Absence Of Jahn-Teller Distortion Research Articles

Real-Space Observation of Ligand Hole State in Cubic Perovskite SrFeO3.

An anomalously high valence state sometimes shows up in transition-metal oxide compounds. In such systems, holes tend to occupy mainly the ligand p orbitals, giving rise to interesting physical properties such as superconductivity in cuprates and rich magnetic phases in ferrates. However, no one has ever observed the distribution of ligand holes in real space. Here, a successful observation of the spatial distribution of valence electrons in cubic perovskite SrFeO3 by high-energy X-ray diffraction experiments and precise electron density analysis using a core differential Fourier synthesis method is reported. A real-space picture of ligand holes formed by the orbital hybridization of Fe 3d and O 2p is revealed. The anomalous valence state in Fe is attributed to the considerable contribution of the ligand hole, which is related to the metallic nature and the absence of Jahn-Teller distortions in this system.

Understanding the thermal stability and bonding characteristic of Li x Ni0.5Mn1.5O4 as cathode materials for lithium-ion battery from first principles

The thermodynamic stability is a very important quantity for the electrode materials, because it is not only related to the electrochemical performances of the materials but also the safety issue of the cells. To evaluate the thermodynamic stability of LixNi0.5Mn1.5O4 (x = 0, 1), the formation enthalpies from elemental phases and oxides were obtained. The values for LiNi0.5Mn1.5O4 were calculated to be −1341.10 and −141.84 kJ mol−1, while those for Ni0.5Mn1.5O4 were −949.11 and −49.21 kJ mol−1. These values are much more negative than those of LiCoO2 and LiNiO2 compounds, indicating that the thermodynamic stability of LixNi0.5Mn1.5O4 is better than the two classic compounds. To clarify the microscopic origin, the density of states, magnetic moments, and bond orders were systematically investigated. The results showed that the excellent thermodynamic stability of LiNi0.5Mn1.5O4 is attributed to the absence of Jahn-Teller distortions, strong electrostatic interactions of Li–O ionic bond, and strong Ni–O/Mn–O ionic-covalent mixing bonds. After lithium extraction, the disappearance of the pure Li–O bonds leads to an increase of formation enthalpy, indicating a decreasing thermodynamic stability for Ni0.5Mn1.5O4 with respect to LiNi0.5Mn1.5O4.

Role of charge transfer configurations in LaMnO3, CaMnO3, and CaFeO3

A simple scheme is proposed to analyze the N-electron wave function obtained in embedded cluster calculations in valence bond terms such as ligand-to-metal charge transfer and non-charge-transfer determinants. The analysis is based on a unitary transformation of pairs of natural orbitals to optimal atomiclike orbitals. The procedure is applied to compare the degree of ionicity in NiO and MnO and to explain the existence or absence of Jahn-Teller distortions in LaMnO3, CaMnO3, and CaFeO3. The authors find that the ground state of LaMnO3 is dominated by non-charge-transfer configurations, whereas the charge transfer configurations dominate the ground state wave function in the other two perovskites.

Electronic structure of the two-dimensional negative charge-transfer materialSr3FeMO7(M=Fe, Co)

We studied the relation between the electronic structure and the physical properties of ${\mathrm{Sr}}_{3}\mathrm{Fe}M{\mathrm{O}}_{7}$ ($M=\mathrm{Fe}$, Co). The main technique used in the study was Fe $2p$, Co $2p$, and O $1s$ x-ray-absorption spectroscopy. The experimental spectra were analyzed in terms of cluster model and band-structure calculations. The analysis of the spectra shows that both ${\mathrm{Sr}}_{3}{\mathrm{FeFeO}}_{7}$ and ${\mathrm{Sr}}_{3}{\mathrm{FeCoO}}_{7}$ are in the negative-charge-transfer regime (this means that their ground states are highly covalent and contain considerable O $2p$ hole character). The Fe ions are in a high-spin state and the Co ions in an intermediate-spin state stabilized by strong hybridization. The electronic structure helps to explain the trends in the electrical conductivity and the observed magnetic moments. Further, the relatively large O $2p$ hole weight in the ground state explains also the absence of Jahn-Teller distortions. ${\mathrm{Sr}}_{3}{\mathrm{FeCoO}}_{7}$ illustrates ferromagnetism and magnetoresistance in a two-dimensional negative-charge-transfer material.

Virtual free ion magnetism and the absence of Jahn-Teller distortion in a linear two-coordinate complex of high-spin iron(II).

Mössbauer spectroscopy and dc magnetization measurements have been used to characterize the low temperature magnetism of a rigorously linear, two-coordinate complex of high-spin Fe(II), Fe[(C(Si(CH3)3]2 (1). The local C-Fe-C chromophore of 1 exhibits novel slow, single-ion paramagnetic relaxation and fully resolved magnetic hyperfine splitting of its zero field Mössbauer spectrum over the range approximately 100 to approximately 50K. The hyperfine field at 4.2 K is 152 T! This is the largest magnetic hyperfine field observed for iron to date regardless of spin, oxidation state, or coordination environment. This observation is attributable to the large unquenched orbital angular momentum corresponding to the degenerate ground (dxy, dx2-y2) orbital pair of 1 in local Dinfinityh symmetry. Maintenance of the ground-state degeneracy is required by the Jahn-Teller theorem leading to the unprecedented result that the magnitude of the magnetic moment of 1's 5Deltag ground state is essentially that of the parent free ion (5D4) ground term.

Investigation of Structural Fatigue in Spinel Electrodes Using In Situ Laser Probe Beam Deflection Technique

The presence of the tetragonal phase at the surface of a particle due to a Jahn-Teller effect was previously reported to be one of the reasons for capacity fade observed during cycling of in the 4 V range. This study reports on the evidence of the onset of a Jahn-Teller effect above 3 V at the surface of particle of thin-film electrodes using in situ laser probe beam deflection technique. The thin film electrode was prepared by electrostatic spray deposition technique. The cyclic deflectograms and cyclic voltammograms were obtained simultaneously, and the potential range responsible for the Jahn-Teller distortion in the 4 V range was determined by differentiating the deflectograms against time. We observed three sets of peaks in both the cathodic and anodic scan of the differential cyclic deflectograms and the magnitudes of these are changed with the scan rate. In the absence of Jahn-Teller distortion, the shape of the differential deflectograms would be identical to that of cyclic voltammograms. Analysis of the differential strain curves obtained from the cyclic deflectograms and the cyclic voltammograms indicates that one of the cathodic peaks of the differential cyclic deflectograms is due to the onset of the Jahn-Teller effect and one of the anodic peaks is due to the relaxation of the tetragonal phase, formed in the previous cathodic scan due to the Jahn-Teller effect, to cubic phase. The Jahn-Teller effect during the cathodic scan is observed to occur around depending on the scan rates, i.e., at slower scan rates the cathodic peak of the differential strain curves became smaller. This could be ascribed to the suppression of the Jahn-Teller effect at near equilibrium conditions. © 2001 The Electrochemical Society. All rights reserved.

Metal-metal bonding in theKSbO3-type oxidesLa4Ru6O19andLa3Ru3O11:A mechanism for band gap formation int2gstates

The electronic structures of ${\mathrm{La}}_{3}{\mathrm{Ru}}_{3}{\mathrm{O}}_{11}$ and ${\mathrm{La}}_{4}{\mathrm{Ru}}_{6}{\mathrm{O}}_{19}$ were calculated using a LMTO-ASA-TB method. Although both compounds have similar conduction networks, ${\mathrm{La}}_{3}{\mathrm{Ru}}_{3}{\mathrm{O}}_{11}$ was found to have a very normal band structure while ${\mathrm{La}}_{4}{\mathrm{Ru}}_{6}{\mathrm{O}}_{19}$ showed many striking features. The ${t}_{2g}$ band is split into three portions of clearly defined orbital origin. The two highest-energy portions are extremely narrow (\ensuremath{\sim}0.15 eV), and each have a capacity of only 1 electron per Ru atom. The highest portion of the ${t}_{2g}$ band is separated from the remainder of the band by a gap of 0.5 eV. All these features occur in the absence of Jahn-Teller distortions. The origin of the gap is instead found to lie in metal-metal bonding due to the unusually short Ru-Ru distance in ${\mathrm{La}}_{4}{\mathrm{Ru}}_{6}{\mathrm{O}}_{19}$ $(2.5\AA{}$ vs $3.0\AA{}$ for ${\mathrm{La}}_{3}{\mathrm{Ru}}_{3}{\mathrm{O}}_{11}).$

Jahn-Teller-like progressions in the absence and presence of Jahn-Teller distortions

Prominent progressions of non-totally symmetric modes may ensue in non-linear molecules in electronic transitions involving degenerate states in the absence of Jahn-Teller distortions. In the presence of Jahn-Teller interactions prominent non-Jahn-Teller progressions may result.

Slow substitution reactions of the copper complex of 2,2′,2″-triaminotriethylamine

The substitution reactions of CuII complexes proceed relatively slowly in the absence of Jahn-Teller distortions.