Complex vs Band Formation in Perovskite Oxides

Abstract

It is argued that there is a critical cation-anion covalent mixing parameter λc such that ligand-field theory is appropriate for λ<λc, but band theory must be used for λ>λc. This provides, therefore, a criterion for distinguishing metallic vs magnetic compounds in those structures, like perovskite, where cation-cation interactions are negligible. It is also argued that λσ>λc can be anticipated where the cations are in a low-spin state. The fact that LaNiO3 contains low-spin NiIII and exhibits no Jahn-Teller distortion suggested that λσ>λc in this compound. Metallic conductivity from −200° to 300°C and Pauli paramagnetism from 4° to 300°K seem to confirm this suggestion. Where λ≈λc, there is the possibility of a phase change in which λ<λc in some directions, λ>λc in others. LaCoO3 seems to illustrate this situation. It undergoes a transition at 1210°K, the cobalt ordering into alternate (111) planes of high-spin Co3+ and planes containing low-spin CoIII. Below 400°K the latter planes contain only CoIII ions. The magnetic Co3+ ions couple antiferromagnetically via Co3+-``diamagnetic CoIIIO6 complex''-Co3+ superexchange to give TN≈90°K.

Phase diagram and isotope effect in(Pr1−yEuy)0.7Kalinov1, Gorbenko2, Талденков3 et al. 2010Phys. Rev. B154200View full textAdd to dashboardCite

“…The most prominent example is LaCoO 3 , which was actively studied and controversially debated for more than 50 years. [5][6][7][8][9][10][11][12][13][14][15][16] It is generally agreed that Co 3+ ions in LaCoO 3 are in the LS state at low temperatures. Above approximately 25 K, a higher-spin state, either IS or HS, becomes thermally populated affecting various physical properties, e.g., magnetic susceptibility χ or thermal expansion α, which both exhibit pronounced peaks in their temperature dependence.…”

“…[5][6][7][8][9][10] The existence of a spin-state change indicates that the difference of the electronic energies, ∆E, between these states is rather small. The most prominent example is LaCoO 3 , which was actively studied and controversially debated for more than 50 years.…”

Lattice normal modes and electronic properties of the correlated metal LaNiO3

“…20 In LaNiO 3 , however, the strong Ni 3d -O 2p covalent interactionsformally it contains Ni 3+ cations in a low-spin 3d 7 configuration (t 6 2g e 1 g )-are anticipated to reduce the correlation effects. 21 Nonetheless, there are clear signatures indicative of important electron-electron interactions in the T 2 dependence of the resistivity and heat capacity measurements. 11,22 Magnetic susceptibility data also reveal enhanced Pauli paramagnetism and effective carrier masses.…”

Spin State Transition inLaCoO3Studied Using Soft X-ray Absorption Spectroscopy and Magnetic Circular Dichroism

“…This directly shows that the currently accepted low-/intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance and inelastic neutron data.PACS numbers: 71.28.+d, 71.70.Ch, 78.70.Dm LaCoO 3 shows a gradual non-magnetic to magnetic transition with temperature, which has been interpreted originally four decades ago as a gradual population of high spin (HS, t 4 2g e 2 g , S = 2) excited states starting from a low spin (LS, t 6 2g , S = 0) ground state [1,2,3,4,5,6,7,8]. This interpretation continued to be the starting point for experiments carried out up to roughly the first half of the 1990's [9,10,11,12].…”