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Spin transition in LaCoO3 investigated by resonant soft X-ray emission spectroscopy
Abstract
The spin transition in LaCoO3 is investigated by temperature-dependent
resonant soft X-ray emission spectroscopy near the Co 2p absorption edges. This
element-specific technique is more bulk sensitive with respect to the
temperature induced spin-state of the Co3+ ions in LaCoO3 than other
high-energy spectroscopic methods. The spin transition is interpreted and
discussed with ab-initio density-functional theory within the fixed-spin moment
method, which is found to yield consistent spectral functions to the
experimental data. The spectral changes for LaCoO3 as a function of temperature
suggest a change in spin-state as the temperature is raised from 85 to 300 K
while the system remains in the same spin state as the temperature is further
increased to 510 K.
... eV. The spectral feature is similar to the Co L 3 -edge RIXS spectra of LS Co 3+ in LaCoO 3 20,21 , but its crystal-field excitation energy is much larger 21 . Tomiyasu et al. reported Co L 3 -edge RIXS of LaCoO 3 single crystal with high energy resolution (~80 meV). ...
Secondary batteries are important energy storage devices for a mobile equipment, an electric car, and a large-scale energy storage. Nevertheless, variation of the local electronic state of the battery materials in the charge (or oxidization) process are still unclear. Here, we investigated the local electronic state of cobalt-hexacyanoferrate (Na$_x$Co[Fe(CN)$_6$]$_{0.9}$), by means of resonant inelastic X-ray scattering (RIXS) with high energy resolution (~100 meV). The L-edge RIXS is one of the most powerful spectroscopic technique with element- and valence-selectivity. We found that the local electronic state around Co$^{2+}$ in the partially-charged Na$_{1.1}$Co$^{2+}$$_{0.5}$Co$^{3+}$$_{0.5}$[Fe$^{2+}$(CN)$_6$]$_{0.9}$ film (x = 1.1) is the same as that of the discharged Na$_{1.6}$Co$^{2+}$[Fe$^{2+}$(CN)$_6$]$_{0.9}$ film (x = 1.6) within the energy resolution, indicating that the local electronic state around Co$^{2+}$ is invariant against the partial oxidization. In addition, the local electronic state around the oxidized Co$^{3+}$ is essentially the same as that of the fully-charged film Co$^{3+}$[Fe$^{2+}$(CN)$_6$]$_{0.3}$[Fe$^{3+}$(CN)$_6$]$_{0.6}$ (x = 0.0) film. Such a strong localization of the oxidized Co$^{3+}$ state is advantageous for the reversibility of the redox process, since the localization reduces extra reaction within the materials and resultant deterioration.
... The Heyd-Scuseria-Ernzerh (HSE06) hybrid functional formalism [151] has been employed for a variety of different spin configurations and also provides an AFM groundstate ordering [72,60,152]. Furthermore, by making use of expanded unit cells (i.e., the supercell approach [153]), in order to remove possible size constraints, more complex magnetic ordering has been investigated for Cr 2 AC, with A = Al, Ge, and Ga. This alternative method provided for Cr 2 AlC a ground-state magnetic ordering with an in-plane AFM spin configuration [154]. ...
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong M\ \C bonds in high-density MC slabs, and relatively weak M\ \A bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity , elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... The Heyd-Scuseria-Ernzerh (HSE06) hybrid functional formalism [151] has been employed for a variety of different spin configurations and also provides an AFM groundstate ordering [72,60,152]. Furthermore, by making use of expanded unit cells (i.e., the supercell approach [153]), in order to remove possible size constraints, more complex magnetic ordering has been investigated for Cr 2 AC, with A = Al, Ge, and Ga. This alternative method provided for Cr 2 AlC a ground-state magnetic ordering with an in-plane AFM spin configuration [154]. ...
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong MC bonds in high-density MC slabs, and relatively weak MA bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity, elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... For the XAS spectra, we used the experimental values for 2p 3/2 /2p 1/2 branching ratio (1.1:1) and for the L 2, 3 peak splitting (8.0 eV), which is somewhat larger than our calculated ab initio spin-orbit splitting of 7.6 eV. Absorption spectra were calculated within the same theoretical scheme used for XES, but with the addition of core-hole effects [29]. Specifically, we generated such a self-consistentfield potential using a × × 2 2 1 hexagonal supercell of 32 atoms containing one core-hole on the investigated element. ...
The anisotropy in the electronic structure of the inherently nanolaminated ternary phase Cr$_{2}$GeC is investigated by bulk-sensitive and element selective soft x-ray absorption/emission spectroscopy. The angle-resolved absorption/emission measurements reveal differences between the in-plane and out-of-plane bonding at the (0001) interfaces of Cr$_{2}$GeC. The Cr $L_{2,3}$, C $K$, and Ge $M_{1}$, $M_{2,3}$ emission spectra are interpreted with first-principles density-functional theory (DFT) including core-to-valence dipole transition matrix elements. For the Ge $4s$ states, the x-ray emission measurements reveal two orders of magnitude higher intensity at the Fermi level than DFT within the General Gradient Approximation (GGA) predicts. We provide direct evidence of anisotropy in the electronic structure and the orbital occupation that should affect the thermal expansion coefficient and transport properties. As shown in this work, hybridization and redistribution of intensity from the shallow $3d$ core levels to the $4s$ valence band explain the large Ge density of states at the Fermi level.
... (ii) The XAS spectrum of Fe metal (x = 0) has narrower 2p 3/2 , and 2p 1/2 absorption peaks, whereas the XAS spectra of the carbon-containing films exhibit small peak A comparison of the integrated peak areas at different carbon contents shows that the 2p 3/2 /2p 1/2 peak ratio is lowest for x = 0.72 (1.9), and x = 0.45 (2.0) in comparison to the lower carbon contents x = 0.26 (2.2), and 0.21 (2.3), while it is higher for bulk Fe x = 0 (2.4). A lower 2p 3/2 /2p 1/2 peak ratio is an indication of higher ionicity (lower conductivity) for the higher carbon contents [26,27]. Note that the 2p 3/2 /2p 1/2 peak ratio is a result of the ionicity for Fe, in the FeC y carbide phase, and not for the entire film that is the sum of both the Fe-rich (∼ FeC y ), and carbon-rich domains in the matrix. ...
We investigate the amorphous structure, chemical bonding, and electrical properties of magnetron sputtered Fe 1−x C x (0.21 x 0.72) thin films. X-ray, electron diffraction and transmission electron microscopy show that the Fe 1−x C x films are amorphous nanocomposites, consisting of a two-phase domain structure with Fe-rich carbidic FeC y , and a carbon-rich matrix. Pair distribution function analysis indicates a close-range order similar to those of crystalline Fe 3 C carbides in all films with additional graphene-like structures at high carbon content (71.8 at% C). From x-ray photoelectron spectroscopy measurements, we find that the amorphous carbidic phase has a composition of 15–25 at% carbon that slightly increases with total carbon content. X-ray absorption spectra exhibit an increasing number of unoccupied 3d states and a decreasing number of C 2p states as a function of carbon content. These changes signify a systematic redistribution in orbital occupation due to charge-transfer effects at the domain-size-dependent carbide/matrix interfaces. The four-point probe resistivity of the Fe 1−x C x films increases exponentially with carbon content from ∼200 µµ cm (x = 0.21) to ∼1200 µµcm (x = 0.72), and is found to depend on the total carbon content rather than the composition of the carbide. Our findings open new possibilities for modifying the resistivity of amorphous thin film coatings based on transition metal carbides through the control of amorphous domain structures.
We present X-ray photoelectron, Co L 2,3 and O K X-ray absorption, as well as Co K β 1,3 X-ray emission spectroscopy results of studies of the spin states of trivalent cobalt ions in single-crystal cobaltite LaCoO 3 . We show that at room temperature, in the bulk of a LaCoO 3 single crystal, Co ³⁺ ions are in the low-spin state, while high-spin Co ²⁺ , high-spin Co ³⁺ , low-spin Co ³⁺ , and probably also intermediate-spin Co ³⁺ ions are localated on the surface.
Mott insulator material, as a kind of strongly correlated electronic system with the characteristic of a drastic change in electrical conductivity, shows excellent application prospects in neuromorphological calculations and has attracted significant attention in the scientific community. Especially, computing systems based on Mott insulators can overcome the bottleneck of separated data storage and calculation in traditional artificial intelligence systems based on the von Neumann architecture, with the potential to save energy, increase operation speed, improve integration, scalability, and three-dimensionally stacked, and more suitable to neuromorphic computing than a complementary metal-oxide-semiconductor. In this review, we have reviewed Mott insulator materials, methods for driving Mott insulator transformation (pressure-, voltage-, and temperature-driven approaches), and recent relevant applications in neuromorphic calculations. The results in this review provide a path for further study of the applications in neuromorphic calculations based on Mott insulator materials and the related devices.
MXenes (transition metal carbides and nitrides/carbonitrides) are a novel type of two-dimensional (2D) materials that are derived from MAX-phase precursors. Their structural and compositional diversity, outstanding electrical conductivity, and tunable surface properties have made MXenes one of the best and most versatile families of 2D materials, with rapidly growing fields of application. Since their inception, MXenes have attracted enormous research interest in various research fields, including physics, chemistry, materials science, nanotechnology, nanomedicine, and environmental science. This chapter briefly introduces fundamental aspects of MXenes such as MAX-phase precursors, MXene derivatives, and methods of synthesis and processing. Various MXene properties such as electronic, optical, magnetic, and mechanical properties are then briefly discussed. The next section briefly describes the current advances of MXene-based composites in various application fields. Finally, the chapter highlights the current challenges and research prospects for the development of MXene-based advanced composite materials for specific applications.
Experimental electronic structure of LaCoO\(_3\) and other RCoO\(_3\) (R = rare earth) in connection with the spin crossover phenomena in LaCoO\(_3\) is reviewed. As the experimental probes, we mostly deal with electron spectroscopic methods, namely photoemission spectroscopy and x-ray absorption spectroscopy. Experimental results are compared with ab initio band-structure calculations and configuration-interaction cluster-model calculations. First, we discuss how electronic-structure measurements can contribute to studying spin crossover phenomena, particularly of LaCoO\(_3\). Then in Sect. 2.2, we give a brief historical review of electron spectroscopic studies on LaCoO\(_3\) in order to highlight the conflict between the two major models, namely the low spin-high spin model and the low spin-intermediate spin-high spin model. Sections 2.3 to 2.5 are devoted to discussing this conflict and how we may be able to resolve it beyond this binarism. In Sect. 2.6, we remark the probing depth of the methods and the surface magnetism of LaCoO\(_3\). In Sect. 2.7, we briefly discuss a part of recent advances both in experiment and theory in this field. We run through the electronic structure of RCoO\(_3\) and other related Co oxides in Sect. 2.8, and finally we give the concluding remarks in Sect. 2.9.
We present results of soft X-ray absorption spectroscopy studies of EuBaCo2O5.5 powders subjected to mechanical impact (milling in a ball mill). We show that in the near-surface region (5–10 nm) of particles of the powder, EuBaCo2O5.5 decomposes into Co3O4, BaCO3, and EuCoO3. A knowledge of the low-spin state of Co³⁺ ions in EuCoO3 and Co3O4 and possibilities of surface-sensitive soft X-ray absorption spectroscopy have allowed to determine the composition of the EuBaCo2O5.5 sample subjected to mechanical impact near its surface, which is inaccessible to standard X-ray phase analysis.
Spin transitions of cobalt ions in LaCoO3 single crystals have been studied by the method of X-ray magnetic circular dichroism (XMCD) at the K- and L2,3-edges of Co³⁺ ions. The orbital momentum of cobalt ions obtained for the K-edge at the 3d level in the region of the spin transition in the temperature range from 25 to 120 K increases by a factor of approximately 1.6, whereas the slope of the magnetization curve value in the same temperature range and magnetic field increases by a factor of more than 10. XMCD experiments at the cobalt L2,3-edges demonstrate gradual growth of the ratio of the orbital momentum to the spin one L/S from 0.48 to 0.53 in the temperature range from 60 K to 120 K.
The spin states of Co$^{3+}$ ions in perovskite-type LaCoO$_3$, governed by complex interplay between the electron-lattice interactions and the strong electron correlations, still remain controversial due to the lack of experimental techniques which can detect directly. In this letter, we revealed the tensile-strain dependence of spin states, $i. e.$ the ratio of the high- and low-spin states, in epitaxial thin films and a bulk crystal of LaCoO$_3$ via resonant inelastic soft x-ray scattering. The tensile-strain as small as 1.0% was found to realize different spin states from that in the bulk.
The local structure and chemical bonding in two-phase amorphous Cr1−xCx nanocomposite thin films are investigated by Cr K-edge (1s) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies in comparison to theory. By utilizing the computationally efficient stochastic quenching (SQ) technique, we reveal the complexity of different Cr-sites in the transition metal carbides, highlighting the need for large scale averaging to obtain theoretical XANES and EXAFS spectra for comparison with measurements. As shown in this work, it is advantageous to use ab initio theory as an assessment to correctly model and fit experimental spectra and investigate the trends of bond lengths and coordination numbers in complex amorphous materials. With sufficient total carbon content (≥ 30 at%), we find that the short-range coordination in the amorphous carbide phase exhibit similarities to that of a Cr7C3±y structure, while excessive carbons assemble in the amorphous carbon phase.
The GdCoO3–δ perovskite is a semiconductor with the energy gap E
g ≈ 0.5 eV from electrical transport measurements. It reveals unusual optical absorption spectra without transparency window expected for semiconductors. Instead we have measured the narrow transmittance peak at the photon energy ε0 = 0.087 eV. To reconcile the transport and optical data we have studied the effect of oxygen vacancies on the electronic structure of the GdCoO3–δ. We have found that oxygen vacancies result in the in-gap states inside the charge-transfer energy gap of the GdCoO3. It is a multielectron effect due to strong electron correlations forming the electronic structure of the GdCoO3–δ. These in-gap states decrease the transparency window and result in a narrow absorption minimum. The predicted temperature dependence of the absorption spectra has been confirmed by our measurements.
In this work we investigate the magnetic, dielectric and charge transport properties of LaCoO3 (LCO) synthesized by two different techniques: microwave assisted and conventionally heated ceramic synthesis. The rapid microwave synthesis conditions are far away from thermodynamic equilibrium and are found to lead to modified crystal defect properties as compared to conventional synthesis. The thermally induced magnetic spin state transition at Ts1 ≈ 80 K is exemplified by temperature (T)-dependent dielectric spectroscopy data, which reveal the appearance of an additional dielectric contribution that is correlated to the transition. Magnetisation, M vs T, and electrical resistivity, ρ vs T, curves show that the additional dielectric phase is strongly influenced by magnetic defects and may be associated with higher spin state clusters in a magnetic spin-state coexistence scenario. We suggest that defects such as oxygen vacancies act as magnetic nucleation centres across the spin state transition Ts1 for the formation of higher spin state clusters in LCO perovskites.
We analyzed the dependence of X-ray absorption spectra for Co 2p orbital of LaCoO3 on the Hubbard U and lattice parameters by using the density functional theory (DFT) method. The X-ray absorption spectra and partial density of states (PDOS) are studied with the different U parameters ranging from 2.5 to 7.8. The characteristic peak position of X-ray absorption spectra is also investigated for different structures: i.e., the O atom positions assuming the displacement by thermal vibrations, the lattice expansion and the crystalline lattice distortion. The effect of the above parameters on the X-ray absorption spectra are discussed on the basis of theoretical calculation results.
We have investigated the effect of particle size on the electrical transport and magnetic properties of La0.5A0.5CoO3 (A=Sr, Ca, and Ba) nanoparticles synthesized by sol–gel technique. A size-induced metal to insulator transition is observed in the resistivity behaviour of Sr- and Ba-doped samples as the dimension changes from higher to lower ones. The magnetoresistance exhibits almost linear behaviour throughout the studied field range. The zero-field-cooled (ZFC) and field-cooled (FC) magnetizations display a broad paramagnetic to ferromagnetic transition at TC with a large magnetic irreversibility. The magnetization results indicate that Co3+ ions are in the intermediate spin state but Co4+ ions stay in a mixture of intermediate and high spin states. The observed frequency dependent shoulder in the in-phase and the peak in the out of phase component of the ac susceptibility indicate the glassy nature of the samples. The analysis of the ac magnetization results suggests that the magnetic behaviour is consistent with the cluster glass model.
High resolution experimental data for resonant soft xray scattering of transition metal compounds are shown. The compounds studied are the ionic transition metal difluorides, ionic and covalent orthovanadates and members of the La1xSrxCoO3 perovskite family. In all compounds we studied the transition metal L2,3 edge and also the ligand (oxygen or fluorine) K edge. For the ionic compounds the transition metal data are in good agreement with atomic multiplet ligand field calculations that include charge transfer effects. Density functional calculations give very useful information to interpret the ligand xray emission data. The experimental metal Lα emission data show that the region between valence and conduction bands in the difluorides has several dexcited states. At the L2edge of the ionic orthovanadates we found the signature of a fast CosterKronig decay process that results in a very localized emission peak. Changes in the oxidation sate in the La1xSrxCoO3 compounds are observed at both the metal L2,3edge and the oxygen K edge absorption spectra.
The electronic structure and chemical bonding of wurtzite-GaN investigated by
N 1s soft x-ray absorption spectroscopy and N K, Ga M1, and Ga M2,3 emission
spectroscopy is compared to that of pure Ga. The measurements are interpreted
by calculated spectra using first-principles density-functional theory (DFT)
including dipole transition matrix elements and additional on-site Coulomb
interaction (WC-GGA+U). The Ga 4p - N 2p and Ga 4s - N 2p hybridization and
chemical bond regions are identified at the top of the valence band between
-1.0 and -2.0 and further down between -5.5 and -6.5 eV, respectively. In
addition, N 2s - N 2p - Ga 4s and N 2s - N 2p - Ga 3d hybridization regions
occur at the bottom of the valence band between -13 and -15 eV, and between
-17.0 and -18.0 eV, respectively. A band-like satellite feature is also found
around -10 eV in the Ga M1 and Ga M2,3 emission from GaN, but is absent in pure
Ga and the calculated ground state spectra. The difference between the
identified spectroscopic features of GaN and Ga are discussed in relation to
the various hybridization regions calculated within band-structure methods.
The electronic structure of the perovskite La1-xSrxCoO3 has been obtained as a function of Sr substitution and volume from a series of generalized-gradient-corrected, full-potential, spin-density-functional band-structure calculations. The energetics of different spin configurations are estimated using the fixed-spin-moment (FSM) method. From the total energy versus spin magnetic moment curve for LaCoO3 the ground state is found to be nonmagnetic with the Co ions in a low-spin (LS) state, a result that is consistent with the experimental observations. Somewhat higher in energy, we find an intermediate-spin (IS) state with spin moment ~1.2muB/%f.u. From the anomalous temperature dependent susceptibility along with the observation of an IS state we predict metamagnetism in LaCoO3 originating from an LS-to-IS transition. The IS state is found to be metallic and the high-spin (HS) state of LaCoO3 is predicted to be a half-metallic ferromagnet. With increasing temperature, which is simulated by a corresponding change of the lattice parameters, we have observed the disappearance of the metamagnetic solution that is associated with the IS state. The FSM calculations on La1-xSrxCoO3 suggest that the hole doping stabilizes the IS state and the calculated magnetic moments are in good agreement with the corresponding experimental values. Our calculations show that the HS state cannot be stabilized by temperature or hole doping since the HS state is significantly higher in energy than the LS or IS state. Hence the spin-state transition in LaCoO3 by temperature/hole doping is from an LS to an IS spin state and the present work rules out the other possibilities reported in the literature.
The ground-state and excited-state properties of the perovskite LaMnO3, the mother material of colossal magnetoresistance manganites, are calculated based on the generalized-gradient-corrected relativistic full-potential method. The electronic structure, magnetism, and energetics of various spin configurations for LaMnO3 in the ideal cubic perovskite structure and the experimentally observed distorted orthorhombic structure are obtained. The excited-state properties such as the optical, magneto-optical, x-ray photoemission, bremsstrahlung isochromat, and x-ray absorption near-edge structure spectra are calculated and found to be in excellent agreement with available experimental results. Consistent with earlier observations the insulating behavior can be obtained only when we take into account the structural distortions and the correct antiferromagnetic ordering in the calculations. The present results suggest that the correlation effect is not significant in LaMnO3 and the presence of ferromagnetic coupling within the ab plane as well as the antiferromagnetic coupling perpendicular to this plane can be explained through the itinerant band picture. Our calculations show that the Mn 3d eg-like electrons are present in the whole valence-band region. We have calculated the hyperfine field parameters for the A-type antiferromagnetic and the ferromagnetic phases of LaMnO3 and compared the findings with the available experimental results. The role of the orthorhombic distortion on electronic structure, magnetism, and optical anisotropy is analyzed.
Polarized and unpolarized neutron measurements have been performed on ${\mathrm{LaCoO}}_{3}$ in a temperature range between 10 and 295 K. A drastic decrease in the magnetic cross section is observed below 150 K. This is the first concrete experimental evidence for the temperature-induced magnetic moment of Co ions in this oxide as proposed in the thermally populated high-spin model. The temperature dependence of the moment is in qualitative agreement with that calculated from the magnetic-susceptibility data.
The electronic structure of the perovskite LaCoO3 for different spin states of Co ions was calculated in the local-density approximation LDA+U approach. The ground state is found to be a nonmagnetic insulator with Co ions in a low-spin state. Somewhat higher in energy, we find two intermediate-spin states followed by a high-spin state at significantly higher energy. The calculations show that Co 3d states of t2g symmetry form narrow bands which could easily localize, while eg orbitals, due to their strong hybridization with the oxygen 2p states, form a broad σ* band. With temperature variation which is simulated by a corresponding change of the lattice parameters, a transition from the low- to intermediate-spin state occurs. This intermediate-spin (occupation t2g5eg1) can develop an orbital ordering which can account for the nonmetallic nature of LaCoO3 at 90 K<T<500 K. Possible explanations of the magnetic behavior and gradual insulator-metal transition are suggested. © 1996 The American Physical Society.
The following questions are considered: How do itinerant d electrons of the Heusler alloys produce magnetic moments that are completely localized on the Mn atoms? What is the microscopic origin of Invar anomalies? Why is elemental antiferromagnetism confined to Cr, Mn, and Fe? Why do the Fe atoms in Fe 3 Si carry two distinctly different magnetic moments? Can we use our understanding of transiton‐metal magnetism to ’’design’’ new magnetic compounds composed of nonmagnetic elements? Our answers to these questions are based on the local‐spin‐density theory of electronic exchange and correlation, implemented using parameter‐free self‐consistent energy‐band calculations.
In solids, linearized augmented plane waves (LAPW's) have proven to be an effective basis for the solution of the Kohn-Sham equations, the main calculational task in the local spin density approximation (LSDA) to density functional theory. The WIEN package uses LAPW's to calculate the LSDA total energy, spin densities, Kohn-Sham eigenvalues, and the electric field gradients at nuclear sites for a broad variety of space groups. Options include retention or omission of non-muffin-tin contributions (hence WIEN is a full-potential or F-LAPW code) and relativistic corrections (full treatment for core states, Scalar-relativistic for valence states).
The different magnetic phases of the bcc and fcc forms of Fe, Co, and Ni are studied by analyzing total-energy surfaces in moment-volume parameter space obtained from energy-band calculations using a local-spin-density approximation. The surfaces, found by calculating total energies while holding both the magnetic moment and the volume fixed, offer a method for studying phases that are inaccessible to traditional self-consistent-field methods. We find that magnetic moments can change discontinuously with volume and that there are ranges of coexistence for different magnetic phases. In the multiphase ranges, these elemental magnetic systems exhibit metamagnetic behavior. Our results show that bcc Co is ferromagnetic for all volumes studied, that fcc Co can exist in either a nonmagnetic or a ferromagnetic phase, and that there is a range of volumes where the two phases can coexist. For Fe, the bcc form exhibits a stable ferromagnetic phase for all volumes considered, but the fcc form can exist in any of three phases-a nonmagnetic, a low-spin, and a high-spin phase-all of which can coexist in limited volume ranges. For Ni, the fcc form exhibits a stable ferromagnetic phase, but the bcc form can exist in both a nonmagnetic and, at expanded volumes, a ferromagnetic phase. The volume ranges for all magnetic phases are clearly identified for the bcc and fcc forms of Fe, Co, and Ni.
In the reported experiments, the partial LCAO 2s-, 2p- and 3 d-density of states are used to analyze the energy bands of the valence states for the compounds MX (M equals Ti,V, X equals C, N, O). The partial density of states is based on energy values which have been obtained by means of Slater-Koster interpolation of APW energy eigenvalues. In order to explain the origin of the main peaks of the X-ray band spectra for the compounds mentioned, the nonmetal K-spectra, the metal K- and L//I//I//I-spectra taken from literature are compared with the l-character densities within the corresponding atomic sphere. This comparison makes it possible to interpret the main features of the X-ray valence band emission spectra of the compounds investigated. The position of the main peaks can be predicted to an accuracy of about 1-2 eV. In the case of VO there is less agreement.
Homogeneous samples of LaCoO 3 were prepared from
coprecipitated precursors; they reveal that single-phase samples are
restricted to a narrow range of the [La]/[Co] ratio of about 1.0. A
first-order phase change near 1200 K only appears where Co 3O
4 is present as an impurity phase. Evolution with temperature
from all low-spin Co(III) at T = 0 K to 50% high-spin Co 3+
and 50% intermediate-spin Co(III) above 650 K is monitored with magnetic
susceptibility, resistance, and thermopower measurements. Evidence is
found for a dynamic ordering of high-spin Co 3+ and low-spin
Co(III) in the intermediate-temperature interval 110 K < T < 350
K. The lifetime of a Co 3+ ion decreases with increasing
temperature to a τ < 10 -8 sec for T > 200 K. The
intermediate-temperature phase is semiconductive with an Eg
≈ 0.14 eV for excitations of hole-electron pairs. The holes are
mobile; the electrons are trapped as Co 2+ on high-spin sites
until they recombine with a mobile hole. The metallic high-temperature
phase for T > 650 K is nucleated and grows with increasing
temperature in the interval 350 K < T < 650 K.
The oxygen nonstoichiometry (δ) of La 1- xSr
xCoO 3- δ (0 < x ≤ 0.6) has been
studied as a function of temperature (573-1673 K) and oxygen partial
pressure (1-10 -3 atm) using TGA and coulometric titration
techniques. The absolute values of oxygen nonstoichiometry were
determined by the direct reduction in a flux of hydrogen (TGA) or by
decreasing PO 2 in a coulometric titration cell.
The boundaries of phase stability of La 0.7Sr
0.3CoO 3-δ and (La 0.7Sr
0.3) 2CoO 4± v were evaluated. A
possible mechanism of disordering processes has been discussed.
Neutron-diffraction measurements have revealeda new anomalous thermal
lattice expansion of LaCoO3 near 500 K thatindicates the
existence of a second spin-state transition, in addition to theone
previously established near 100 K. The anomalous expansion and
thetemperature dependence of the Co magnetic moments are
successfullyinterpreted in a wide temperature range based on a simple
model assuminglow-spin (LS, S=0), intermediate-spin (IS, S=1), and
high-spin(HS, S=2) states of Co atoms.The first spin transition, near
100 K, is from LS to IS, andthe second, near 500 K, is from IS to a
mixed state of IS and HS.The fitted model parameters indicate that the
initially-large energy differencebetween IS and HS states decreases
towards zero as the secondtransition proceeds. The large drop in
resistivity associated with the lattertransition appears to be
correlated with the population of the HS state.
Precision x-ray measurements of the motions of the atoms in LaCoO3 every 50°C from room temperature to 1000°C indicate that the crystal space group is R3¯c below 375°C and R3¯ above 375°C. In the symmetry R3¯ there are two distinguishable octahedral cobalt positions, CoI having larger crystalline fields than CoII. Since high- and low-spin cobalt ions are simultaneously present, this indicates preferential long-range ordering of low-spin cobalt at CoI sites, and of high-spin cobalt at CoII sites for T>375°C. Calorimetric data show a first-order transition at Tt=937°C and a higher order transition in the temperature interval 125
59Co and 139La NMR measurements have been performedto study the microscopic magnetic properties closely relatedwith the spin state of a trivalent cobalt ion in LaCoO3.The temperature-independent 59Co and 139La Knightshifts below ˜ 30 K clearly revealed the presence of anonmagnetic ground state at low temperatures. We calculated thetemperature dependence of the magnetic susceptibility chiand the 59Co Knight shift 59K in a single ion modelwith the low-spin 1A1 ground state above which thehigh-spin 5T2 state is located. Then, both the trigonalcrystal-field and spin-orbit interactions are taken intoaccount. From this analysis, we infer the presence of anantiferromagnetic exchange interaction between the high-spinstates. The characteristic temperature dependence of the59Co hyperfine coupling constant is also discussed.
The crystal structure and properties of La1 − xSrxCoO3 − y with strontium contents ranging from x = 0.1 to x = 0.7 have been studied. The lattice parameters were measured as a function of temperature (4.2–400 K) and the crystal structure was found to change from rhombohedral (at low temperatures and values of x) to cubic. While LaCoO3 is paramagnetic the oxides in the composition range 0.2 < x < 0.6 are soft ferromagnets. The strontium additions are compensated by the formation of Co4+ (cobalt ions with one positive effective charge, CoCo.) and oxygen vacancies (Vo..). From the results it is concluded that the relative importance of oxygen vacancies increases with increasing temperature and decreasing oxygen activity. As a result the concentration of electronic charge carriers — and the resultant electrical conductivity — decrease with increasing temperature. The defect structure is discussed and it is concluded that defect associations — probably between oxygen vacancies and strontium ions — and formation of microdomains of perovskite-related phases are important aspects of the overall structure of these perovskite phases.
X-ray photoemission measurements of the core levels and valence electronic structure of LaCoO3 and La0.5Ca0.5CoO3 high quality epitaxial films are presented. Shifts of the core levels and main valence band features are consistent with a doping-induced change in the chemical potential. Oxygen states are found to significantly contribute to a peak in the valence band at 1 eV binding energy, verifying earlier results of cluster calculations. A Fermi level crossing of this same band upon doping is observed, yielding a high Fermi level density of states.
We present soft-x-ray-absorption spectra (XAS) of LaCoO3 taken at different temperatures (80–630 K). The shape of the multiplets in the Co 2p XAS spectra conveys information on the symmetry and spin of the ground state. The O 1s XAS spectra are related to unoccupied metal bands through covalent mixing. The changes in the spectra taken at different temperatures provide information on the spin-state transition in this compound. At low temperature, 80 and 300 K, the material is in a highly covalent low-spin state. The main contribution to the ground state in this case is given by t2g6(1A1) with an occupancy of 0.56. At higher temperature, 550 and 630 K, the results indicate a gradual transition to a mixed-spin state. The main contribution to the high-spin part of the mixture is given by t2g4eg2(5T2) with an occupancy of 0.71. There is no evidence of charge disproportionation in the temperature range 80–630 K. The O 1s XAS spectra reflect important changes in the unoccupied Co 3d bands across the spin-state transition.
We have studied the electronic structure of LaCoO3 by photoemission spectroscopy and x-ray absorption spectroscopy (XAS). The Co 2p core-level and valence-band photoemission spectra display satellite structures indicating a strong electron-correlation effect. The Co 2p core-level photoemission, the valence-band photoemission, and the O 1s XAS spectra have been analyzed using a configuration-interaction cluster model for the initial-state configurations of the low-spin (LS: 1A1), intermediate-spin (IS: 3T1) and high-spin (HS: 5T2) states and their mixtures. The ground state of LaCoO3 in the LS state is found to have heavily mixed d6 and d7L-̱ character, reflecting the strong covalency. The magnetic susceptibility has been analyzed for various level orderings of the LS, IS, and HS states. From the analyses of the photoemission spectra and the magnetic susceptibility data, the temperature-induced paramagnetism in LaCoO3 above ∼90 K is most likely due to a gradual LS-to-IS transition.
We present measurements of the magnetic susceptibility and of the thermal expansion of a LaCoO3 single crystal. Both quantities show a strongly anomalous temperature dependence. Our data are consistently described in terms of a spin-state transition of the Co3+ ions with increasing temperature from a low-spin ground state (t2g6eg0) to an intermediate-spin state (t2g5eg1) without (100–500 K) and with (>500 K) orbital degeneracy. We attribute the lack of orbital degeneracy up to 500 K to (probably local) Jahn-Teller distortions of the CoO6 octahedra. A strong reduction or disappearance of the Jahn-Teller distortions seems to arise from the insulator-to-metal transition around 500 K.
We report the valence-band and Co 2p photoemission spectra of ${\mathrm{LaCoO}}_{3}$ obtained at different temperatures (100, 300, and 573 K). Analysis of the core-level spectrum in terms of a configuration interaction model suggests that both low- and high-spin states coexist at the low temperature (100 K). It also indicates that there is a decrease in the low-spin contribution at 573 K related to local structural changes. Photoemission spectra of the valence-band region further support this interpretation.
Crystallographic, magnetic, and electrical studies of the system La1−xSrxCoO3.00±0.01 for 0≤x≤0.5 give indirect evidence for the presence of chemical inhomogeneities separating strontium‐free regions, where localized ``3d'' electrons occur at thermally excited high‐spin Co3+ ions, from strontium‐rich regions, where the ``3d'' electrons are collective and give ferromagnetism at low temperatures. These different regions occur within the same rhombohedral perovskite crystal and appear to represent two different electronic phases within the same crystallographic phase. A schematic band model for the ferromagnetic phase is presented.
The pressure dependence of the 100 K spin-state transition in
LaCoO3 is investigated through magnetization measurement
under pressures of up to 18.2 kbar. The energy gap, Δ, between the
low-spin ground state and the excited magnetic state (intermediate spin)
increases remarkably with increasing pressure. The linear part of the
pressure dependence of Δ, which is dominant below 5 kbar, is
interpreted consistently by the volume expansion due to the spin-state
transition under ambient pressure. In the higher pressure region, a
quadratic pressure dependence is dominant, which suggests a very steep
volume dependence of the energy of the excited magnetic state under the
highly compressed condition.
The cubic Laves phase of YCo2 is near a transition from a paramagnetic to a magnetically ordered state. Self-consistent energy-band calculations yield the total energy and the magnetic moment as functions of volume. The new fixed spin-moment (FSM) method, which allows one to calculate the total energy as an explicit function of the magnetisation, is introduced. At the theoretical equilibrium volume YCo2 is found to be a strongly enhanced Pauli paramagnet, but at a slightly larger lattice constant a metamagnetic transition seems possible. Its occurrence can be understood on the basis of a double minimum in the total energy obtained from the present FSM calculations, which lead to an estimated critical field of about 350 T. In the magnetic state the Y moment is coupled antiferromagnetically to the Co moment.
Co K-edge near-edge X-ray absorption spectra are reported for RECoO3 (RE=Y, Ho, Gd, Nd) at 300 K and for LaCoO3 in the temperature range (140<or=T<or=830 K). The pre-edge structure associated with Co 1s to 3d excitation is found to be comparable for all the cobaltates at room temperature, consistent with the proposed similarity of their electronic structure. For LaCoO3, temperature-dependent changes are observed in the pre-edge structure. These data, when compared with previously published UV photoelectron spectra and calculated density of states, are found to be consistent with a proposed model for the higher-order semiconductor-to-metal transition of LaCoO3 which involves increased sigma *- pi * band overlap with temperature.
Soft x‐ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close‐lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x‐ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flipper I wiggler beamline at HASYLAB in Hamburg using a new grazing incidence instrument designed specifically for this experiment. The photon flux at the Flipper I station (typically 5×10<sup>1</sup><sup>2</sup> photons per second on the sample with a 1% bandpass) is enough to allow soft x‐ray fluorescence spectra to be recorded at relatively high resolution and within reasonable accumulation times (typically, the spectra presented in this work were recorded in 30 min). The spectrometer is based on a new concept which allows the instrument to be quite small, still covering a large wavelength range (10–250 Å). The basic idea involves the use of several fixed mounted gratings and a large two‐dimensional detector. The grating arrangement provides simple mounting within a limited space and, in particular, large spectral range. The detector can be moved in a three‐axis coordinate system in order to cover the-
different Rowland curves defined by the different gratings. The arrangement permits the use of gratings with different radii, which further facilitate the achievement of optimum performance over a large range. Two‐dimensional detection is used to allow a large solid angle, without suffering from loss of resolution due to imaging errors. The detector is based on five 2‐in. MCPs with resistive anode read out. The sensitivity of the detector, which is normally very low for soft x rays, especially at grazing angles, is enhanced by CsI coating and by using an entrance electrode.
A design of a small size grazing incidence instrument is presented, which offers large spectral range and high resolution without sacrificing luminosity. The instrument is particularly suited for use at synchrotron radiation sources since it can be conveniently attached to existing experiment chambers. The basic idea of the design is the use of fixed mounted gratings of diffent radii and groove densities and a big two-dimensional position sensitive detector mounted on a x−y angle table. The design is discussed in some detail and performance is presented.
Metal-insulator transitions are accompanied by huge resistivity changes,
even over tens of orders of magnitude, and are widely observed in
condensed-matter systems. This article presents the observations
and current understanding of the metal-insulator transition with
a pedagogical introduction to the subject. Especially important are
the transitions driven by correlation effects associated with the
electron-electron interaction. The insulating phase caused by the
correlation effects is categorized as the Mott Insulator. Near the
transition point the metallic state shows fluctuations and orderings
in the spin, charge, and orbital degrees of freedom. The properties
of these metals are frequently quite different from those of ordinary
metals, as measured by transport, optical, and magnetic probes. The
review first describes theoretical approaches to the unusual metallic
states and to the metal-insulator transition. The Fermi-liquid theory
treats the correlations that can be adiabatically connected with
the noninteracting picture. Strong-coupling models that do not require
Fermi-liquid behavior have also been developed. Much work has also
been done on the scaling theory of the transition. A central issue
for this review is the evaluation of these approaches in simple theoretical
systems such as the Hubbard model and t-J models. Another key issue
is strong competition among various orderings as in the interplay
of spin and orbital fluctuations. Experimentally, the unusual properties
of the metallic state near the insulating transition have been most
extensively studied in d-electron systems. In particular, there is
revived interest in transition-metal oxides, motivated by the epoch-making
findings of high-temperature superconductivity in cuprates and colossal
magnetoresistance in manganites. The article reviews the rich phenomena
of anomalous metallicity, taking as examples Ti, V, Cr, Mn, Fe, Co,
Ni, Cu, and Ru compounds. The diverse phenomena include strong spin
and orbital fluctuations, mass renormalization effects, incoherence
of charge dynamics, and phase transitions under control of key parameters
such as band filling, bandwidth, and dimensionality. These parameters
are experimentally varied by doping, pressure, chemical composition,
and magnetic fields. Much of the observed behavior can be described
by the current theory. Open questions and future problems are also
extracted from comparison between experimental results and theoretical
achievements.
The spin gap energy ($\approx${} 30 meV) associated with the low-spin ($S=0$) to high-spin ($S=2$) transition in LaCo${\mathrm{O}}_{3}$ was proved to be considerably smaller than the charge gap energy ($\approx${} 0.1 eV) which was estimated by optical spectroscopy. Hole doping in the low-spin ground state of LaCo${\mathrm{O}}_{3}$ leads to formation of localized magnetic polarons with unusually high spin number ($S=10$-${}16$), which can be viewed as a precursor of the doping-induced ferromagnetic metallic state.
A method to determine the absorption coefficient near the onset of core-electron transitions for concentrated samples using fluorescence-yield (FY) detection is presented. Measuring the FY signal for different experimental geometries, we are able to calculate the true absorption coefficient. Thus we are able to correct fully for saturation effects present in FY spectra of concentrated samples. The technique is demonstrated for Co and a buried layer of CoSi2.
The electron emission spectrum resulting from thermal collisions of He*(23S) atoms with LaCoO3 was measured as well as the hnu-dependent photoemission spectra. The He* spectrum shows that the lower-lying Co 3d derived (main) bands are strongly suppressed relative to the O 2p derived bands, reflecting the spatial distribution of the initial Co 3d wave functions. On the other hand, the higher-lying Co 3d derived (satellite) band is anomalously enhanced in the He* spectrum. These findings are discussed on the basis of the initial and final configuration interactions, using a Co2O11 cluster.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
Spin transition in LaCoO3investigated etc
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