Changes in electronic structure upon Li insertion reaction of monoclinic Li3Fe2(PO4)3

The electrochemical lithium insertion reaction of monoclinic Li(3)Fe(2)(PO(4))(3) as cathode materials of lithium-ion batteries was investigated from the viewpoint of the electronic structure around Fe and the polyanion unit (PO(4)). Fe K-edge and L(III,II)-edge XAS measurements revealed that Fe(3+) was reduced to Fe(2+) upon Li insertion. In addition, O K-edge and P K-edge XAS also showed spectral changes upon Li insertion, which corresponded to changes in the electronic structure of the PO(4) polyanion unit. The ab initio density functional calculation was performed within the GGA and LDA+U methods. The LDA+U method reproduced well the cell potential upon lithium intercalation into Li(3)Fe(2)(PO(4))(3), whereas the GGA method underestimated the intercalation. The calculated electronic structure of Li(3)Fe(2)(PO(4))(3) described strong P 3p-O 2p covalent bonding, while weak hybridization was indicated in Fe 3d-O 2p. Moreover, the difference in electronic density between Li(3)Fe(2)(PO(4))(3) and the lithiated model indicated that the polarization effect between inserted Li and oxygen induced the changes in the electronic structure around the polyanion unit.     

Changes in electronic structure upon lithium insertion into Fe2(SO4)3 and Fe2(MoO4)3 investigated by X-ray absorption spectroscopy

Changes in electronic structure upon electrochemical lithium insertion into two iron compounds, namely, rhombohedral Fe2(SO4)3 with a NASICON-type structure and monoclinic Fe2(MoO4)3, were investigated using X-ray absorption spectroscopy (XAS). Fe K-edge and L(III)- and L(II)-edge XAS revealed that the rearrangement of Fe d electrons or rehybridization of Fe d-O p bonding took place accompanied by the reduction of Fe ions upon Li insertion for both samples and that a larger change in spectra was observed in Fe2(SO4)3. In addition, the changes in the electronic structure of the polyanion units XO4(2-) (X = S or Mo) after Li insertion were also investigated by O K-edge and S K-edge or Mo L(III)-edge XAS. The results indicated that the electronic structure around oxygen markedly changed in Fe2(MoO4)3, while no significant change was observed in Fe2(SO4)3.     

New lithium-ion conducting perovskite oxides related to (Li, La)TiO3

We describe the synthesis and lithium-ion conductivity of new perovskite-related oxides of the formulas, LiCa_1.65□_0.35Ti_1.3B_1.7O₉ (B =Nb,Ta)(I,II), LiSr₂Ti_2.5W_0.5O₉ (III) and LiSr_1.65□_0.35Ti_2.15W_0.85O₉(IV). OxidesI andII crystallize in orthorhombic (GdFeO₃-type) structure, while oxidesIII andIV possess cubic symmetry. All of them exhibit significant lithium-ion conduction at high temperatures, the highest conductivity of ∼ 10⁻²S/cm at 800°C among the oxides is exhibited by the composition IV. The results are discussed in the light of previous work on lithium-ion conducting perovskite oxides containingd ₀ cations.     

Crystal structure and diffusion path in the fast lithium-ion conductor La(0.62)Li(0.16)TiO3

We report the results of a neutron powder diffraction study of the La(0.62)Li(0.16)TiO3 perovskite that determined the diffusion path of lithium cations at room temperature. At 77 K, the Li cations are located at the 2c site (Wycoff notation of the Cmmm space group) on the (002) La deficient layer, while, at room temperature, they are spread over a wide area and migrate following the 2c-4f-2c or 2c-2d-2c tie line on the (002) layer. The probability density of Li cations has a minimum between the 2c and 4f or between the 2c and 2d positions on the diffusion path in contrast to the previous reports where the bottleneck has been thought to be located at the 2c, 2d, and 4f positions. On the basis of the present structural model, the Li-cation conductivity is discussed in terms of a two-dimensional bond-percolation model for Li-cation diffusion. It was found that the vacancy at the La site is essential for the Li-cation conduction.     

NMR study of Li+ ion dynamics in the perovskite Li(3x)La(1/3-x)NbO3

(7)Li and (6)Li nuclear magnetic resonance (NMR) experiments are carried out on the perovskite Li(3x)La(1/3-x)NbO(3). The results are compared to those obtained on the titanate Li(3x)La(2/3-x)TiO3 (LLTO) in order to investigate the effect, on the lithium ion dynamics, of the total substitution of Nb(5+) for Ti(4+) in the B-site of the ABO(3) perovskites. The XRD patterns analysis reveals that this substitution leads to a change in the distribution of the La(3+) ions in the structure. La(3+) ions distribution is very important, in regard to ionic conductivity, because these immobile ions can be considered as obstacles for the long-range Li+ motion. If compared to the titanates, the compounds of the niobate solid solution have a bigger unit cell volume, a smaller number of La(3+) ions, and a higher number of vacancies. These should favor the motion of the mobile ions into the structure. This is not experimentally observed. Therefore, the interactions between the mobile species and their environment greatly influence their mobility. (7)Li and (6)Li NMR relaxation time experiments reveal that the Li relaxation mechanism is not dominated by quadrupolar interaction. (7)Li NMR spectra reveal the presence of different Li+ ion sites. Some Li+ ions reside in an isotropic environment with no distortion, some others reside in weakly distorted environments. T(1), T(1)(rho), and T(2) experiments allow us to evidence two motions of Li+. As in LLTO, T(1) probes a fast motion of the Li+ ions inside the A-cage of the perovskite structure and T(1)(rho) a slow motion of these ions from A-cage to A-cage. At variance with what has been observed in LLTO, these different Li+ ions can be differentiated through the spin-lattice relaxation times, T(1) and T(1)(rho), as well as through the transverse relaxation time, T(2).     

Surface electronic structure transitions at high temperature on perovskite oxides: the case of strained La0.8Sr0.2CoO3 thin films

In-depth probing of the surface electronic structure on solid oxide fuel cell (SOFC) cathodes, considering the effects of high temperature, oxygen pressure, and material strain state, is essential toward advancing our understanding of the oxygen reduction activity on them. Here, we report the surface structure, chemical state, and electronic structure of a model transition metal perovskite oxide system, strained La(0.8)Sr(0.2)CoO(3) (LSC) thin films, as a function of temperature up to 450 °C in oxygen partial pressure of 10(-3) mbar. Both the tensile and the compressively strained LSC film surfaces transition from a semiconducting state with an energy gap of 0.8-1.5 eV at room temperature to a metallic-like state with no energy gap at 200-300 °C, as identified by in situ scanning tunneling spectroscopy. The tensile strained LSC surface exhibits a more enhanced electronic density of states (DOS) near the Fermi level following this transition, indicating a more highly active surface for electron transfer in oxygen reduction. The transition to the metallic-like state and the relatively more enhanced DOS on the tensile strained LSC at elevated temperatures result from the formation of oxygen vacancy defects, as supported by both our X-ray photoelectron spectroscopy measurements and density functional theory calculations. The reversibility of the semiconducting-to-metallic transitions of the electronic structure discovered here, coupled to the strain state and temperature, underscores the necessity of in situ investigations on SOFC cathode material surfaces.     

On the Location of Li(+) Cations in the Fast Li-Cation Conductor La(0.5)Li(0.5)TiO(3) Perovskite

Lithium cations sit in the center of the oxygen "windows" defined by the vertex-sharing TiO(6) octahedra of the title compound (see picture), as shown from neutron diffraction data. The octahedra are tilted to optimize the bond distances between the atoms. The unusual coordination and the partially unoccupied sublattice (occupancy factors 1/6 and (1/2) for Li and La, respectively) account for the high mobility of the Li(+) cations.     

Structural and surface analysis of unsupported and alumina-supported La(Mn,Fe,Mo)O3 perovskite oxides

A series of bulk and Al₂O₃-supported perovskite oxides of the type LaMn_1???x???y Fe _x Mo _y O₃ (x?=?0.00?0.90 and y?=?0.00–0.09) were synthesized by the citric acid complexation–gelation method followed by annealing in air at 800 °C. For all samples, the local environment and the chemical state and concentration of surface species were determined. Mössbauer spectra revealed the only presence of octahedral Fe³⁺ ions dispersed in the perovskite structure, however well-crystallized together with a poorly crystalline LaFeO₃ phases were detected for larger substitutions (x?=?0.90). A similar picture was obtained for Mo-loaded (y?=?0.02 and 0.05) samples but a new phase most likely related to Fe³⁺ ions dispersed aside from the perovskite structure was found for larger substitutions (y?=?0.09). Together with these structures, supported samples showed the presence of LaFeO₃ nanoparticles. Finally, photoelectron spectroscopy indicated that the chemical state and composition of the samples in the surface region (2–3 nm) approaches that of the bulk. For the unsupported substituted samples, iron (and molybdenum) enters into the perovskite structure while manganese tends to be slightly segregated. Moreover, in supported perovskites, a fraction of Mo and La atoms interact with the alumina surface. All these oxides were active in methane combustion and best performance was recorded for the Fe-rich composition (x?=?0.9) in which both Mn³⁺ and Mo³⁺ ions were in the same proportion (y?=?0.05).     

Band structure and electronic properties of lithium azide (LiN3)

Ab initio Hartree–Fock band structure and molecular calculations have been performed to study the electronic structure of LiN₃ in a monoclinic C 2/m crystal structure. The total energy, band structure, density of states, and charge densities are computed. The calculated lattice energy (energy to separate the ions infinitely apart) of 8.6 eV agrees very well with 8.45 eV deduced from Madelung and London polarizability energies. The calculated split of the N 1s core bands of 5.0 eV compares favorably with the experimental X-ray photoelectron value of 4.4 eV. This good agreement is not contributed to crystalline environment effects as proposed in earlier MO studies of N where the best values obtained were 5.1, 5.8, and 6.3 eV, but to the quality of the nitrogen core basis set. The calculated valence density of states supports one of two competing interpretations that peak III observed in the X-ray photoelectron spectrum arises from contaminations or other extrinsic states.     

Investigation on the arrangement of lithium ions in LixLa(1/3)NbO3 with perovskite structure

The relationship between the Li arrangement and the electrochemical behavior has been examined as a function of composition x in electrochemically lithiated A-site deficient perovskite, Li(x)()La(1/3)NbO(3). The cell potential diagram and powder X-ray diffraction (XRD) study indicated that the Li ions are inserted into the vacant Perovskite A-site with an electrochemical reaction. In addition, the derivatives of the cell potential diagram showed three cathodic peaks, indicating a stepwise Li insertion mechanism takes place. Such a stepwise behavior would be ascribed to the changes in arrangement of inserted Li ions in the Perovskite lattice, since the XRD patterns of pristine La(1/3)NbO(3) showed that the La arrangement in La(1/3)NbO(3) was ordered along the c-axis, causing two kinds of A-site vacancies. To reveal the changes in the arrangement of Li ions, the entropy measurement of the reaction was performed by both the electrochemical and the calorimetric techniques. Moreover, the formation energy of the Perovskite structure with various Li arrangements was compared by using an ab initio calculation. The results of experiment and computation suggested that the electrochemical reaction proceeded via two kinds of superstructures of Li(1/6)La(1/3)NbO(3) and Li(1/2)La(1/3)NbO(3) due to the ordered arrangement of Li ions.     

Ionic conduction of lithium for Perovskite-type compounds, Li x La(1− x )/3NbO3 and (Li0.25La0.25)1− x Sr0.5 x NbO3

Perovskite-type compounds, Li _x La_{(1− x )/3}NbO₃ and (Li_0.25La_0.25)_{1− x} Sr_{0.5 x} NbO₃ as lithium ionic conductors, were synthesized by a solid-state reaction. From powder X-ray diffraction, the solid solution ranges of the two compounds were determined to be 0≤x≤0.25 and 0≤x≤0.125, respectively. In the Li _x La_{(1− x )/3}NbO₃ system, the ionic conductivity of lithium at room temperature, σ₂₅, exhibited a maximum value of 4.7 × 10⁻⁵ S · cm⁻¹ at x = 0.10. However, because of the decrease in the lattice parameters with increasing Li concentration x˙, σ₂₅ of the samples decreased with increasing x from 0.10 to 0.25. Also, in the (Li_0.25La_0.25)_{1− x} Sr_{0.5 x} NbO₃ system, the lattice parameter increased with the increase of Sr concentration and the σ₂₅ achieved a maximum (7.3 × 10⁻⁵ S · cm⁻¹ at 25 °C) at x = 0.125. Received: 12 September 1997 / Accepted: 15 November 1997     

Experimental visualization of lithium conduction pathways in garnet-type Li(7)La(3)Zr(2)O(12)

The evolution of the Li-ion displacements in the 3D interstitial pathways of the cubic garnet-type Li(7)La(3)Zr(2)O(12), cubic Li(7)La(3)Zr(2)O(12), was investigated with high-temperature neutron diffraction (HTND) from RT to 600 °C; the maximum-entropy method (MEM) was applied to estimate the Li nuclear-density distribution. Temperature-driven Li displacements were observed; the displacements indicate that the conduction pathways in the garnet framework are restricted to diffusion through the tetrahedral sites of the interstitial space.     

On the influence of the vacancy distribution on the structure and ionic conductivity of A-site-deficient Li(x)Sr(x)La(2/3-x)TiO3 perovskites

The crystal structure and dielectric properties of slowly cooled A-site-deficient perovskites Li(x)Sr(x)La(2/3-x)□(1/3-x)TiO(3) (0.04 ≤ x ≤ 0.33) have been investigated by powder X-ray diffraction (XRD), impedance spectroscopy, and (7)Li NMR techniques. In this series, nominal vacancies decrease with Li content, but the total amount of A-site vacancies, n(t) = Li + □, participating in conduction processes remains basically constant. Rietveld analysis of the XRD patterns showed a change of symmetry from orthorhombic to tetragonal when the lithium and strontium contents increased above x = 0.08 and from tetragonal to cubic above x = 0.16. Structural modifications are mainly due to the cation vacancy ordering along the c axis, which disappear gradually when the lithium content increases. In agreement with the structural information, two lithium signals with different quadrupole constants are detected in (7)Li NMR spectra of orthorhombic/tetragonal phases, which have been associated with lithium in two crystallographic z/c = 0 and 1/2 planes of perovskites. In cubic samples, only a single narrow component, indicative of mobile species, was detected. Lithium motion was thermally activated, with activation energies going from 0.35 to 0.38 eV. Evolution of the bulk dc-conductivity preexponential factors along the series showed a maximum that has been first related to the dependence of lithium hopping on the lithium and vacancy concentrations. Finally, changes in the vacancy ordering, produced along the series, affect the dimensionality of the conductivity, indicating that not only the amount of vacancies but also its distribution are relevant.     

Crystal and electronic structure and magnetic properties of divalent europium perovskite oxides EuMO3 (M = Ti, Zr, and Hf): experimental and first-principles approaches

A comparative study of the crystal and electronic structure and magnetism of divalent europium perovskite oxides EuMO(3) (M = Ti, Zr, and Hf) has been performed on the basis of both experimental and theoretical approaches playing complementary roles. The compounds were synthesized via solid-state reactions. EuZrO(3) and EuHfO(3) have an orthorhombic structure with a space group Pbnm at room temperature contrary to EuTiO(3), which is cubic at room temperature. The optical band gaps of EuZrO(3) and EuHfO(3) are found to be about 2.4 and 2.7 eV, respectively, much larger than that of EuTiO(3) (0.8 eV). On the other hand, the present compounds exhibit similar magnetic properties characterized by paramagnetic-antiferromagnetic transitions at around 5 K, spin flop at moderate magnetic fields lower than 1 T, and the antiferromagnetic nearest-neighbor and ferromagnetic next-nearest-neighbor exchange interactions. First-principles calculations based on a hybrid Hartree-Fock density functional approach yield lattice constants, band gaps, and magnetic interactions in good agreement with those obtained experimentally. The band gap excitations are assigned to electronic transitions from the Eu 4f to Mnd states for EuMO(3) (M = Ti, Zr, and Hf and n = 3, 4, and 5, respectively).     

Positive and negative magnetodielectric effects in A-site ordered (BiMn3)Mn4O12 perovskite

A-site ordered perovskite (BiMn3)Mn4O12 was synthesized through a high-pressure synthesis route at 5 GPa and found to exhibit two magnetic transitions and to show either a positive or a negative magnetodielectric effect depending on the temperature range/magnetic state.     

Electrochemical insertion of lithium into the ramsdellite-type oxide Li2Ti3O7: influence of the Li2Ti3O7 particle size

The effect of a milling process on the electrochemical performance of Li₂Ti₃O₇ electrodes has been investigated by the galvanostatic intermittent titration technique (GITT) and AC impedance spectroscopy. The insertion ratio is slightly increased by the milling treatment and a value of x _Li=1.25 per mol Li₂Ti₃O₇ has been determined. The average potential during insertion is close to 1.5 V/Li. The analysis of impedance data obtained at equilibrium during insertion and deinsertion shows two relaxation processes and a diffusion phenomenon at low frequency according to the Frumkin-Melik-Gayakazian model. Cycling experiments of batteries using this material were performed with unmilled and milled particles. Composite electrodes containing different amounts of electroactive material added to a binder and a conductive additive have also been prepared in order to check the effect of grinding on the cyclability of the compound. Interesting electrochemical performances have been determined with such electrodes: lithium uptake up to 1.25 Li per Li₂Ti₃O₇, low irreversible capacity loss between the first and the following cycles, good stability upon cycling even after 50 cycles. However, the milled process has not improved significantly the electrochemical performance of the Li₂Ti₃O₇ electrodes. Electronic Publication     

A new type of orthoborates: ASr4La3(BO3)6 (A = Li,Na)

Two new rare earth containing orthoborate crystals ASr₄La₃(BO₃)₆ (A = Li, Na) have been obtained by spontaneous nucleation from high-temperature melts of A₂O–SrO–La₂O₃–B₂O₃–AF. X-ray diffraction analyses show that they both crystallize in the rhombohedral space group R-3 with cell parameters of a = 12.309(7) Å, c = 9.316(7) Å and a = 12.4049(13) Å, c = 9.348(2) Å for the Li and Na compounds respectively. Similar to the large A′₆MM′(BO₃)₆ family, these compounds are all related to the structure of Sr₃Y(BO₃)₃ with La and Sr statistically occupy the Sr site, and the alkaline elements and remaining Sr enter the ordered Y1 and Y2 sites, which can be approximately represented as (La_2.91Sr_3.09)(La_0.09Sr_0.91)Li[B₆O₁₈] and (La_2.85Sr_3.15)(La_0.15Sr_0.85)Na[B₆O₁₈]. The characteristic of the structure is that the La/Sr and isolated BO₃ groups form a network with tunnels along the c-axis where the alkaline A and Sr ions alternatively reside. The optical transmission spectrum shows that the ultraviolet absorption edge of NaSr₄La₃(BO₃)₆ crystal is about 193 nm and Raman spectra reveal that both crystals possess sharp peaks at 930 cm⁻¹.