Intermediate spin state of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Co</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Co</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>ions in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>0.5</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ba</mml:mi></mml:mrow><mml:mrow><mml:mn>0.5</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CoO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>evidenced by Jahn-Teller distortions

From high resolution neutron and synchrotron powder diffraction studies performed on the ideal cubic perovskite compound ${\mathrm{La}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{CoO}}_{3},$ we have observed the onset of a long-range tetragonal phase accompanying a para-ferromagnetic transition at ${T}_{C}\ensuremath{\sim}180 \mathrm{K}.$ The lowering of the crystal symmetry is the direct signature of a cooperative and static Jahn-Teller (JT) distortion of the ${\mathrm{CoO}}_{6}$ octahedra. The JT effect is favored by the intermediate spin-state configuration, ${t}_{2g}^{5}{e}_{g}^{1}$ and ${t}_{2g}^{5}{e}_{g}^{1}\underset{\ifmmode\bar\else\textasciimacron\fi{}}{L},$ of the ${\mathrm{Co}}^{3+}{(d}^{6})$ and ${\mathrm{Co}}^{4+}{(d}^{5})$ species occurring in the structure. At ${\mathrm{T}}_{C},$ the material exhibits a sudden increase of metallicity and a substantial peak in the magnetoresistivity. When further cooling down, the gradual ordering of the ${d}_{{3z}^{2}\ensuremath{-}r2}$ orbitals leads to a metal-insulator-like transition at ${T}_{\mathrm{MI}}\ensuremath{\sim}120 \mathrm{K}.$

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