A Novel Mixed Valent Chromium-Layered Oxide with Peculiar Magnetic Properties: Sr4.5Cr2.5O9

A new mixed valent chromium-layered oxide, Sr4.5Cr2.5O9, with a structure closely related to the "0201-1201" structure of the superconducting cuprate TlBa2-xLa2+xCu2O9, has been synthesized at normal pressure. This orthorhombic phase (a=10.7959(2) ?, b=5.4012(1) ?, and c=30.0755(6) ?) consists of the regular intergrowth of [Sr2CrO4]0201 blocks and [(Sr0.5Cr0.5)Sr2CrO5]1201 blocks. It differs from the thallium cuprate by the 1:1 ordering of Sr and Cr species within the intermediate layer of the "1201" block, so that at this level CrO4 tetrahedra, containing Cr6+ species, are obtained. Moreover, at the boundary between the "1201" and "0201" blocks, CrO6 octahedra alternate with CrO5 pyramids, both occupied by Cr3+ species. The magnetic measurements show an original metamagnetic transition from an antiferromagnetic to a weak ferromagnetic state. These properties are interpreted in terms of strong intralayer and weak interlayer magnetic couplings between Cr3+ species through a superexchange mechanism.