X-ray diffraction study and theoretical calculations on the X-phase, Al₉(Co, Ni)₄

Abstract

The structure of the X-phase was solved based on single-crystal X-ray diffraction data. The previously unknown ternary compound in the system Al–Co–Ni is representative of a new structure type with Pearson symbol mS26, lattice parameters a = 12.146(2) Å, b = 4.0702(5) Å, c = 7.652(1) Å, β = 105.88(1)°, V = 363.83(9) ų, and spacegroup C2/m. The stability of the monoclinic structure and the Co/Ni ordering was studied by first-principles total-energy calculations. The lowest-energy variant has composition Al₁₈Co₅Ni₃. There are indications that the X-phase is an entropy-stabilized high-temperature phase.

Acknowledgements

W.S and S.K. gratefully acknowledge financial support from grant SNF 200020-105158, M.M. from grant VEGA-2/5096/25 and M.W. from NSF grant DMR-0111198. W.S. would like to thank Yuri Grin for valuable discussions.