Crystal and Magnetic Structures of the Chain Antiferromagnet CaFe₄Al₈

The crystal structure of CaFe₄Al₈ was studied by X-ray single crystal and powder diffraction as well as high-resolution neutron powder diffraction. CaFe₄Al₈ crystallizes with a tetragonal CeMn₄Al₈-type structure, an ordered variant of the ThMn₁₂-type (Pearson symbol tI26, space group I4/ mmm, a = 8.777(1), c = 5.077(1) Å). Similarly to the well-known A15-type superconductors, the structure of CaFe₄Al₈ contains one-dimensional chains of d-metal atoms, which are parallel to the crystallographic fourfold axis. CaFe₄Al₈ is paramagnetic at room temperature and exhibits long-range antiferromagnetic ordering at about 180 K, combined with a short-range ordered spin arrangement. The magnetic structure, determined by powder neutron diffraction at 4 K, shows that the magnetic moments on the Fe atoms form mirror-inverted chains along the c-direction and are slightly canted from the axis.