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.     

Mechanistic studies on the electrochemically lithiated spinel LiCuVO4

The lithiation mechanism of the spinel LiCuVO₄ was studied by X-ray diffraction, XPS, and electrochemical measurements using the lithium cell with the spinel cathode. The lithiation proceeded by the following steps: (1) in the multiphasic reaction for x < 1.5 in Li_1+xCuVO₄, the LiCuVO₄ spinel transforms to a new phase, Li_2.5Cu_0.5VO₄, and Cu metal; (2) in the monophasic electrochemical displacement reaction for 1.5 < x < 2.0, the copper ions extrude from Li_2.5Cu_0.5VO₄ with lithium intercalation, which forms Li₃VO₄ and Cu metal; (3) in the intercalation reaction for 2.0 < x < 5.0, lithium ions intercalate into Li₃VO₄ with several reaction steps. The new phase, Li_2.5Cu_0.5VO₄, lithiated reversibly with the electrochemical displacement between copper and lithium ions.     

Crystal Structures of Double Vanadates LiCoVO4 and Li0.5Co1.25VO4

The crystal structures of vanadates Li_1-2xCo_1+xVO₄ with x = 0 and 0.25 have been studied by a full pattern analysis. It has been shown that in cubic spinel LiCoVO₄ (space group Fd3m), the 8a tetrahedral sites contain a majority of vanadium and a small amount of lithium; all cobalt, lithium, and a small amount of vanadium occupy the 16d octahedral sites. Li_0.5Co_1.25VO₄ crystals belong to the rhombic system (Imma space group) with unit cell parameters a = 5.939(1), b = 5.810(1), and c = 8.303(1). On substitution of lithium by cobalt according to the scheme 2Li⁺ Co²⁺ + , half of the lithium and 70% of the vacancies formed are in the 4a octahedral sites, and onethird of lithium and most of cobalt occupy the 4d octahedral sites. The 4e tetrahedral sites are completely occupied by vanadium and lithium in a ratio of 0.92/0.08. The interatomic distances in LiCoVO₄ and Li_0.5Co_1.25VO₄ are calculated, and the sizes of lithium ion transport channels are evaluated.     

Caractérisation de la solution solide In1−xLi3xVO4 (0 < x ≤ 0,4) dans le système InVO4Li3VO4 et examen des systèmes CrVO4Li3VO4 et InVO4CrVO4

In the InVO₄Li₃VO₄ system, a continuous solid solution In_1−xLi⁽⁶⁾_a□⁽⁶⁾_bLi⁽⁴⁾_c□⁽⁶⁾_dVO₄ exists between InVO₄ and In_0.6Li_1.2VO₄, with a + b = x, c + d = 1, and a + c = 3x. The solid solution is of two types: in the first, 0 < x ≤ 0.33, a = 0, interstitial Li⁺ ions are in the vacant tetrahedral sites of InVO₄; in the second, 0.33 < x ≤ 0.4, c = 1, Li⁺ ions are also in the octahedral sites vacated by In³⁺. The ionic conductivity measured for some compositions is weak, 10⁻⁷ (Ω cm)⁻¹ at 493 K. Solid solution has not been found between CrVO₄ and Li₃VO₄, although CrVO₄ is isostructural with InVO₄. Mutual solid solution between CrVO₄ and InVO₄ is extremely limited. Yellow and weakly hygroscopic monocrystals have been synthetized for R₂Li₃(VO₄)₃ compositions (R = In,Cr,Fe); their chemical formula can be symbolized by LiVO₃: R³⁺. The R³⁺ percentage was too low to be detected by analysis of electronic densities based on X-ray diffraction intensities.     

The crystal structures of the lithium-inserted metal oxides Li0.5TiO2 anatase,LiTi2O4 spinel,and Li2Ti2O4

The crystal structures of three lithium titanates by neutron diffraction powder profile analysis were determined. The tetragonal anatase form of TiO₂ becomes orthorhombic on ambient-temperature lithium insertion to Li_0.5TiO₂ due to the formation of TiTi bonds. The lithium partially occupies the highly distorted octahedral interstices in the anatase framework in fivefold-coordination with oxygen. Cubic LiTi₂O₄ formed by heating Li_0.5TiO₂ anatase has a normal spinel structure with Li in the tetrahedral sites. In Li₂Ti₂O₄ formed by reacting LiTi₂O₄ spinel with n-BuLi at ambient temperature, the titanium remains in the spinel positions but the lithium is displaced, filling all the available octahedral sites.     

Etude par RMN de la mobilité du lithium dans trois oxydes à structure pseudo-bidimensionnelle: Li8SnO6, Li7NbO6 et Li6In2O6

The lithium ion mobility in three solid electrolytes (Li₈SnO₆, Li₇NbO₆, and Li₆In₂O₆) has been studied by NMR at several resonance frequencies from 170 to 500°K. The ⁷Li quadrupolar lineshape evolution shows the predominant influence on the conductivity mechanism of the vacancies in the octahedral sites of the oxygen close packing. In Li₈SnO₆, which has no vacancies, the lithium ions situated in the tetrahedral sites have the highest mobility. Spin-lattice relaxation times are in good agreement with the hypothesis of a Li₇NbO₆ 2D conductivity. The values of the activation energy, increasing from Li₇NbO₆ to Li₆In₂O₆ and to Li₈SnO₆, are found to be three times lower than those obtained from conductivity measurements.     

Structure cristalline du vanadate mixte In0,6Li1,2VO4

A mixed vanadate In_0.6Li_1.2VO₄ has been synthesized from an equimolecular mixture of InVO₄ and Li₃VO₄. The exact chemical formula has been determined by a crystal structure refinement. Crystallographic data are: a = 5.763(1), b = 8.742(2), c = 6.385(3)Å, Z = 4, d_calc = 3.97 g cm^?3. Seven hundred twenty-six reflections have been used for structure determination and refinement, to a final value R = 0.019, after absorption and extinction corrections. InO₆ octahedra and VO₄ tetrahedra are linked together in the same three-dimensional network that exists in InVO₄. Nevertheless, a partial substitution of In³⁺ by Li⁺ and an insertion of Li⁺ in tetrahedral interstices occur. Vacancies exist, either in the octahedral and tetrahedral positions, In_0.6Li⁽⁶⁾_0.3□⁽⁶⁾_0.1Li⁽⁴⁾_0.9□⁽⁴⁾_0.1VO₄, or solely in the tetrahedral positions, In_0.6Li⁽⁶⁾_0.4Li⁽⁴⁾_0.8□⁽⁴⁾_0.2VO₄.     

Structural and magnetic properties of orthorhombic LixMnO2

Rietveld refinement of the crystal and magnetic structures of Li_xMnO₂ (x = 0.98, 1.00, 1.02) are performed using neutron and X-ray measurements. A significant structural disorder due to the presence of manganese ions in lithium positions (Mn_Li) and lithium ions in manganese ones (Li_Mn) is found to be a common feature of Li_0.98MnO₂, Li_1.00MnO₂, and Li_1.02MnO₂.An essential anisotropy of the thermal-expansion coefficients of the lithium manganese oxides is observed in the temperature range of 1.5–300 K. Furthermore, the distortion of the oxygen octahedral environment around the manganese ions decreases when the temperature lowers. This is attributed to the strong exchange interactions between parallel exchange-coupled Mn chains. First-principles calculations of the effective exchange-interaction parameters in Li₁₆Mn₁₆O₃₂ confirm the essential antiferromagnetic interactions between the chains. In addition, a hypothetical (Li₁₅Mn)Mn₁₆O₃₂ structure where a lithium atom located between the Mn double layers is replaced by a manganese atom is considered. The calculations reveal that the presence of such defects results in appearance of a ferromagnetic component that agrees with the magnetic measurements.     

Revised phase diagram of Li2MoO4-ZnMoO4 system, crystal structure and crystal growth of lithium zinc molybdate

Orthorhombic lithium zinc molybdate was first chosen and explored as a candidate for double beta decay experiments with ¹⁰⁰Mo. The phase equilibria in the system Li₂MoO₄-ZnMoO₄ were reinvestigated, the intermediate compound Li₂Zn₂(MoO₄)₃ of the α-Cu₃Fe₄(VO₄)₆ (lyonsite) type was found to be nonstoichiometric: Li_2−2xZn_2+x(MoO₄)₃ (0≤x≤0.28) at 600 °C. The eutectic point corresponds to 650 °C and 23 mol% ZnMoO₄, the peritectic point is at 885 °C and 67 mol% ZnMoO₄. Single crystals of the compound were prepared by spontaneous crystallization from the melts and fluxes. In the structures of four Li_2−2xZn_2+x(MoO₄)₃ crystals (x=0; 0.03; 0.21; 0.23), the cationic sites in the face-shared octahedral columns were found to be partially filled and responsible for the compound nonstoichiometry. It was first showed that with increasing the x value and the number of vacancies in M3 site, the average M3-O distance grows and the lithium content in this site decreases almost linearly. Using the low-thermal-gradient Czochralski technique, optically homogeneous large crystals of lithium zinc molybdate were grown and their optical, luminescent and scintillating properties were explored.     

Lithium ions in the van der Waals gap of Bi2Se3 single crystals

Insertion/extraction of lithium ions into/from Bi₂Se₃ crystals was investigated by means of cyclic voltammetry. The process of insertion is reflected in the appearance of two bands on voltammograms at ∼1.7 and ∼1.5 V, corresponding to the insertion of Li⁺ ions into octahedral and tetrahedral sites of the van der Waals gap of these layered crystals. The process of extraction of Li⁺ ions from the gap results in the appearance of four bands on the voltammograms. The bands 1 and 2 at ∼2.1 and ∼2.3 V correspond to the extraction of a part of Li⁺ guest ions from the octahedral and tetrahedrals sites and this extraction has a character of a reversible intercalation/deintercalation process. A part of Li⁺ ions is bound firmly in the crystal due to the formation of negatively charged clusters of the (LiBiSe₂.Bi₃Se₄⁻) type. A further extraction of Li⁺ ions from the van der Waals gap is associated with the presence of bands 3 and 4 placed at ∼2.5 and ∼2.7 V on the voltammograms as their extraction needs higher voltage due to the influence of negative charges localized on these clusters.     

Etude par RMN de la localisation des cations dans des phases ferroélectriques non-stoechiométriques derivées de LiTaO3

A ⁷Li NMR investigation of nonstoechiometric ferroelectric phases derived from LiTaO₃ has been performed on three solid solutions of formulation $\text{Li}{}_{\mathrm{1+x}}\text{Ta}{}_{\mathrm{<mtext>1?</mtext><mtext>x</mtext><mtext>5</mtext>}}\text{O}{}_3$, Li_1+xTa_1?xTi_xO₃, and Li_1?xTa_1?3x Ti_4xO₃. For the first one, based on the substitution of 1 Ta⁵⁺ by 5 Li⁺, the existence of Li⁺ in both octahedral and tetrahedral sites is confirmed. It is not excluded that the 5 Li⁺ form a small cluster within seven sites (one octahedral position and six tetrahedral ones) in the vicinity of the substituted Ta⁵⁺. For the second solid solution a large variation of the ⁷Li quadrupolar spectrum with composition has been detected, such behavior is related to the great decrease in T_c near the x = 0.10 composition.     

Photomagnetic effects in single-crystal Ru-doped lithium ferrite

Single crystals of Li_0·5Fe_2·5O₄:Ru (0.01 – 0.03 atoms per formula unit) are found to exhibit a light-induced change in magnetic permeability and coercive force at 77 K. A light-enhanced disaccommodation is also found in these crystals.Magnetization measurements on a polycrystalline series of samples with the formula Li_0.5Fe_2.5?x Ru_xO₄ (x = 0.00?0.17) have shown that the Ru-ion is in a low-spin state on octahedral sites. In addition, magnetic, electrical, and optical data are given for lithium ferrite crystals doped with 0.00 – 0.03 Ru-atoms per formula unit, including nonphotomagnetic samples.     

Li2MTi6O14 (M=Sr,Ba): new anodes for lithium-ion batteries

Isostructural Li₂MTi₆O₁₄ (M=Sr, Ba) materials, prepared by a solid state reaction method, have been investigated as insertion electrodes for lithium battery applications. These titanate compounds have a structure that consists of a three-dimensional network of corner- and edge-shared [TiO₆] octahedra, 11-coordinate polyhedra for the alkali-earth ions, and [LiO₄] tetrahedra in tunnels that also contain vacant tetrahedral and octahedral sites. Electrochemical data show that these compounds are capable of reversibly intercalating four lithium atoms in a three-stage process between 1.4 and 0.5 V vs. metallic lithium. The electrodes provide a practical capacity of approximately 140 mAh/g; they are, therefore, possible alternative anode materials to the lithium titanate spinel, Li₄Ti₅O₁₂. The lithium intercalation mechanism and crystal structure of Li₂MTi₆O₁₄ (M=Sr, Ba) electrodes are discussed and compared with the electrochemical and structural properties of Li₄Ti₅O₁₂. The area-specific impedance (ASI) of Li/Li₂SrTi₆O₁₄ cells was found to be significantly lower than that of Li/Li₄Ti₅O₁₂ cells.     

Magnetic interactions in CoM3+xGa2−xO4 spinel solid solutions: I: CoRhxGa2−xO4

The magnetic susceptibility of polycrystalline solid solutions CoRh_xGa_2?xO₄ with a spinel structure have been measured between 4.2 and 1000°K. The magnetic properties have been found to vary with the composition x as a consequence of the variation in the distribution of Co²⁺ ions among tetrahedral and octahedral sites. The low-temperature magnetic behavior reveals an antiferromagnetic order and the concomitant presence of finite clusters of exchange-coupled Co²⁺ ions and of isolated paramagnetic ions.     

The structure of Li3.4Si0.7S0.3O4 by powder neutron diffraction

The structure of Li_4?2xSi_1?xS_xO₄ (x ≈ 0.32) has been determined from neutron powder diffraction studies at room temperature, 350, and 700°C. This compound, which is a member of the series of ionic-conducting solid solutions formed between Li₄SiO₄ and Li₂SO₄, is isostructural with Li₃PO₄. The space group is Pmnb, with a = 6.1701(1), b = 10.6550(2), c = 5.0175(1)Å at room temperature. The distribution of lithium ions suggests the occurrence of a defect cluster in which the inclusion of an interstitial lithium ion causes displacements of the adjacent lithium ions of the normal Li₃PO₄ structure. There appears to be little variation of the structure with temperature.     

Order-disorder transition in the complex lithium spinel Li2CoTi3O8

Li₂CoTi₃O₈ has an ordered Li₂BB′₃O₈ spinel structure, space group P4₃32, at room temperature with 3:1 ordering of Ti and Li on the octahedral sites, and Li, Co disordered over the tetrahedral site. Rietveld refinement of variable temperature neutron powder diffraction data has shown an order-disorder phase transition in Li₂CoTi₃O₈ which commences at ∼500 °C with Li and Co mixing on the tetrahedral and 4-fold octahedral sites and is complete at a first order structural discontinuity at ∼915 °C. The fraction of Ti on the 12-fold octahedral site exhibits a small decrease with increasing temperature, which may suggest that the disordering involves all three cations. Above 930 °C, the structure, space group Fd3¯m, has Li, Co and Ti sharing a single-octahedral site and Li, Co sharing a tetrahedral site, although Co still exhibits a preference for tetrahedral coordination. A labelling scheme for ordered and partially ordered 3:1 spinels is devised which focuses on the occupancy of the Li,B cations.     

Synthese et propriétés de conduction ionique des phases Li8−2xCaxCeO6 (0 < x ≤ 0,5) et Li6CaCeO6

The solid solution Li_8−2xCa_xCeO₆ (0 < x ≤ 0,5) and the definite phase Li₆CaCeO₆ have been obtained at 800°C through a study of Li---Ca---Ce---O system. Electrical measurements on the doped phases Li^tetr.₆ [Li_2-2xCa_xCe□]^oct.O₆ show that the conductivity varies slightly with the creation of vacancies in the octahedral layers. This result unambiguously confirms the following diffusion mechanism: the conduction is assumed essentially by lithium ions located in the tetrahedral layers. The compound Li₆CaCeO₆ is isostructural with Li₆In₂O₆. The cell is trigonal, Å, c = 10,603 Å, c/a = 1,058₇, and Z = 6. This new quaternary phase, which belongs to the same structural family of oxides of the type Li₈MO₆, either pure or doped with calcium, may be represented by the formula Li^tetr.₆[Ca Ce□]^oct.O₆. Electrical and structural data are correlated for this compound.     

Infrared Spectroscopic Study of LiCoO2 Thin Films

Site disorder of Co³⁺ ions in sputtered films of lithium cobaltite has been examined using infrared spectroscopy. Both transmission and reflectance modes have been used to characterize the nature of IR absorption. It is found that Co³⁺ in the sputtered films occupy two types of octahedral sites that differ in the nature of second-neighbor environment. Li⁺ ions exhibit two bands, which may arise from tetrahedral and octahedral site occupancies or from the presence of disordered regions in the films.     

Desordre atomique,magnétique et électronique dans les ferrites Zn1−xGexFe2O4 (x = 0,25 − 0,5 − 0,75)

Given the interest in magnetite, it is tempting to try modulating the electron concentration of its octahedral subarray through the x value in Zn_1−xGe_xFe₂O₄, providing that Zn and Ge remain localized on the tetrahedral sites. This is what Gerard and Grandjean concluded from the Mössbauer effect measurements; they claim that double exchange is generalized over the octahedral sites. But Miyahara and Sai have deduced from X-ray diffraction that some iron is on the tetrahedral site. We have therefore reexamined this question with a larger span of techniques. X-Ray diffraction measurements allow an evaluation of τ in M_1−τFe_τ[M_τFe_2−τ]O₄, with M = Zn, Ge: τ = 0.30 − 0.30 and 0.08 for x = 0.25 − 0.5 − 0.75, respectively. Magnetic measurements reveal an important disorder: the magnetization, much lower than expected from an octahedral double exchange, decreases very slowly in the temperature range 100–600 K, and depends on the applied field; saturation is difficult to reach; a high coercitive field and after-effect are recorded. The Mössbauer measurements do not preclude a small tetrahedral signal. They also display a progressive ordering of Fe during cooling. The electrical conductivity, much more activated than in magnetite, is typical of small polarons with Anderson localization; there is no electron ordering, Verwey type, below 110 K. All the data are consistent with the occupation of a fraction of tetrahedral sites by iron.     

Anomalous site disorder in metal hydrides

We point out that existing measurements in fcc metal hydrides LaH_x and YH_x of intrinsic disorder x₀(2) (octahedral site occupation at the stoichiometric composition x = 2) appear to be highly inconsistent with one another by thermodynamic and spectroscopic techniques. The high-temperature thermodynamic data give x₀(2) ～ 0.01, while NMR, ESR, and neutron scattering have x₀(2) an order of magnitude larger at room temperature, where conventional wisdom indicates it should be less. This is explained by a model in which large-amplitude hydrogen vibrations at high temperature prevent occupation of both a tetrahedral and neighbor octahedral sites. Hence x₀(2) decreases at high temperature, and good agreement is found with reasonable values of the parameters.