A new layered cobaltite (Ga1/3Co2/3)(2)Sr2CoO6 6+delta with high spin Co3+ : modulated structure and physical properties

摘要

Investigations in the Sr-Co-Ga-O system by means of transmission electron microscopy techniques have allowed the detection of a new layered cobaltite. Its structure has been obtained by combining high-resolution images and powder X-ray diffraction. The modulated structure of this novel oxide, (Ga1/3Co2/3)(2)Sr2CoO6+delta is derived from the perovskite with a = a(p)root 2, b = a(p)root 2, c = 19.2 angstrom and a modulation vector q* = q(1)a* + q(2)c*. For the as-prepared samples, the delta value is close to 0.4 and the modulated structure (q(1) approximate to 1/3 and q(2) = 1) can be described in an ideal orthorhombic 3D supercell Bb2b with the unit cell parameters a = 3a(p)root 2 = 16.3030(5) angstrom, b = 5.4725(2) angstrom and c = 19.2034(4) angstrom. The layer stacking consists in an intergrowth between a [SrCoO3] perovskite-type block and a block of triple layers, [(SrO)(CO2/3Ga1/3O1 + delta/2)(Co2/3Ga1/3O1 + delta/2)] derived from [AO] rock-salt type layers. The magnetic study shows that the Co3+ cations exhibit a high spin state in this structure. The theta(p) value, -570 K, suggests that antiferromagnetic interactions dominate in this compound as in the brownmillerite cobaltites. Nonetheless, its resistivity rho(300 K) = 10(-1) Omega cm is lower than that of the brownmillerite compounds. Since the positive Seebeck coefficient indicates the presence of holes (Co4+) in the CoO2 conducting layers, the existence of weak ferromagnetic Co3+-O-Co4+ interactions developing below approximate to 150 K is proposed to explain the electronic properties of this new oxide. (c) 2005 Elsevier Inc. All rights reserved.