MS16-P16 Crystal structure of AlCr2 from single-crystal X-ray diffraction data Milica Milosavljevic (Max-Planck-Institut fuer Chemische Physik fester Stoffe, Dresden, Germany) Yuri Grin (Max Planck-Institute for Chemical Physics of Solids, Dresden, Germany) Horst Borrmann (Max Planck-Institute for Chemical Physics of Solids, Dresden, Germany)email: milicamilosavljevic87@gmail.comIn course of extensive investigations of the Al - Cr phase diagram AlCr2 was commonly assigned as Cr-richest binary compound, however, stable below about 900°C only [1]. Based on powder diffraction data MoSi2 structure type was assigned, but no detailed structural analysis has been described till now. The phase diagram is indicating particular problems to obtain single crystals of this particular intermetallic compound. In a multi-step sintering procedure, carefully monitored by powder diffraction as well as scanning electron microscopy (SEM), finally well-defined samples were obtained. Within these samples large grains measuring up to 100 µm could be identified and selected ones were cut out using focused-ion beam (FIB) methods. Those pieces proved to be single crystals with good quality and allowed for the first detailed structural investigation of AlCr2. Lattice parameters a = 3.0056 and c = 8.6449 Å for the body-centered tetragonal unit cell are in excellent agreement with those given in [1] for the ideal composition. The structural model refines well in space group I4/mmm without indications of mixed or anti-site occupancies. The c/a ratio at 2.876 is much larger than the typical value for MoSi2 at 2.437 and quite close to the ideal value for a threefold bcc-type superstructure. The disordered solid solution of Al in Cr existing at higher temperatures adapts indeed a bcc-type structure. A detailed comparison with the structure of MoSi2 is currently hampered by the fact that structural parameters given by different authors are varying quite significantly [2, 3].
In an optional way the structural arrangement may be discussed on the basis of hexagonal AlCr2 layers, which are stacked in ABAB sequence along [110] direction. This allows for interesting comparison with hexagonal CrSi2 and orthorhombic TiSi2 type arrangements in which the same type of layers is stacked in ABCABC or ABCDABCD sequence, respectively.
 
References:

[1] Ellner, M., Braun, J., Predel, B. (1989) Z. Metallkd. 80, 374 – 383.

[2] Christensen, A. N. (1993) J. Crystal Growth 129, 266 – 268.

[3] Harada, Y., Morinaga, M., Saso, D., Takata, M., Sakata, M. (1998) Intermetallics 6, 523 – 527.
Keywords: intermetallic compound, single crystals, AlCr2