oxygen atoms from three malonates, one oxygen atom from a water molecule and one nitrogen atom of hmt ligand. Each malonate binds a metal ion with its two oxygen atoms in a chelating mode and connects to adjacent two metal ions with another two oxygen atoms to form an infinite wavy layer. The layers are bridged by μ₂-hmt molecules to form a three-dimensional framework with channels. The magnetic susceptibility data show there is a weak antiferromagnetic exchange interaction in the complex [Cu₂(C₃H₂O₄)₂(H₂O)₂(μ₂-hmt)]_n.     

Etude par diffraction neutronique de la phase “Cs4−xYb12F40−x” (0 ≤ x ≤ 1): Hypothèse structurale

The structure of the phase Cs_4?xYb₁₂F_40?x(0 ≤ x ≤ 1) has been determined by a single-crystal neutron diffraction study. It has been solved in the space group P6₃mc and refined to the best R factor of 0.0535 for the formula Cs_3.4Yb₁₂F_39.4 (324 independent reflections). Three edge-sharing pentagonal bipyramids surrounding three ytterbium atoms form Yb₃F₁₆ groups and the structure is described as the superposition, according to the sequence A₁A₂B₁B₂A₁A₂…, of sheets of corner-sharing Yb₃F₁₆ groups with a possible transformation of bipyramids into octahedra in the A₂ and B₂ layers. These sheets are joined together by the axial fluorine atoms of the bipyramids or octahedra. Cesium atoms are located in the tunnels formed by their stacking. It is shown that the Cs_4?xYb₁₂F_40?x phase (0 ≤ x ≤ 1) is an intermediate step of the Cs_4?xYb₁₂F_40?x solid solution observed with 0 ≤ x ≤ 2 and corresponds to a superstructure of the high-temperature YbF₃ phase.     

Etude structurale des composés MxTiSe2 (M = Fe,Co, Ni)

New compounds M_xTiSe₂ have been prepared with M = Fe (x ? 0.66), M = Co or Ni (x ? 0.50). The metal M is located in vacant octahedral sites of the TiSe₂ host lattice (hexagonal unit cell a′, c′). An ordering of vacancies occurs if x ? 0.20. With M = Co or Ni (x = 0.50) and with M = Fe (0.25 ? x ? 0.66) isotypic compounds of Ti₃Se₄ can be obtained (M₃ □ X₄ type; monoclinic unit cell a ≈ a′ √3, b ≈ a′, c ≈ 2c′). The compounds Fe_0.38TiSe₂ and Co_0.38TiSe₂ (hexagonal unit cell a ≈ a′ √3, c ≈ 2c′) are of the M₂ □ X₃ type, variety 2c′. The Fe_0.25TiSe₂ and Co_0.25TiSe₂ monoclinic unit cells (a ≈ 2a′ √3, b ≈ 2a′, c ≈ 2c′) allow us to assume, for these two compounds, a structure of the M₅ □ ₃X₈ type, variety 2c′, identical to the Ti₅Se₈ one. The compound Ni_0.25TiSe₂ has an hexagonal unit cell (a ≈ 2a′, c ≈ 3c′); it belongs to a so-called 3c′ variety of the M₅ □ ₃X₈ type.     

Crystal structure and magnetic properties of Ce7Ni5±xGe3±xIn6 and Pr7Ni5±xGe3±xIn6

The indides Ce₇Ni_5±xGe_3±xIn₆ and Pr₇Ni_5±xGe_3±xIn₆ were synthesized from the elements by arc-melting of the components. Single crystals were grown via special annealing sequences. Both structures were solved from X-ray single crystal diffraction data: new structure type, P6/m, Z=1, a=11.385(2), c=4.212(1) Å, wR₂=0.0640, 634F² values, 25 variables for Ce₇Ni_4.73Ge_3.27In₆ and a=11.355(6), c=4.183(2) Å, wR₂=0.0539, 563F² values, 25 variables for Pr₇Ni_4.96Ge_3.04In₆. Both indides show homogeneity ranges through Ni/Ge mixing (M sites). This new structure type can be derived from the AlB₂ structure type by a substitution of the Al and B atoms by CeM₁₂ and NiIn₆Ce₃ polyhedra (tricapped trigonal prism). Magnetic susceptibility measurements on a polycrystalline sample of Ce₇Ni₅Ge₃In₆ indicated Curie-Weiss like paramagnetic behavior down to 1.71 K with the effective magnetic moment slightly reduced in relation to the value expected for trivalent cerium ions. No magnetic ordering is evident.     

Synthesis and solid state studies on Mb2Sb2O7 and (Mn1−xCdx)2Sb2O7 pyrochlores

Pyrochlore oxides of the type Mn₂Sb₂O₇ and (Mn_1?xCd_x)₂Sb₂O₇ have been synthesized by high-temperature solid state reactions and characterized by X-ray diffraction and chemical analysis. X-Ray diffraction studies showed that the compound Mn₂Sb₂O₇ has a rhombohedrally distorted pyrochlore structure. In the solid solutions (Mn_1?xCd_x)₂Sb₂O₇, the phases with x ≥ 0.6 are cubic. Magnetic and ¹²¹Sb Mössbauer studies indicate that all the Mn and Sb are present in the +2 and +5 state occupying A and B sites, respectively, in the pyrochlore structure. Electrical measurements indicate that the compounds are insulators or semiconductors exhibiting p-type behavior. The stoichiometry and probable cause of the rhombohedral distortion in Mn₂Sb₂O₇ and solid solutions are discussed.