The crystal structure of Al_20.6Ta_22.4 and its relation to Frank–Kasper phases

Abstract

The monoclinic phase in the Al–Ta system has been identified by means of X-ray single-crystal structure analysis as Al_{19+ x} Ta_{24− x} (x = 1.6), Pearson symbol mP86. Al_20.58Ta_22.42 crystallizes in the space group P2₁/n, with a = 987.86 pm, b = 990.12 pm, c = 1489.45 pm and β = 99.958°. Twinned crystals of the title compound can be obtained by iodine-promoted reactions of the elements. In the experiments described here, the reactions took place in sealed tantalum ampoules at 1400°C. Single-phase samples were obtained by a metallurgical powder method, which minimizes aluminium loss by incongruent vapourization. The composition was found to span the range 0.51 ≤ x _Ta ≤ 0.53. Al_{19+ x} Ta_{24− x} decomposes in the solid state at 1450°C into Al₆₉Ta₃₉ and σ-AlTa₂. The crystal structure is analyzed in relation to tetrahedral close-packed structures. The kinship with Cr₃Si-type and H-type structures is accentuated.

Acknowledgments

The authors are grateful for technical assistance provided by Daniel Kreß with the preparation of some of the samples and by Clemens Pietzonka with the EDX measurements. Financial support by Fonds der Chemischen Industrie is appreciated.