Intermediate spin state of Co3+ and Co4+ ions in La0.5Ba0.5CoO3 evidenced by Jahn-Teller distortions

From high resolution neutron and synchrotron powder diffraction studies performed on the ideal cubic perovskite compound La0.5Ba0.5CoO3, we have observed the onset of a long-range tetragonal phase accompanying a para-ferromagnetic transition at TC∼180 K. The lowering of the crystal symmetry is the direct signature of a cooperative and static Jahn-Teller (JT) distortion of the CoO6 octahedra. The JT effect is favored by the intermediate spin-state configuration, t2g5eg1 and t2g5eg1 L, of the Co3+(d6) and Co4+(d5) species occurring in the structure. At TC, the material exhibits a sudden increase of metallicity and a substantial peak in the magnetoresistivity. When further cooling down, the gradual ordering of the d3z2-r2 orbitals leads to a metal-insulator-like transition at TMI∼120 K