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

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)2Sr2CoO6+δ is derived from the perovskite with image, image, image and a modulation vector image. For the as-prepared samples, the image value is close to 0.4 and the modulated structure (image and image) can be described in an ideal orthorhombic 3D supercell image with the unit cell parameters image, image and image. The layer stacking consists in an intergrowth between a [SrCoO3] perovskite-type block and a block of triple layers, [(SrO)(Co2/3Ga1/3O1+δ/2)(Co2/3Ga1/3O1+δ/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 image value, −570 K, suggests that antiferromagnetic interactions dominate in this compound as in the brownmillerite cobaltites. Nonetheless, its resistivity image 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 ≈150 K is proposed to explain the electronic properties of this new oxide.