Intermediate spin state of Co3+ and Co4+ ions in La0.5Ba0.5CoO3 evidenced by Jahn-Teller distortions -: art. no. 060401

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 T(C)similar to180 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, t(2g)(5)e(g)(1) and t(2g)(5)e(g)(1)L,of the Co3+ (d(6)) and Co4+ (d(5)) species occurring ring in the structure. At 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) orbitals leads to a metal-insulator-like transition at T(MI)similar to120 K.