Sodium and Manganese Stoichiometry of P2‐Type Na2/3MnO2

To realize a reversible solid‐state Mn^III/IV redox couple in layered oxides, co‐operative Jahn–Teller distortion (CJTD) of six‐coordinate Mn^III (t_2g³–e_g¹) is a key factor in terms of structural and physical properties. We develop a single‐phase synthesis route for two polymorphs, namely distorted and undistorted P2‐type Na_2/3MnO₂ having different Mn stoichiometry, and investigate how the structural and stoichiometric difference influences electrochemical reaction. The distorted Na_2/3MnO₂ delivers 216 mAh g^?1 as a 3 V class positive electrode, reaching 590 Wh (kg oxide)^?1 with excellent cycle stability in a non‐aqueous Na cell and demonstrates better electrochemical behavior compared to undistorted Na_2/3MnO₂. Furthermore, reversible phase transitions correlated with CJTD are found upon (de)sodiation for distorted Na_2/3MnO₂, providing a new insight into utilization of the Mn^III/IV redox couple for positive electrodes of Na‐ion batteries.     

An investigation of the Cr1−xMnxO2 series and the characterization of orthorhombic CrMnO4

Several members of the Cr_1?xMn_xO₂ series were prepared in the tetrahedral anvil press by the reaction of CrO₂ with MnO₂. The tetragonal, rutile-type products were single-phase and have been characterized by crystallographic and magnetic measurements. The results are consistent with the formulations Cr⁴⁺_1?2xCr³⁺ Mn⁵⁺O₂ for 0 ? x ? 0.5. At low manganese concentration, x < 0.20, the magnetic moments are consistent with ferromagnetic contribution from Mn⁵⁺. A two-phase product was noted at the composition x = 0.90. The CrMnO₄ composition was found to have a powder pattern similar to that of orthorhombic PtO₂.     

Thermochemistry of Li1+xMn2−xO4 (0?x?1/3) spinel

Lithium substituted Li_1+xMn_2−xO₄ spinel samples in the entire solid solution range (0?x?1/3) were synthesized by solid-state reaction. The samples with x<0.25 are stoichiometric and those with x?0.25 are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na₂O·4MoO₃ at 974 K was performed to determine their enthalpies of formation from constituent binary oxides at 298 K. The cubic lattice parameter was determined from least-squares fitting of powder XRD data. The variations of the enthalpy of formation from oxides and the lattice parameter with x follow similar trends. The enthalpy of formation from oxides becomes more exothermic with x for stoichiometric compounds (x<0.25) and deviates endothermically from this trend for oxygen-deficient samples (x?0.25). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn³⁺ to Mn⁴⁺ versus formation of oxygen vacancies). For stoichiometric spinels, the oxidation of Mn³⁺ to Mn⁴⁺ is dominant, whereas for oxygen-deficient compounds both mechanisms are operative. The endothermic deviation is ascribed to the large endothermic enthalpy of reduction.     

High-resolution X-ray diffraction, DIFFaX, NMR and first principles study of disorder in the Li2MnO3-Li[Ni1/2Mn1/2]O2 solid solution

X-ray diffraction patterns of Li[Li_(1−2x)/3Ni_xMn_2/3−x/3]O₂ show reflections between 20° and 35°, 2θ (CuKα) due to Li₂MnO₃-like ordering of the transition metal (Ni, Mn and Li) layers. The ordering is rarely perfect, resulting in characteristic broadening and changes in the intensities of these reflections. ⁶Li MAS NMR studies of Li₂MnO₃ show that the [Li_1/3Mn_2/3] layers are well ordered, the disorder arising from the stacking of the layers in the c-direction. DIFFaX was used to model the Li₂MnO₃ XRD data and extract the stacking fault frequency. The results show that even well-ordered cation layers may show only weak or no superstructure reflections, if there is no or little ordering in the c-direction.     

Hydrothermal synthesis of two perovskite rare-earth manganites, HoMnO3 and DyMnO3

Two perovskite rare-earth manganites RMnO₃ (R=Ho, Dy) were synthesized from the hydrothermal redox reactions of KMnO₄ and MnCl₂ at 250 °C and characterized by means of X-ray diffraction, scanning electron microscopy and SQUID. They are orthorhombic, whereas the hexagonal phases, which were competitive strongly with the orthorhombic phases in solid-state reactions, are avoided in the hydrothermal systems. The pure metastable manganites may serve as a model for understanding the magnetisms of Jahn-Teller distortion and charge ordering. This new synthetic approach leaves many rooms for new doped or undoped RMnO₃ compounds.     

A Sodium Manganese Oxide Cathode by Facile Reduction for Sodium Batteries

A nonstoichiometric sodium manganese oxide (Na_xMnO_2+δ) cathode useful for sodium batteries was synthesized by an ambient‐temperature strategy that involved facile reduction of aqueous sodium permanganate in sodium iodide and subsequent heat treatment at 600 °C. Combined powder X‐ray diffraction and synchrotron X‐ray diffraction analyses confirmed the annealed sample to belong to a Na_xMnO₂ phase with a P2‐hexagonal structure. The ICP‐AES results confirmed the stoichiometry of the sample to be Na_0.53MnO_2+δ. Electron microscopy studies revealed the particle size of the electrode to be in the range of a few hundred nanometers. The Na_0.53MnO_2+δ cathode delivered an average discharge capacity of 170 mA h g^?1 with a stable plateau at 2.1 V for the initial 25 cycles versus sodium. Ex situ XANES studies confirmed the reversible intercalation of sodium into Na_0.53MnO_2+δ and suggested the accommodation of over‐stoichiometric Mn⁴⁺ ions to contribute towards the performance of the electrode.     

Synthesis and phase transition of Li-Mn-O spinels with high Li/Mn ratio by thermo-decomposition of LiMnC2O4(Ac)

LiMnC₂O₄(Ac) precursor in which Li⁺ and Mn²⁺ were amalgamated in one molecule was prepared by solid-state reaction at room-temperature using manganese acetate, lithium hydroxide and oxalic acid as raw materials. By thermo-decomposition of LiMnC₂O₄(Ac) at various temperatures, a series of Li_1+y[Mn_2−xLi_x]_16dO₄ spinels were prepared with Li₂MnO₃ as impurities. The structure and phase transition of these spinels were investigated by XRD, TG/DTA, average oxidation state of Mn and cyclic voltammeric techniques. Results revealed that the Li-Mn-O spinels with high Li/Mn ratio were unstable at high temperature, and the phase transition was associated with the transfer of Li⁺ from octahedral 16c sites to 16d sites. With the sintering temperature increasing from 450 to 850 °C, the phase structure varied from lithiated-spinel Li₂Mn₂O₄ to Li₄Mn₅O₁₂-like to LiMn₂O₄-like and finally to rock-salt LiMnO₂-like. A way of determining x with average oxidation state of Mn and the content of Li₂MnO₃ was also demonstrated.     

High-temperature neutron diffraction study of the cation ordered perovskites TbBaMn2O5+x and TbBaMn2O5.5−y

The miscibility of TbBaMn₂O_5+x and TbBaMn₂O_5.5−y has been investigated at 100-600 °C using in situ powder neutron diffraction. No miscibility is observed, and the two phases remain oxygen stoichiometric (x,y=0) at 600 °C. Structure refinement results show that neither material undergoes a phase transition in this temperature range. TbBaMn₂O₅ is Mn²⁺/Mn³⁺ charge ordered and any charge melting transition is >600 °C. This symmetry-broken charge ordering is remarkably robust in comparison to that in other oxides.     

Magnetic Properties of LnMnO3 (Ln=Ho, Er, Tm, Yb, and Lu)

Magnetic data are presented for LnMnO₃ (Ln=Ho, Er, Tm, Yb, and Lu) having the hexagonal crystal structure of P6₃cm. DC magnetization measurements show that magnetic order is not clearly observed for Ln=Ho-Yb, while an antiferromagnetic transition of the Mn³⁺ moments is found at ∼90 K for LuMnO₃, where the Lu³⁺ ion has no 4f localized moment. This is ascribed to both the paramagnetism of Ln³⁺ and the suppression of magnetization in the Mn³⁺ sublattices arising from strong antiferromagnetic interactions between Mn³⁺. Deviation from the Curie-Weiss law at low temperatures indicates the onset of antiferromagnetism. Some magnetization data of Ca-substituted compounds, Ln_0.5Ca_0.5MnO₃, which have the different crystal structure of orthorhombic Pnma, are also discussed briefly.     

Electrochemical intercalation and deintercalation of NaxMnO2 bronzes

Various Na_xMnO₂ bronzes have been electrochemically deintercalated. Na_0.40MnO₂ has a channel structure which is maintained for a large intercalation range (0.30 ≤ × ≤ 0.58). In order to explain the upper intercalation limit, an ordered sodium distribution between two types of Na⁺ sites is proposed. Na_0.70MnO₂ and α-NaMnO₂ have lamellar structures of P2 and 0′3 types. During intercalation the original P2 type is maintained for 0.45 ≤ × ≤ 0.85 while two reversible structural transitions are observed from α-NaMnO₂. A similar behavior occurs during the deintercalation of the high-temperature β-NaMnO₂ variety. In each case of the structural transition the double octahedral layers remain unchanged. Electronic localization (increased by Mn³⁺ Jahn—Teller effect) tends to trap the Na⁺ ions and therefore increases the relaxation time of the investigated materials.     

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.     

Influence of Co doping on properties of Pr0.8Na0.2Mn(1−y)CoyO3 perovskites

The influence of the cobalt substitution for manganese ions in the series of the perovskites Pr_0.8Na_0.2Mn_(1−x)Co_xO₃ (0?x?0.1) was investigated. The study of electric and magnetic properties was carried out on sintered polycrystalline samples. The composition of x=0.04 exhibits an insulator to metal-like (I-M) transition at ∼106 K, connected with a ferromagnetic arrangement. For x=0.1, however, an insulating behavior persists down to low temperatures in spite of the transition to the bulk ferromagnetism. The observed properties are related to an acting of the cobalt ions as point defects. They disturb the tendency to charge ordering and instead of the antiferromagnetic arrangement typical for x=0 ferromagnetic double-exchange interactions Mn³⁺-O²⁻-Mn⁴⁺ and Mn^3.5+δ-O²⁻-Co²⁺, decisive for the resulting behavior, arise.     

Synthesis, structure and magnetic behavior of a new three-dimensional Manganese phosphite-oxalate: [C2N2H10][Mn2(OH2)2(HPO3)2(C2O4)]

A novel manganese phosphite-oxalate, [C₂N₂H₁₀][Mn₂^II(OH₂)₂(HPO₃)₂(C₂O₄)] has been hydothermally synthesized and its structure determined by single-crystal X-ray diffraction. The structure consists of neutral manganese phosphite layers, [Mn(HPO₃)]_∞, formed by MnO₆ octahedra and HPO₃ units, cross-linked by the oxalate moieties. The organic cations occupy the middle of the 8-membered one-dimensional channels. Magnetic studies indicate weak antiferromagnetic interactions between the Mn²⁺ ions.     

Disproportionation of stoichiometric LiMn2O4 on annealing in oxygen

The evolution of stoichiometric LiMn₂O₄ upon annealing under oxygen pressures in the range 0.2-5 atm at moderate temperature (450°C) was studied with a combination of thermogravimetry, X-ray and neutron diffraction. It is shown that such treatments result in a slight, but significant mass increase. Structural analyses show that the resulting spinel is a manganese-deficient spinel phase with lower cell parameter and higher manganese valence, and that the expelled manganese forms Mn₂O₃. The presence of this second phase, which was not identified in a recent study of oxygen annealing on this compound (Nakamura and Kajiyama, Solid State Ionics 133 (2000) 195), is compatible with the initial stoichiometry and does not require any oxygen vacancies in the initial LiMn₂O₄, as supposed earlier. The most likely formula of the resulting lithium-rich spinel with increased manganese valence is Li(Mn_2−ε□_ε)O₄ with ε in the range 0.02-0.03 at 5 atm O₂.     

微晶Na0.7MnO2.05的制备及其在水溶液中的充放电特性研究

利用十二核锰簇合物[Mn₁₂O₁₂(CH₃COO)₁₆(H₂O)₄]为前驱物,通过先碱解再灼烧的方法合成了一种钠锰氧化合物Na_0.7MnO_2.05。扫描电子显微镜(SEM)观察结果表明产物由微米级的扁平棒状晶体组成。电化学测试表明,Na_0.7MnO_2.05是一种性能比较优良的超级电容器电极材料。在0.5 mol·L^-1 Na₂SO₄电解质溶液中和0~0.8 V电位窗口范围内,具有良好的循环稳定性能,充放电速率为0.125A·g^-1时单电极比电容达121 F·g^-1。     

Effects of Metal Ion Sources on Synthesis and Electrochemical Performance of Spinel LiMn2O4 Using Tartaric Acid Gel Process

The effect of lithium and manganese ions on the synthesis, phase purity, and electrochemical properties of tartaric acid gel processed lithium manganese oxide spinel were investigated. The poor bonding between both lithium and manganese ions with tartaric acid was shown by the FT-IR analysis when lithium nitrate and/or manganese nitrate were used as sources. Li₂MnO₃ and Mn₂O₃ impurities formed in addition to lithium manganese oxides when nitrate salts were used as the sources. When acetate salts were used as sources for the lithium and manganese ions, single-phase LiMn₂O₄ was obtained. These results indicate that homogeneous bonding between acetate salt and tartaric acid was formed. The capacity of single-phase LiMn₂O₄ calcined at 500°C was 117 mAh/g which was much higher than those containing Mn₂O₃ and Li₂MnO₃ impurity compounds. Thus, sources of lithium and manganese ions play an important role in the synthesis and electrochemical behaviors of lithium manganese oxide spinel.     

Hydrothermal Synthesis and Spectroscopic and Magnetic Behavior of the Mn7(HOXO3)4(XO4)2(X=As, P) Compounds. Crystal Structure of Mn7(HOAsO3)4(AsO4)2

The Mn₇(HOXO₃)₄(XO₄)₂ (X=As, P) compounds have been synthesized by using hydrothermal conditions. The arsenate phase was obtained under autogeneous pressure at 170°C. However, more drastic conditions at both pressure and temperature were necessary in the attainment of the phosphate compound. The crystal structure of Mn₇(HOAsO₃)₄(AsO₄)₂ was solved using single-crystal data. The unit-cell parameters are a=6.810(3) Å, b=8.239(2) Å, c=10.011(4) Å, α=104.31(2)°, β=108.94(3)°, γ=101.25(2)°. Triclinic, P-1 with Z=1. The isostructural Mn₇(HOPO₃)₄(PO₄)₂ phase was characterized from X-ray powder diffraction techniques. The crystal structure of both compounds consists of zig-zag chains constructed by dimeric edge-sharing Mn₂O₁₀ octahedra linked through the MnO₅ trigonal bipyramids. The three-dimensional framework is completed by the connection between isolated MnO₆ entities to the dimers octahedra and trigonal bipyramids. The existence of hydrogenarsenate and hydrogenphosphate anions has been confirmed by IR and Raman spectroscopies. Magnetic measurements indicate the existence of antiferromagnetic interactions in both compounds, which are slightly stronger in the arsenate phase.     

Synthesis of o-LiMnO2 by Microwave Irradiation and StudyIts Heat Treatment and Lithium Exchange

Microwave irradiation of a suspension of γ-MnOOH in a 4 mol dm⁻³ LiOH solution brought about a rapid formation of semicrystalline orthorhombic LiMnO₂ (o-LiMnO₂) within 30 min at 120°C. Cubic Li_1.6Mn_1.6O₄ was obtained by heating o-LiMnO₂ at 400°C; lithium could be topotactically extracted from Li_1.6Mn_1.6O₄ with acid to form cubic H_1.6Mn_1.6O₄.     

Crystal structures and magnetic properties of CeAu4Si2 and CeAu2Si2

Single crystals of CeAu₄Si₂ and CeAu₂Si₂ have been grown out of ternary fluxes rich in Au, and the former, also by sintering the stoichiometric composition at 750 °C. The single-crystal X-ray refinement result for CeAu₄Si₂ is orthorhombic, Cmmm (No. 65, Z=2), different from a tetragonal result found from an X-ray powder diffraction refinement [H. Nakashima, et al., J. Alloys Compds. 424 (2006) 7]. For CeAu₂Si₂, this is the first report of the stoichiometric crystalline phase, in the known tetragonal I4/mmm structure. The anisotropic field- and temperature-dependent magnetizations, as well as specific heat and resistivity data are compared. Although both compounds have related structural packing, they present unique magnetic features. CeAu₂Si₂ is a typical antiferromagnet with T_N=8.8(1) K and CeAu₄Si₂ features a ferromagnetic component below T_c=3.3(1) K. Both phases have effective moments close in value to that of free Ce³⁺.