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• W2033599281 abstract "Precision x-ray measurements of the motions of the atoms in LaCo${mathrm{O}}_{3}$ every 50ifmmode^circelsetextdegreefi{}C from room temperature to 1000ifmmode^circelsetextdegreefi{}C indicate that the crystal space group is $Roverline{3}c$ below 375ifmmode^circelsetextdegreefi{}C and $Roverline{3}$ above 375ifmmode^circelsetextdegreefi{}C. In the symmetry $Roverline{3}$ there are two distinguishable octahedral cobalt positions, ${mathrm{Co}}_{mathrm{I}}$ having larger crystalline fields than ${mathrm{Co}}_{mathrm{II}}$. Since high- and low-spin cobalt ions are simultaneously present, this indicates preferential long-range ordering of low-spin cobalt at ${mathrm{Co}}_{mathrm{I}}$ sites, and of high-spin cobalt at ${mathrm{Co}}_{mathrm{II}}$ sites for $T>375$ifmmode^circelsetextdegreefi{}C. Calorimetric data show a first-order transition at ${T}_{t}=937$ifmmode^circelsetextdegreefi{}C and a higher order transition in the temperature interval $125<T<375$ifmmode^circelsetextdegreefi{}C, which is also manifest in a large Debye-Waller factor in this interval and in a plateau in the curve of reciprocal susceptibility versus temperature. At about 650ifmmode^circelsetextdegreefi{}C there is some evidence, from differential thermal-analysis data, of another higher order transition. The electrical conductivity increases with increasing temperature below 650ifmmode^circelsetextdegreefi{}C, but much more rapidly in the interval $125<T<650$ifmmode^circelsetextdegreefi{}C than below 125ifmmode^circelsetextdegreefi{}C. It is nearly temperature-independent in the interval $650<T<937$ifmmode^circelsetextdegreefi{}C and is continuous through the first-order transition. However, above 937ifmmode^circelsetextdegreefi{}C the resistivity increases with temperature as in a metal. The space group remains $Roverline{3}$ and the pseudocubic cell edge is continuous through the first-order phase change, but the rhombohedral angle drops abruptly from 60.4ifmmode^circelsetextdegreefi{} to 60ifmmode^circelsetextdegreefi{} and the ${mathrm{La}}^{3+}$ ions are shifted discontinously along the $c$ axis toward a ${mathrm{Co}}_{mathrm{I}}$ ion. Similar ${mathrm{La}}^{3+}$-ion displacements occur in the temperature interval $400<T<650$ifmmode^circelsetextdegreefi{}C. The Debye-Waller factor decreases by an order of magnitude on going to the high-temperature phase. It is pointed out that crystal-field and band theory should describe two different thermodynamic states of electrons: localized and collective. The data are interpreted to indicate (1) that the first-order phase change at ${T}_{t}=937$ifmmode^circelsetextdegreefi{}C is a localized-electron $ensuremath{leftrightarrows}$ collective-electron phase change for electrons in orbitals of ${e}_{g}$ symmetry, higher temperatures introducing a Fermi surface and partial disproportionation between high- and low-spin cations at ${mathrm{Co}}_{mathrm{II}}$ and ${mathrm{Co}}_{mathrm{I}}$ positions; (2) that the number of charge carriers is constant through the transition, because the number of localized charge carriers is saturated below ${T}_{t}$ and just above ${T}_{t}$ the bandwidth of the collective-electron states is $ensuremath{Delta}ensuremath{epsilon}<k{T}_{t}$; (3) that the mobilities of the charge carriers are also continuous through the transition, the activation energy for a localized-electron hop becoming $ensuremath{ll}mathrm{kT}$ at $Tensuremath{le}937$ifmmode^circelsetextdegreefi{}C; (4) that the higher-order transition in the interval $125<T<375$ifmmode^circelsetextdegreefi{}C represents a region of short-range order, the ordered phase occurring at higher temperatures where the populations of high- and low-spin cobalt ions approach one another; and (5) that exciton transfer is an important mechanism in LaCo${mathrm{O}}_{3}$." @default.
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• W2033599281 title "First-Order Localized-Electron<mml:math xmlns:mml=http://www.w3.org/1998/Math/MathML display=inline><mml:mi>⇆</mml:mi></mml:math>Collective-Electron Transition in LaCo<mml:math xmlns:mml=http://www.w3.org/1998/Math/MathML display=inline><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant=normal>O</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>" @default.
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• W2033599281 doi "https://doi.org/10.1103/physrev.155.932" @default.
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