A new layered cobaltite (Ga 1/3Co 2/3) 2Sr 2CoO 6+δ with high spin Co 3+: modulated structure and physical properties

Abstract

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, (Ga 1/3Co 2/3) 2Sr 2CoO 6+ δ is derived from the perovskite with a = a p √ 2 , b = a p √ 2 , c = 19.2 Å and a modulation vector q * = q 1 a * + q 2 c * . For the as-prepared samples, the δ value is close to 0.4 and the modulated structure ( q 1 ≈ 1 / 3 and q 2 = 1 ) can be described in an ideal orthorhombic 3D supercell Bb 2 b with the unit cell parameters a = 3 a p √ 2 = 16.3030 ( 5 ) Å , b = 5.4725 ( 2 ) Å and c = 19.2034 ( 4 ) Å . The layer stacking consists in an intergrowth between a [SrCoO 3] perovskite-type block and a block of triple layers, [(SrO)(Co 2/3Ga 1/3O 1+ δ /2)(Co 2/3Ga 1/3O 1+ δ /2)] derived from [AO] rock-salt type layers. The magnetic study shows that the Co 3+ cations exhibit a high spin state in this structure. The θ p value, −570 K, suggests that antiferromagnetic interactions dominate in this compound as in the brownmillerite cobaltites. Nonetheless, its resistivity ρ 300 K = 1 0 - 1 Ω cm is lower than that of the brownmillerite compounds. Since the positive Seebeck coefficient indicates the presence of holes (Co 4+) in the CoO 2 conducting layers, the existence of weak ferromagnetic Co 3+–O–Co 4+ interactions developing below ≈150 K is proposed to explain the electronic properties of this new oxide.