Spin-State Transition In LaCoO3 Research Articles

Direct Evidence of the Symmetry Change of Co-3d Orbitals Associated with the Spin-State Transition in LaCoO3 by X-ray Compton Scattering

We have investigated the electron momentum density of Co-3d electrons in LaCoO3 using X-ray Compton scattering in order to demonstrate the symmetry change of the Co(3d) electron orbital states through the spin-state transition. The electron momentum density reconstructed from the Compton profiles indicates the symmetry change in the 3d electron-orbital states between below and above 100 K, which provides the first microscopic direct evidence for the orbital symmetry change of occupied electronic state associated with the spin-state transition in LaCoO3. The reproduced electron orbital states show a covalent bond with O-2p orbitals, which is responsible for the collectiveness in the characteristics of the spin-state transition.

Density Functional Theory Calculation of Spin-State Transition in LaCoO3

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.

Strain coupling mechanisms and elastic relaxation associated with spin state transitions inLaCoO3

Advantage is taken of the wealth of experimental data relating to the evolution with temperature of spinstates of Co3 + in LaCoO3 in order to undertake a detailed investigation of the mechanisms by which changes inelectronic structure can influence strain, and elastic and anelastic relaxations inperovskites. The macroscopic strain accompanying changes in the spin state inLaCoO3 is predominantly a volume strain arising simply from the change in effective ionic radius of theCo3 + ions. This acts to renormalize the octahedral tilting transition temperature in a mannerthat is easily understood in terms of coupling between the tilt and spin order parameters.Results from resonant ultrasound spectroscopy at high frequencies (0.1–1.5 MHz) revealstiffening of the shear modulus which scales qualitatively with a spin order parameterdefined in terms of changing Co–O bond lengths. From this finding, in combination withresults from dynamic mechanical analysis at low frequencies (0.1–50 Hz) and data from theliterature, four distinctive anelastic relaxation mechanisms are identified. The relaxationtimes of these are displayed on an anelasticity map and are tentatively related to spin-spinrelaxation, spin-lattice relaxation, migration of twin walls and migration of magneticpolarons. The effective activation energy for the freezing of twin wall motion below ∼ 590 K at low frequencieswas found to be 182 ± 21 kJ mol − 1 (1.9 ± 0.2 eV) which is attributed to pinning by pairs of oxygen vacancies, though thelocal mechanisms appear to have a spread of relaxation times. It seemsinevitable that twin walls due to octahedral tilting must have quite differentcharacteristics from the matrix in terms of local spin configurations ofCo3 + . A hysteresis in the elastic properties at high temperatures furtheremphasizes the importance of oxygen content in controlling the properties ofLaCoO3.

Spin‐State Transition in LaCoO3 and Related Materials

AbstractFor Abstract see ChemInform Abstract in Full Text.

Thermodynamical analysis of spin-state transitions inLaCoO3: Negative energy of mixing to assist thermal excitation to the high-spin excited state

Thermodynamical analysis of spin-state transitions in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">La</mml:mi><mml:mi mathvariant="normal">Co</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>: Negative energy of mixing to assist thermal excitation to the high-spin excited state

Itinerant metamagnetism and possible spin transition in LaCoO3 by temperature/hole doping

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 ∼1.2μB/%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.

Spin State Transition in LaCoO3 with Temperature or Strontium Doping as Seen by XAS

An X-ray absorption spectroscopy study of the polycrystalline La1−xSrxCoO3 (x=0, 0.25, 0.5) series at O K-edge was performed above and below the various magnetic transition temperatures. A crystal field analysis is proposed to explain the observed fine structure changes. Temperature-dependent O K-edge of LaCoO3 shows evidence of smooth and continuous spin transition of Co3+ from low to intermediate spin states. This result is confirmed by cobalt K-edge measurements. Upon strontium doping, O K-edges of the La1−xSrxCoO3 (x=0, 0.25, 0.5) series suggested a mixed (LS–IS) Co3+/IS Co4+ cobalt ion configuration. The fundamental mechanism governing the magnetotransport properties shown by the compounds is then discussed.

Structural Changes Accompanying the Spin-State Transition in LaCoO3: A variable Temperature Co-K-EXAFS Study

Variable-temperature Co-K-EXAFS measurements establish the occurrence of two cobalt sites, probably associated with the two spin states, above 400K. The population of the two Co sites becomes equal around 670 K, at which temperature the change in space group occurs.

Near-neighbour impurity effect on the spin-state transitions in LaCoO3 at low temperature (12&lt;T&lt;300 K)

The low-temperature magnetic susceptibility behaviour of LaCoO3 prepared under different conditions as well as substituted samples such as LaCo0.95M0.05O3 (M=Al, Ga, Cr, Fe, Mn, Ni) and La0.98Sr0.02CoO3 have been investigated in the temperature range 12–300 K. Earlier interpretations of the magnetic susceptibility have been reexamined. In the case of LaCoO3 samples containing Al, Ga, Cr and Fe impurities spin-state transitions involving a temperature independent activation energy (∼0.01 eV) are observed atT<200 K. Analysis of the data indicates that either the excited state has an intermediate-spin (t 2g 5 e g 1 ) configuration or only half the Co ions are involved in the activated transition to the high-spin (t 2g 4 e g 2 ) configuration. Al3+, and Cr3+ increases the activation energy considerably. Substitution of L ≠ 0 ions such as Mn3+ and Ni3+ or Co4+ (low-spin) seems to introduce ferromagnetic interactions and stabilizes the paramagnetic state. LaCoO3, when Co is substituted by Mn (5%) or La is substituted by Sr (2%) show giant magnetic moments. When Co is substituted by Ni (5%) a ferromagnetic ground state is observed.