Showing papers on "Valence (chemistry) published in 2000"
TL;DR: In this article, the absolute energy positions of conduction and valence band edges were compiled for about 50 each semiconducting metal oxide and metal sulfide minerals, and the relationship between energy levels at mineral semiconductor-electrolyte interfaces and the activities of these minerals as a catalyst or photocatalyst in aqueous redox reactions were reviewed.
Abstract: The absolute energy positions of conduction and valence band edges were compiled for about 50 each semiconducting metal oxide and metal sulfide minerals. The relationships between energy levels at mineral semiconductor-electrolyte interfaces and the activities of these minerals as a catalyst or photocatalyst in aqueous redox reactions are reviewed. The compilation of band edge energies is based on experimental flatband potential data and complementary empirical calculations from electronegativities of constituent elements. Whereas most metal oxide semiconductors have valence band edges 1 to 3 eV below the H2O oxidation potential (relative to absolute vacuum scale), energies for conduction band edges are close to, or lower than, the H2O reduction potential. These oxide minerals are strong photo-oxidation catalysts in aqueous solutions, but are limited in their reducing power. Non-transition metal sulfides generally have higher conduction and valence band edge energies than metal oxides; therefore, valence band holes in non-transition metal sulfides are less oxidizing, but conduction band electrons are exceedingly reducing. Most transition-metal sulfides, however, are characterized by small band gaps (<1 eV) and band edges situated within or close to the H2O stability potentials. Hence, both the oxidizing power of the valence band holes and the reducing power of the conduction band electrons are lower than those of non-transition metal sulfides.
3,145 citations
TL;DR: In this article, the authors used time-of-flight neutron powder diffraction to determine the correct crystal structures for Sr4Fe4O11 (orthorhombic, space group Cmmm, a=10.974(1) A, b=7.702(1), and c=5.473(1)-A) and Sr8Fe8O23 (tetragonal, I4/mmm) through comparison of the goodness of fit for Rietveld refinements of candidate models and bond-length distributions.
409 citations
TL;DR: In this article, the electronic structures and work functions of pristine and Cs-intercalated single-walled carbon nanotube bundles were investigated using ultraviolet photoemission spectroscopy.
Abstract: The electronic structures and the work functions of pristine and Cs-intercalated single-walled carbon nanotube bundles were investigated using ultraviolet photoemission spectroscopy. The valence bands of the pristine bundles were considerably altered from those of graphite. A spectral shift to the higher binding energy side was observed in the Cs-intercalated sample. The work function of the pristine bundles was found to be 4.8 eV, which is 0.1–0.2 eV larger than that of graphite. A drastic decrease of the work function to about 2.4 eV was observed in the Cs-intercalated sample.
365 citations
TL;DR: Current progress in applying EELS for quantitative determination of Mn and Co valences in magnetic oxides, including valence state transition, quantification of oxygen vacancies, refinement of crystal structures, and identification of the structure of nanoparticles are reported.
330 citations
TL;DR: In this paper, the authors adopt a pragmatic approach to matters involving chemical bonding and use bond-valence theory to deal with the sulfite and thiosulfate groups, where one of the S atoms is an anion and is involved in a tetrahedral group.
Abstract: Sulfur is the fifteenth most abundant element in the continental crust of the Earth (260 ppm), and the sixth most abundant element in seawater (885 ppm). Sulfur (atomic number 16) has the ground-state electronic structure [Ne]3 s 2 3p 4 , and is the first of the group VIB elements in the periodic table (S, Se, Te, Po). In minerals, sulfur can occur in the formal valence states S2−, S, S4+, and S6+, corresponding to the sulfide minerals, native sulfur , the sulfite minerals, and the sulfate minerals. In the sulfide minerals, S2− functions as a simple anion (e.g. CuFeS2, chalcopyrite) and as a compound S2 anion (e.g. FeS2, pyrite). In the sulfosalts, S2− functions as a component of a complex anion (e.g. AsS3 in tennantite, Cu12As4S13). In the sulfite minerals, S4+ has four valence electrons available for chemical bonding, and occurs in triangular pyramidal coordination with O. In the sulfate minerals, S6+ has six valence electrons available for bonding, and occurs in tetrahedral coordination with O. In addition, there are the thiosulfate minerals, in which S is in the hexavalent state, but is coordinated by three O2− anions and one S2− anion. Chemists frequently write the thiosulfate group as S2O3; however, we write it as SO3S to emphasize that one of the S atoms is an anion and is involved in a tetrahedral group. Although the focus of this chapter is the sulfate minerals, we will deal also with the sulfite and thiosulfate minerals, as they occur in the same types of geochemical environments.
We adopt a pragmatic approach to matters involving chemical bonding. We use bond-valence theory …
287 citations
TL;DR: The participation of 4f electrons in the bonding of the lanthanide complexes Ln(NR2)3; R = H, SiH3, has been investigated at the DFT level as mentioned in this paper.
Abstract: The participation of 4f electrons in the bonding of the lanthanide complexes Ln(NR2)3; R = H, SiH3, has been investigated at the DFT level. Structural parameters obtained with small core (f electrons in the valence) and large core (f electrons in the core) effective core potentials (ECPs) suggest the nonparticipation of the f electrons to the Ln−N bonding. A methodological study has been carried out on the lanthanide contraction with various ab initio methods using large core ECPs. The calculated lanthanide contraction (0.180 A) is in excellent agreement with the experimental value (0.179 A). Comparison of calculated structural parameters with available X-ray data shows that calculations with large core ECPs and density functional methods quantitatively reproduce the bonding at the lanthanide.
282 citations
TL;DR: The determination of the bulk Ce 4f electronic states of these compounds resolves differences, and the power of this technique is demonstrated by applying it to the cerium compounds CeRu2Si2 and CeRu 2.
Abstract: Electron correlations are known to play an important role in determining the unusual physical properties of a variety of compounds Such properties include high-temperature superconductivity, heavy fermion behaviour and metal-to-insulator transitions High-resolution photoelectron spectroscopy (PES) provides a means of directly probing the electronic states (particularly those near the Fermi level) in these materials, but the short photoelectron mean free paths (< or = 5 A) associated with the low excitation energies conventionally used (< or = 120 eV) make this a surface-sensitive technique Now that high-resolution PES is possible at much higher energies, with mean free paths as long as 15 A (ref 6), it should become feasible to probe the bulk electronic states in these materials Here we demonstrate the power of this technique by applying it to the cerium compounds CeRu2Si2 and CeRu2 Previous PES studies of these compounds revealed very similar spectra for the Ce 4f electronic states, yet it is expected that such states should be different owing to their differing degrees of hybridization with other valence bands Our determination of the bulk Ce 4f electronic states of these compounds resolves these differences
243 citations
TL;DR: In this article, high-level ab initio calculations have been performed on dihydrogen-bonded complexes with hydrogen fluoride (HF) as a proton-donating molecule and simple molecules as proton acceptors.
Abstract: High-level ab initio calculations have been performed on dihydrogen-bonded complexes with hydrogen fluoride (HF) as a proton-donating molecule and simple molecules as proton-acceptors (CH4, SiH4, BeH2, MgH2, LiH, and NaH). MP4(SDQ)/6-311++G** and QCISD(T)/6-311++G** results show that H-bond energies for such systems are significant. For example, the H-bond energy is −11.9 kcal/mol for the LiH···HF complex at the QCISD(T)/6-311++G** level of theory; the basis set superposition error (BSSE) was included, and the geometry of the complex was optimized at the QCISD/6-311++G** level. The relationships between the geometrical parameters of these complexes are in good agreement with those obtained from the bond valence model. The BSSE is taken into account in all levels of calculations. A comparison of the results of the calculations shows that the MP2/6-311++G** level of theory is sufficient for a description of dihydrogen-bonded complexes. Additionally, Bader's theory is included in the analysis of the investig...
226 citations
TL;DR: The equilibrium geometries and bond-dissociation energies for loss of one CO and loss of six CO from TM(CO)6q (TMq = Hf2-, Ta-, W, Re+, Os2+, Ir3+) have been calculated at the BP86 level using Slat...
Abstract: The equilibrium geometries and bond-dissociation energies for loss of one CO and loss of six CO from TM(CO)6q (TMq = Hf2-, Ta-, W, Re+, Os2+, Ir3+) have been calculated at the BP86 level using Slat...
217 citations
TL;DR: A chemical view of spin magnetic phenomena in finite and infinite systems using the concepts of bonding and electronic shielding is presented and the exceptional occurence of ferromagnetism only in the first transition series appears to parallel the special main-group chemistry of the first long period.
Abstract: A chemical view of spin magnetic phenomena in finite (atoms and molecules) and infinite (transition metals and their alloys) systems using the concepts of bonding and electronic shielding is presented The concept is intended to serve as a semiquantitative signpost for the synthesis of new ferromagnets After a concise overview of the historic development of related theories developed within the physics community, the consequences of spin-spin coupling (made manifest in the exchange or Fermi hole) in atoms and molecules are explored Upon moving to a paramagnetic state, the majority/minority spin species become more/less tightly bound to the nucleus, resulting in differences in the energies and spatial extents of the two sets of spin orbitals By extrapolating well-known arguments from ligand-field theory, the paucity of ferromagnetic transition metals arises from quenching the paramagnetism of the free atoms due to strong interatomic interactions in the solid state Critical valence electron concentrations in Fe, Co, and Ni, however, result in local electronic instabilities due to the population of antibonding states at the Fermi level varepsilon(F) Removal of these antibonding states from the vicinity of varepsilon(F) is the origin of ferromagnetism; in the pure metals this results in strengthening the chemical bonds In the 4d and 5d transition metals, the valence d orbitals are too well shielded from the nucleus, so a transition to a ferromagnetic state does not result in sufficiently large changes to occur Thus, the exceptional occurence of ferromagnetism only in the first transition series appears to parallel the special main-group chemistry of the first long period A connection between ferromagnetism in the transition metals and Pearson's absolute hardness eta is easily established and shows that ferromagnetism appears only when eta<02 eV in the nonmagnetic calculation As expected from the principle of maximum hardness, Fe, Co, and Ni all become harder upon moving to the more stable ferromagnetic state Magnetism in intermetallic alloys follows the same path Whether or not an alloy contains ferromagnetic elements, the presence of antibonding states at varepsilon(F) serves as a "fingerprint" to indicate a ferromagnetic instability The differences in the sizes of the local magnetic moments on the constituent atoms of a ferromagnetic alloy can be understood in terms of the relative contributions to the density of states at varepsilon(F) in the nonmagnetic calculations Appropriately parameterized, nonmagnetic, semi-empirical calculations can also be used to expose the ferromagnetic instability in elements and alloys These techniques, which have become relatively commonplace, can be used to guide the synthetic chemist in search of new ferromagnetic materials
197 citations
TL;DR: In this paper, the preparation of LiCoyMnxNi1−x−yO2 from LiOH·H2O, Ni(OH)2 and γ-MnOOH in air was studied in detail.
TL;DR: In this paper, a simple valence model coupled with Coulomb energy and Lennard-Jones 9−6 functional terms was optimized to yield accurate prediction of structural, vibrational, and thermophysical properties for these molecules.
Abstract: As a part of the COMPASS force field development, a number of small inorganic molecules were parametrized for condensed-phase applications. Using a simple valence model coupled with Coulomb energy and Lennard-Jones 9−6 functional terms, the parameters were optimized to yield accurate prediction of structural, vibrational, and thermophysical properties for these molecules. Extended validation on liquid nitrogen (N2) and carbon dioxide (CO2) in normal and supercritical conditions demonstrates that the present force field is capable of predicting various thermophysical properties in a very broad range of experimental conditions.
TL;DR: In this article, Hartree−Fock calculations have been performed on the bcc lattices of Li, Na, K, V, and on the fcc ones of Al, Ca, Sc, Cu in order to investigate the topological properties of the electron charge density and the electron localization function.
Abstract: Periodic Hartree−Fock calculations have been performed on the bcc lattices of Li, Na, K, V, and on the fcc ones of Al, Ca, Sc, Cu in order to investigate the topological properties of the electron charge density and of the electron localization function, ELF All systems are calculated to be conductors It is found that the existence of nonnuclear attractors of the electron charge density gradient field first evidenced in lithium clusters is not a prerequisite for metallic behavior They are missing the V and Cu cells The topology of ELF is characterized by di- or polysynaptic valence basins The value of ELF at the valence basin attractors η(ra) is rather low: typically less than 06, which is consistent with an antiparallel pairing close to that of a homogeneous electron gas At the index 1 saddle points located on the separatrices between valence basins, the ELF value η(rs) is very close to that at the valence attractors η(ra) The isosurface η(r) = η(rs) − e defines a reducible valence domain which
TL;DR: In this article, evidence for the formation of a valencefluctuation state of iron, formally denoted as Fe2.5+, is presented for the Sr2FeMoO6−w double perovskite, known for exhibiting a very large magnetoresistance.
Abstract: In this letter evidence for the formation of a valence-fluctuation state of iron, formally denoted as Fe2.5+, is presented. The system under study is the Sr2FeMoO6−w double perovskite, known for exhibiting a very large magnetoresistance. Samples of Sr2FeMoO6−w were synthesized by means of an encapsulation technique utilizing an Fe getter technique and characterized by 57Fe Mossbauer spectroscopy. From 5 K to room temperature the Mossbauer spectrum is dominated by a component with hyperfine parameter values between those expected for high-spin Fe3+ and high-spin Fe2+.
TL;DR: The coordination of antimony in the SnO2 nanocrystallites has been investigated by extended x-ray absorption fine structure measurements (EXAFS) at the Sb K-edge at 5 K while its valence state was determined by near edge X-ray infrared fine structure measurement (XANES) at Sb L1 edge as mentioned in this paper.
Abstract: Colloids of nanocrystalline tin dioxide containing 9.1 at. % and 16.7 at. % antimony have been prepared by the coprecipitation method. High-resolution transmission electron microscopy (TEM) images show crystalline particles in the 2–6 nm size regime. X-ray powder diffraction patterns of nanocrystalline powders obtained by drying the colloids and heating to 100 °C indicate the same rutile lattice structure known from bulk SnO2. On heating to 500 °C in air, the nanocrystalline powder shows a slight increase in particle size but especially a change in color from yellowish to bluish which is accompanied by the development of n-type conductivity. The coordination of antimony in the SnO2 nanocrystallites has been investigated by extended x-ray absorption fine structure measurements (EXAFS) at the Sb K-edge at 5 K while its valence state was determined by near edge x-ray absorption fine structure measurements (XANES) at the Sb L1 edge. The Sb higher neighbor shell distances in the doped material differ from the ...
TL;DR: In this article, the effect of Coulomb interaction on the superconducting transition temperature was investigated in an extended periodic Anderson model with the f-c Coulomb interactions, and it was shown that the sharp valence change is caused by U fc with moderate strength of the order of the conduction electrons.
Abstract: Properties of an extended periodic Anderson model with the f-c Coulomb interaction U fc is studied as a model for CeCu 2 Ge 2 and related compounds which is cosidered to exhibit a sharp valence change under pressure. The problem is treated by extending the slave-boson and large- N expansion method to treat the present system. It is shown that, as the f-level e f is increased relative to the Fermi level, the sharp valence change is caused by the effect of U fc with moderate strength of the order of the bandwith of conduction electrons. The superconducting transition temperature T c due to the valence-fluctuation exchange is estimated on the slave-boson fluctuation approximation. In the model with spherical Fermi surface, T c exhibits sharp peak as a function of e f , simulating the effect of pressure, for the d -wave pairing channel.
TL;DR: Two series of contracted all-electron basis sets are presented for Rb to Xe which were obtained by full optimization of atomic SCF energies and are of split valence type and better than double zeta quality for remaining valence orbitals.
Abstract: Two series of contracted all-electron basis sets are presented for Rb to Xe which were obtained by full optimization of atomic SCF energies. The smaller, economic basis is of split valence type. The larger is of triple
zeta type
for the 4d
atomic orbitals and better than double zeta quality for remaining valence orbitals.
TL;DR: The defect chemistry and transport properties of single-crystalline 9.5 mol% Y2O3-stabilized ZrO2 selectively doped with 3D transition metal or rare earth ions, are analyzed in this article.
TL;DR: In this article, a detailed quantum chemical analysis of the underlying principles of hyperfine coupling in 3D transition metal complexes has been carried out and the explicit evaluation of one and two-electron integrals for some atomic systems has been used to understand the spin polarization of the core shells.
Abstract: A detailed quantum chemical analysis of the underlying principles of hyperfine coupling in 3d transition metal complexes has been carried out. The explicit evaluation of one- and two-electron integrals for some atomic systems has been used to understand the spin polarization of the core shells. While spin polarization enhances the exchange interaction of the 2s and 2p shells with the singly occupied orbitals, the opposite spin polarization of the 3s and 3p shells arises from the required orthogonality to the 2s and 2p shells, respectively. Core-shell spin polarization in molecules is found to be proportional to the spin population in the valence 3d orbitals but to depend little on other details of bonding. In contrast, the spin polarization of the valence shell depends crucially on the overlap between the singly occupied and certain doubly occupied valence orbitals. Large overlap leads to pronounced spin polarization of these orbitals and, among other things, likely to spin contamination when using UHF wa...
TL;DR: In this paper, the effect of the Coulomb interaction on the superconducting transition temperature was investigated in an extended periodic Anderson model with the f-c Coulomb interactions and it was shown that the sharp valence change is caused by the effect with moderate strength of the order of the conduction electrons.
Abstract: Properties of an extended periodic Anderson model with the f-c Coulomb interaction $\Ufc$ is studied as a model for \Ge and related compounds which is cosidered to exhibit a sharp valence change under pressure. The problem is treated by extending the {\it slave-boson} and large-N expansion method to treat the present system. It is shown that, as the f-level $\epsilon_{\rm f}$ is increased relative to the Fermi level, the sharp valence change is caused by the effect of $\Ufc$ with moderate strength of the order of the bandwith of conduction electrons. The superconducting transition temperature $T_\rc$ due to the valence-fluctuation exchange is estimated on the slave-boson fluctuation approximation. In the model with spherical Fermi surface, $T_\rc$ exhibits sharp peak as a function of $\epsilon_{\rm f}$, simulating the effect of pressure, for the d-wave pairing channel.
TL;DR: In this paper, a comprehensive study of the electronic structure of group-III nitrides (AlN, GaN, InN, and BN) crystallizing in the wurtzite, zinc-blende, and graphitelike hexagonal (BN) structures is presented.
Abstract: A comprehensive study of the electronic structure of group-III nitrides (AlN, GaN, InN, and BN) crystallizing in the wurtzite, zinc-blende, and graphitelike hexagonal (BN) structures is presented. A large set of the x-ray emission and absorption spectra was collected at the several synchrotron radiation facilities at installations offering the highest possible energy resolution. By taking advantage of the linear polarization of the synchrotron radiation and making careful crystallographic orientation of the samples, the bonds along c axis ({pi}) and ''in plane'' ({sigma}) in the wurtzite structure could be separately examined. Particularly for AlN we found pronounced anisotropy of the studied bonds. The experimental spectra are compared directly with ab initio calculations of the partial density of states projected on the cation and anion atomic sites. For the GaN, AlN, and InN the agreement between structures observed in the calculated density of states (DOS) and structures observed in the experimental spectra is very good. In the case of hexagonal BN we have found an important influence of insufficient core screening in the x-ray spectra that influences the DOS distribution. The ionicity of the considered nitrides is also discussed. (c) 2000 The American Physical Society.
TL;DR: The types of structures formed when two or more cations occupy the NaCl unit cell in a non-random manner are reviewed in this article, with reference to the cation arrangement in layers of close-packed octahedra.
Abstract: The types of structures formed when two or more cations occupy
the sodium sites in the NaCl unit cell in a non-random manner are reviewed
with reference to the cation arrangement in layers of close-packed octahedra
and the cation arrangement in the NaCl subcell. Factors influencing the stability
of particular structure types, including bond valence and radius ratios, are
discussed.
TL;DR: In this paper, the bulk structure of molybdenum trioxide (MoO3−x) under reductive and oxidative reaction conditions was investigated in situ with time-resolved X-ray absorption spectroscopy (XAS).
TL;DR: The crystal chemistry of ternary equiatomic europium compounds EuTX (T = transition metal; X = element of the III, IV, or V main group) is reviewed in this article.
Abstract: The crystal chemistry of ternary equiatomic europium compounds EuTX (T = transition metal; X = element of the III, IV, or V main group) is reviewed. Besides preparation techniques we have especially focused on the structure−property relationships and peculiarities in chemical bonding. A main interest in these compounds is the valence state of the europium atoms. The magnetic susceptibility and electrical conductivity data, 151Eu Mossbauer spectroscopic results and LIII X-ray absorption experiments are discussed in detail.
TL;DR: In this article, the turnover frequency (TOF) for conversion of neo-pentane was determined for Pt in Y zeolite with different numbers of protons and La C3 ions, different Si/Al ratios and with non-framework Al being present.
Abstract: The turnover frequency (TOF) for conversion of neo-pentane was determined for Pt in Y zeolite with different numbers of protons and La C3 ions, different Si/Al ratios and with non-framework Al being present. Comparing Pt/NaY to Pt/H-NaY and Pt/K-USY with Pt/H-USY, respectively, shows an increase in the ln (TOF) which is proportional to the number of protons. Compared to NaY, the TOF of Pt in non-acidic NaLaY zeolite is about 25 times higher, which indicates also a strong influence on the charge of the cations on the TOF of Pt. The 20 times increase in the Pt TOF for K-USY compared to NaY is attributed to the effect of a higher Si/Al ratio and non-framework Al in the K-USY. EXAFS data collected on Pt/NaY and Pt/H-USY showed platinum particles consisting of 14‐20 atoms on an average. These results were confirmed by HRTEM, which also showed that the Pt particles were dispersed inside the zeolite. The EXAFS data indicate that the metal particles are in contact with the oxygen ions of the support. The peak in the Fourier transform of the atomic XAFS (AXAFS) spectrum of the Pt/H-USY is larger in intensity than the corresponding peak of the Pt/Na-Y data. A detailed analysis of the L2 and the L3 X-ray absorption near edge structure revealed a shape resonance due to the Pt‐H anti-bonding state (AS) induced by chemisorption of hydrogen on the surface of the platinum metal particles. The difference in energy (Eres) between the AS and the Fermi-level (EF) is 4.7 eV larger for Pt/H-USY than for Pt/NaY. Both the AXAFS spectra and the shape resonances of the Pt-NaY and the Pt/H-USY catalysts provide direct experimental evidence of how the support properties determine the electronic structure of the platinum metal particles. Previous AXAFS and shape resonance work lead to a model in which the position in energy of the Pt valence orbitals is directly influenced by changes in the potential (i.e. electron charge) of the oxygen ions of the support and how the proton density affects this oxygen charge. This work shows that the potential of the oxygen ions is also a function of the Si/Al ratio of the support and the polarisation power of the charge compensating cations (H C ,N a C ,L a 3C and extra-framework Al); the metal particles experience an interaction which is determined by several properties of the support. The data further reveal how the change in the Pt electronic structure directly influences the catalytic properties of the catalyst. While the TOF is dependent on the metal‐support interaction, the hydrogenolysis selectivity is determined by the Pt particle size, and increases linearly with increasing dispersion, or decreasing particle size. ©2000 Elsevier Science B.V. All rights reserved.
TL;DR: In this paper, the authors employed the combined laser MBE (CLMBE) method to fabricate epitaxial ZnO thin films doped with all the 3d transition metal ions and studied the solubility behavior of 3d ions from the viewpoints of ionic radius and valence state.
TL;DR: Comparison of the present iminato (UNPR3) calculations with previous extended Hückel work on d block imido (MNR) systems reveals that in all cases there is little or no preference for linearity over bending at the nitrogen when R is (a) only sigma-bound to the nitrogen and (b) sterically unhindered.
Abstract: The electronic and geometric structures of the title species have been studied computationally using quasi-relativistic gradient-corrected density functional theory. The valence molecular orbital ordering of UO22+ is found to be πg < πu < σg ≪ σu (highest occupied orbital), in agreement with previous experimental conclusions. The significant energy gap between the σg and σu orbitals is traced to the “pushing from below” mechanism: a filled−filled interaction between the semi-core uranium 6p atomic orbitals and the σu valence level. The U−N bonding in UON+ and UN2 is significantly more covalent than the U−O bonding in UON+ and UO22+. UO(NPH3)3+ and U(NPH3)24+ are similar to UO22+, UON+, and UN2 in having two valence molecular orbitals of metal−ligand σ character and two of π character, although they have additional orbitals not present in the triatomic systems, and the U−N σ levels are more stable than the U−N π orbitals. The inversion of U−N σ/π orbital ordering is traced to significant N−P (and P−H) σ c...
TL;DR: In this article, single-phase compounds Li2Mn3MO8 (M = Mg, Co, Ni, Cu) have been synthesized and investigated as replacements of LiMn2O4 for lithium intercalation below 3 V.
Abstract: Single-phase compounds Li2Mn3MO8 (M = Mg, Co, Ni, Cu) have been synthesized and investigated as replacements of LiMn2O4 for lithium intercalation below 3 V. They all retain the spinel structure, with cation ordering on the octahedral M (16d) site for M = Mg only. Cell parameters vary as Co < Ni < Mg ≈ Cu < Mn and average M–O bond lengths as Co ≈ Ni < Cu < Mg < Mn. Lithium was intercalated both chemically and electrochemically. Electrochemical potential step spectroscopy shows features typical of a two-phase intercalation reaction, in spite of a manganese valence range mostly above the accepted Jahn–Teller distortion limit (50% Mn3+). The tetragonal distortion is only noticeable at high intercalation levels. It yields c/a distortion values much lower for M = Co or Ni than for unsubstituted LiMn2O4. However, no improvement in electrochemical cyclability was obtained. Magnetic susceptibility measurements show features typical of frustrated systems, as expected for the 16d sublattice, and confirm that chemical intercalation reaches lithium contents close to the theoretical limit (one additional Li per AB2O4 formula unit). For cobalt substitution, bond length and Curie constant analysis both lead to a charge distribution Li2[(Mn4+)2Mn3+Co3+]O8 rather than Li2[(Mn4+)3Co2+]O8.