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 $T_C\sim 180\hspace{0.5em}\mathrm{K}.$ 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 $t_{2g}^5{e}_g^1\underset{¯}{L},$ 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 $d_{{3z}^2-r2\mathrm{}}$ orbitals leads to a metal-insulator-like transition at $T_{\mathrm{MI}}\sim 120\hspace{0.5em}\mathrm{K}.$

• Received 27 September 2001

DOI:https://doi.org/10.1103/PhysRevB.65.060401