o′o′-Al13Co4, a new quasicrystal approximant

The crystal structure of o′o′-Al13Co4, a new Al13TM4 (TM = transition metal) modification was determined by single-crystal X-ray diffraction. The phase, with Pearson symbol oP204−x (x = 6), crystallizes in the orthorhombic space group Pnma (No. 62) with lattice parameters a = 28.890(4) Å, b = 8.138(2) Å and c = 12.346(1) Å. The structure, which can be seen as a stacking of four flat and puckered atomic layers along the pseudo-tenfold bb axis, is closely related to o-Al13Co4 and m-Al13Co4. First-principles total-energy calculations indicate that all these related Al13Co4 modifications, some of them stabilized by vacancies, seem to be slightly unstable at low temperatures.