Solvothermal Synthesis of Superfine Li1−xMn2O4−σ Powders

Superfine Li_1−xMn₂O_4−σ powders were successfully synthesized by the alcohol-thermal method using 0.01 mol of MnO₂, 0.01mol of LiOH·H₂O, and 0.06mol of NaOH as starting materials at 160-200°C. The products are characterized by XRD, TEM, ED, BET, and ICP. Results show that the Li_0.74Mn₂O_3.74 powder prepared at 200°C has an average size of 180 nm with BET surface areas of 16.44 m²/g. A possible formation mechanism is proposed. It was concluded that the alcohol acts not only as the solvent but also as the reducing agent in the synthesis of Li_1−xMn₂O_4−σ powders. The effects of reaction temperature and the contents of NaOH and LiOH on the formation of single phase Li_1−xMn₂O_4−σ were investigated.     

The electronic structure of the CuRh1−xMgxO2 thermoelectric materials: An X-ray photoelectron spectroscopy study

A detailed X-ray photoelectron spectroscopy study has been performed for the CuRh_1−xMg_xO₂ (x=0, 0.04 and 0.10) series for a better understanding of the role of the Mg²⁺ substitution on the electrical properties and the value of the Seebeck coefficient. This study is based on an analysis of different compounds such as Rh₂O₃, Sr₂RhO₄ and CuCrO₂ in order to characterize different oxidation states (Rh³⁺ and Rh⁴⁺ in octahedral oxygen environment and Cu⁺ in a dumbbell O-Cu-O coordination). The Cu2p signal of copper in the non-doped compounds CuCrO₂ and CuRhO₂ reveals different electronic structures. An evolution of the Cu2p core signal with the increase of Mg²⁺ content in the CuRh_1−xMg_xO₂ is highlighted by XPS. The differences observed, especially for the Cu2p core peaks are discussed for the non-doped compounds CuCrO₂ and CuRhO₂ as for the CuRh_1−xMg_xO₂ series upon Mg²⁺ substitution.     

Synthesis and Electrochemical Studies of Spinel LiNi1−xMnxVO4

Spinel compound LiNi_1−xMn_xVO₄ (0≤x≤0.4) had been prepared by using the moist chemical method. X-ray diffraction spectra showed that the lattice constant increased with x in the LiNi_1−xMn_xVO₄, XPS spectra indicating that Li1s had a chemical shift towards lesser binding energy, and manganese in LiNi_1−xMn_xVO₄ existing as the mixed valence of Mn²⁺ and Mn³⁺. The electrochemical charge and discharge testing at a current density of 0.1 mA/cm² between the potentials of 4.0 and 3.0 V vs Li/Li⁺ in 1 mol/dm³ LiPF₆/EC+DEC (1:1 by volume) at 25°C showed that LiNi_1−xMn_xVO₄ cell has a better rechargeability, but a lower cell voltage of 4.0 V vs Li/Li⁺ than that without the doping sample, and the capacity and the cycle efficiency of the Li/LiNi_1−xMn_xVO₄ cells increased with x in the LiNi_1−xMn_xVO₄.     

Luminescent and structural properties of the series Ba6−xEuxTi2+xTa8−xO30 and Ba4−yKyEu2Ti4−yTa6+yO30

The series Ba_6−xEu_xTi_2+xTa_8−xO₃₀ and Ba_4−yK_yEu₂Ti_4−yTa_6+yO₃₀ have been synthesized at 1400°C in air. They exhibit efficient excitation at about 400 nm and typical emission of Eu³⁺ at about 580-620 nm, form solid solutions within 0.0?x?2.0 and 0?y?4 respectively, and crystallized in P4/mbm at room temperature with Eu atoms occupied at centrosymmetric site (0, 0, 0). Their conductivity is very low (2.8×10⁻⁶ Ω⁻¹ cm⁻¹ at 740°C for Ba₆Ti₂Ta₈O₃₀).     

Li and Li MAS NMR Studies on Fast Ionic Conducting Spinel-Type Li2MgCl4, Li2-xCuxMgCl4, Li2-xNaxMgCl4, and Li2ZnCl4

⁶Li and ⁷Li MAS NMR spectra including 1D-EXSY (exchange spectroscopy) and inversion recovery experiments of fast ionic conducting Li₂MgCl₄, Li_2-xCu_xMgCl₄, Li_2-xNa_xMgCl₄, and Li₂ZnCl₄ have been recorded and discussed with respect to the dynamics and local structure of the lithium ions. The chemical shifts, intensities, and half-widths of the Li MAS NMR signals of the inverse spinel-type solid solutions Li_2-xM^I_xMgCl₄ (M^I=Cu, Na) with the copper ions solely at tetrahedral sites and sodium ions at octahedral sites and the normal spinel-type zinc compound, respectively, confirm the assignment of the low-field signal to Li^tet of inverse spinel-type Li₂MgCl₄ and the high-field signal to Lioct as proposed by Nagel et al. (2000). In contrast to spinel-type Li_2-2xMg_1+xCl₄ solid solutions with clustering of the vacancies and Mg²⁺ ions, the Cu⁺ and Na⁺ ions are randomly distributed on the tetrahedral and octahedral sites, respectively. The activation energies due to the various dynamic processes of the lithium ions in inverse spinel-type chlorides obtained by the NMR experiments are E_a=6.6-6.9 and ΔG^*>79 KJ mol⁻¹ (in addition to 23, 29, and 75 kJmol-1 obtained by other techniques), respectively. The largest activation energy of >79 KJ mol⁻¹ corresponds to hopping exchange processes of Li ions between the tetrahedral 8a sites and the octahedral 16d sites. The smallest value of 6.6-6.9 KJ mol⁻¹, which was derived from the temperature dependence of both the spin-lattice relaxation times T₁ and the correlation times τ_C of Li^tet, reveals a dynamic process for the Li^tet ions inside the tetrahedral voids of the structure, probably between fourfold 32e split sites around the tetrahedral 8a site.     

Vacancy ordering and oxygen dynamics in oxide ion conducting La1−xSrxGa1−xMgxO3−x ceramics: Ga, Mg and O NMR

The oxygen vacancies distribution in the rigid lattice and the thermally activated motion of oxygen atoms are studied in La_1−xSr_xGa_1−xMg_xO_3−x (x=0.00; 0.05; 0.10; 0.15 and 0.20) compounds. For that ⁷¹Ga, ²⁵Mg and ¹⁷O NMR was performed from 100 K up to 670 K, and ion conductivity measurements were carried out up to 1273 K. The comparison of the electric field gradients at the Ga- and Mg-sites evidences that oxygen vacancies appear exclusively near gallium cations as a species trapped below room temperature in local clusters, GaO_5/2-□-GaO_5/2. These clusters decay at higher temperature into mobile constituents of the structural octahedra Ga(O_5/6□_1/6)_6/2. At the same time, the nearest octahedral oxygen environment of magnesium cations persists at different doping levels. The case of two adjacent vacant anion sites is found highly unlikely within the studied doping range. The thermally activated oxygen motion starts to develop above room temperature as is observed from both the motional narrowing of ¹⁷O NMR spectra and the ¹⁷O nuclear spin-lattice relaxation rate. The obtained results show that two types of motion exist, a slow motion and a fast one. The former is a long-range diffusion whereas the latter is a local back and forth oxygen jumps between two adjacent anion sites. These sites are strongly differentiated by the probability of the vacancy formation, like the vacant apical site and the occupied equatorial site in the orthorhombic compositions x <0.15.     

Mn environment in layered Li[Mg0.5−xNixMn0.5]O2 oxides monitored by EPR spectroscopy

X-band and high-frequency EPR spectroscopy were used for studying the manganese environment in layered Li[Mg_xNi_0.5−xMn_0.5]O₂, 0?x?0.5. Both layered LiMg_0.5Mn_0.5O₂ and monoclinic Li[Li_1/3Mn_2/3]O₂ oxides (containing Mn⁴⁺ ions only) were used as EPR standards. The EPR study was extended to the Ni-substituted analogues, where both Ni²⁺ and Mn⁴⁺ are paramagnetic. For LiMg_0.5−xNi_xMn_0.5O₂ and Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂, an EPR response from Mn⁴⁺ ions only was detected, while the Ni²⁺ ions remained EPR silent in the frequency range of 9.23-285 GHz. For the diamagnetically diluted oxides, LiMg_0.25Ni_0.25Mn_0.5O₂ and Li[Li_0.10Ni_0.35Mn_0.55]O₂, two types of Mn⁴⁺ ions located in a mixed (Mn-Ni-Li)-environment and in a Ni-Mn environment, respectively, were registered by high-field experiments. In the X-band, comparative analysis of the EPR line width of Mn⁴⁺ ions permits to extract the composition of the first coordination sphere of Mn in layered LiMg_0.5−xNi_xMn_0.5O₂ (0?x?0.5) and Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂ (x>0.2). It was shown that a fraction of Mn⁴⁺ are in an environment resembling the ordered “α,β”-type arrangement in Li_1−δ1Ni_δ1[Li_{(1−2x)/3+δ1}Ni_2x/3−δ1)_α(Mn_(2−x)/3Ni_x/3)_β]O₂ (where and δ₁=0.06 were calculated), while the rest of Mn⁴⁺ are in the Ni,Mn-environment corresponding to the Li_1−δ2Ni_δ2[Ni_1−yMn_y]O₂ () composition with a statistical Ni,Mn distribution. For Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂ with x?0.2, IR spectroscopy indicated that the ordered α,β-type arrangement is retained upon Ni introduction into monoclinic Li[Li_1/3Mn_2/3]O₂.     

Structural Study of Li1+x−yNb1−x−3yTix+4yO3 Solid Solutions

The structures of Li_1+x−yNb_1−x−3yTi_x+4yO₃ solid solutions within the so-called M-phase field in the Li₂O-Nb₂O₅-TiO₂ system were investigated using high-resolution transmission electron, microscope (HRTEM) and single-crystal X-ray diffraction. The results demonstrated that the phase field is not a solid solution but rather a homologous series of commensurate intergrowth structures with LiNbO₃-type (LN) slabs separated by single [Ti₂O₃]²⁺ corundum-type layers. The thickness of the LN slab decreases with increasing Ti-content from ∼55 to 3 atomic layers in the metastable H-Li₂Ti₃O₇ end-member. The LN slabs accommodate a wide range of Ti⁴⁺/Nb⁵⁺ substitution, and for a given homolog the distribution of Ti and Nb is not uniform across the slab. A single-crystal X-ray diffraction study of a structure composed of nine-layer LN slabs revealed preferential segregation of Ti to the slab surfaces which apparently provides partial compensation for the charge on the adjacent [Ti₂O₃]²⁺ corundum layers. The extra cations in phases with x>0 are accommodated through the formation of Li-rich Li₂MO₃-type layers in the middle of the LN slabs. The fraction of layers with extra cations increases with increasing Ti-content in the structure.     

Formation, sintering, and electrical conductivity of perovskite Sm(Cr1−xMgx)O3 (0?x?0.23) prepared by the hydrazine method

The presence of SmCrO₄ is experimentally established. In Mg²⁺-substituted SmCrO₃, single-phase perovskite Sm(Cr_1−xMg_x)O₃, where x=0-0.23, are formed at ∼830°C by decomposition of Sm(Cr_1−xMg_x)O₄ which crystallizes at 530-570°C from amorphous materials prepared by the hydrazine method. Sm(Cr_1−xMg_x)O₃ solid solution powders consisting of submicrometer-size particles are sinterable; dense materials can be fabricated by sintering for 2 h at 1700°C in air. The relative densities, grain sizes, and electrical conductivities increase with increased Mg²⁺ content. Sm(Cr_0.77Mg_0.23)O₃ materials exhibit an excellent direct current electrical conductivity of 2.2×10³ S m⁻¹ at 1000°C.     

Polaron morphologies in SrFe1−xTixO3−δ

The morphologies of the charge carriers in the perovskite system SrFe_1−xTi_xO_3−δ are explored by transport and magnetic measurements. Oxygen vacancies are present in all samples, but they do not trap out the Fe³⁺ ions they introduce. The x=0.05 composition was prepared with three different values of δ. They all show small-polaron conduction above 225 K; but where there is a ratio c=Fe⁴⁺/Fe<0.5, the polaron morphology appears to change progressively with decreasing temperature below 225 K to two-Fe polarons that become ferromagnetically coupled in an applied magnetic field at lower temperatures; With an applied field of 2500 Oe, divergence of the paramagnetic susceptibility for zero-field-cooled and field-cooled samples manifests a greater stabilization of ferromagnetic pairs on cooling in the applied field. With a c>0.5, the data are consistent with a disproportionation reaction 2Fe⁴⁺=Fe³⁺+Fe(V)O_6/2 that inhibits formation of two-Fe polarons and, on lowering the temperature, creates Fe³⁺-Fe(V)-Fe³⁺ superparamagnetic clusters.     

Chemical Synthesis and Properties of Spinel Li1−xCo2O4−δ

Spinel Li_1−xCo₂O_4−δ samples with 0.44≤(1−x)≤1 have been synthesized by chemically extracting lithium with the oxidizer NO₂BF₄ in acetonitrile medium from the LT-LiCoO₂ synthesized at 400°C. Rietveld analysis of the X-ray diffraction data reveals that the Li_1−xCo₂O_4−δ samples adopt the normal cubic spinel structure with a cation distribution of (Li_1−x)_8a[Co₂]_16dO_4−δ. Redox iodometric titration data indicate that the LT-LiCoO₂ tends to lose oxygen on extracting lithium and the spinel Li_1−xCo₂O_4−δ samples are oxygen-deficient. Both infrared spectroscopic and magnetic susceptibility data suggest that the LiCo₂O_4−δ spinel is metallic with itinerant electrons. The tendency to lose oxygen on extracting lithium from the LT-LiCoO₂ and the observed metallic behavior of the spinel LiCo₂O_4−δ are explained on the basis of a qualitative band diagram.     

Structural characterization and physical properties of the system La1.33LixCrxTi2−xO6

New phases which arise from partial substitution of Ti⁴⁺ by Cr³⁺ and Li⁺ of the compound La_2/3TiO₃ have been obtained, giving rise to the series La_1.33Li_xCr_xTi_2−xO₆ (x=0.66, 0.55 and 0.44). These phases adopt a perovskite-type structure as deduced from their structural characterization. Rietveld's analyses of neutron diffraction data show that it is orthorhombic (S.G. Pbnm) with ordered domains. Conductivity has been examined by complex impedance spectroscopy and it increases with increasing lithium and chromium content. These materials behave as mixed conductors with low activation energies. Magnetic susceptibility variation with temperature shows antiferromagnetic interactions at the lowest temperatures.     

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.     

Effects of gold catalysts and thermal evaporation method modifications on the growth process of Zn1−xMgxO nanowires

In this paper, we investigate the roles of gold catalysts and thermal evaporation method modifications in the growth process of Zn_1−xMg_xO nanowires. Zn_1−xMg_xO nanowires are fabricated on silicon substrates with and without using a gold catalyst. Characterizations reveal that Mg acts in a self-catalyst role during the growth process of Zn_1−xMg_xO nanowires grown on catalyst-free substrate. The optical properties and crystalline quality of the Zn_1−xMg_xO nanowires are characterized by room temperature photoluminescence (PL) measurements and Raman spectroscopy, respectively. The Raman and PL studies demonstrate that the Zn_1−xMg_xO nanowires grown using the catalyst-free method have good crystallinity with excellent optical properties and have a larger band-gap in comparison to those grown with the assistance of gold.     

Structural and electrical changes in NdSrNiO4−δ by substitute nickel with copper

A novel series of the formula NdSrNi_1−xCu_xO_4−δ were synthesized for various values of x ranging from 0 to 1 in 1 atm of O₂ gas flow using conventional solid-state methods and were characterized by powder X-ray diffraction and electrical resistivity measurements. The compounds have been shown to adopt the K₂NiF₄-type structure. The oxygen stoichiometry of the compounds was determined from thermo-gravimetric analysis (TGA). An analysis of the micro-structure of the neodymium strontium nickel copper oxide is described. All the samples were semi-conducting from room temperature down to 77 K. The effect of Cu²⁺ incorporation on the structural and electrical properties of NdSrNi_1−xCu_xO_4−δ, 0?x?1, are discussed in terms of Jahn-Teller distortion of the (Ni/Cu)O₆ octahedra and mixed valence character of copper.     

Neutron powder diffraction and first-principles computational studies of CuLixMg2−x (x≅0.08), CuMg2, and Cu2Mg

A small addition of Li changes the orthorhombic structure of CuMg₂ to hexagonal CuLi_xMg_2−x (x=0.08). Determining the Li content of the ternary phase and Li atomic positions was our main objective for this work. For this reason we performed neutron diffraction at several different temperatures below and above room temperature. The results obtained on two neutron powder diffractometers were compared with X-ray diffraction (XRD) data, and with first-principles calculations. The first-principles calculations are in good agreement with Rietveld-refined data from neutron diffraction, but do not show a marked preference for one of several possible Li sites. The pair distribution function (PDF) fitting is consistent with Li substituting only Mg1 (1/2, 0, z). Interstitial spaces in the structure of CuMg₂ and of CuLi_xMg_2−x were also considered, but are unlikely to be occupied by Li. Neutron diffraction data for binary CuMg₂ and Cu₂Mg were also obtained.     

Structure and photoluminescence properties of Ce-doped novel silicon-oxynitride Ba4−zMzSi8O20−3xN2x (M=Mg, Ca, Sr)

Structural and photoluminescence properties of undoped and Ce³⁺-doped novel silicon-oxynitride phosphors of Ba_4−zM_zSi₈O_20−3xN_2x (M=Mg, Sr, Ca) are reported. Single-phase solid solutions of Ba_4−zM_zSi₈O_20−3xN_2x oxynitride were synthesized by partial substitutions of 3O²⁻→2N³⁻ and Ba→M (M=Mg, Ca, Sr) in orthorhombic Ba₂Si₄O₁₀. The influences of the type of alkaline earth ions of M, the Ce³⁺ concentration on the photoluminescence properties and thermal quenching behaviors of Ba₃MSi₈O_20−3xN_2x (M=Mg, Ca, Sr, x=0.5) were investigated. Under excitation at about 330 nm, Ba₃MSi₈O_20−3xN_2x:Ce³⁺ (x=0.5) exhibits efficient blue emission centered at 400-450 nm in the range of 350-650 nm owing to the 5d→4f transition of Ce³⁺. The emission band of Ce³⁺ shifts to long wavelength by increasing the ionic size of M due to the modification of the crystal field, as well as the Ce³⁺ concentrations due to the Stokes shift and energy transfer or reabsorption of Ce³⁺ ions. Among the silicon-oxynitride phosphors of Ba₃MSi₈O_18.5N:Ce³⁺, M=Sr_0.6Ca_0.4 possesses the best thermal stability probably related to its high onset of the absorption edge of Ce³⁺.     

Crystal structure, thermal expansion and conductivity of anisotropic La1−xSrxGa1−2xMg2xO3−y (x=0.05, 0.1) single crystals

Crystal structure and anisotropy of the thermal expansion of single crystals of La_1−xSr_xGa_1−2xMg_2xO_3−y (x=0.05 and 0.1) were measured in the temperature range 300-1270 K. High-resolution X-ray powder diffraction data obtained by synchrotron experiments have been used to determine the crystal structure and thermal expansion. The room temperature structure of the crystal with x=0.05 was found to be orthorhombic (Imma, Z=4, a=7.79423(3) Å, b=5.49896(2) Å, c=5.53806(2) Å), whereas the symmetry of the x=0.1 crystal is monoclinic (I2/a, Z=4, a=7.82129(5) Å, b=5.54361(3) Å, c=5.51654(4) Å, β=90.040(1)°). The conductivity in two orthogonal directions of the crystals has been studied. Both, the conductivity and the structural data indicate three phase transitions in La_0.95Sr_0.05Ga_0.9Mg_0.1O_2.92 at 520-570 K (Imma-I2/a), 770 K (I2/a-R3c) and at 870 K (R3c-R-3c), respectively. Two transitions at 770 K (I2/a-R3c) and in the range 870-970 K (R3c-R-3c) occur in La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85.     

Comparison of the Chemical Stability of Li1−xCoO2 and Li1−xNi0.85Co0.15O2 Cathodes

The chemical stability of the layered Li_1−xCoO₂ and Li_1−xNi_0.85Co_O.15O₂ cathodes is compared by monitoring the oxygen content with lithium content (1−x) in chemically delithiated samples. The Li_1−xCoO₂ system tends to lose oxygen from the lattice at deep lithium extraction while the Li_1−xNi_0.85Co_0.15O₂ system does not lose oxygen at least for (1−x)>0.3. This difference seems to result in a lower reversible (practical) capacity (140 mA h/g) for LiCoO₂ compared to that for LiNi_0.85Co_0.15O₂ (180 Ma h/g). The loss of significant amount of oxygen leads to a sliding of oxide layers and the formation of a major P3 and a minor O1 phase for the end member CoO_2−δ with δ=0.33. In contrast, Ni_0.85Co_0.15O_2−δ with a small amount of δ=0.1 maintains the initial O3 layer structure.