The new polymorph of tetraaluminium chromium, Al4Cr, designated as the ω-phase, was obtained as the product from a high-pressure sintering (HPS) process of a stoichiometric Al4Cr mixture. The crystal structure is isotypic with Al4Mo and Al4W. The unit cell of ω-Al4Cr is much smaller than any of the other reported Al4Cr polymorphs, containing 30 rather than several hundred atoms.

During a high-pressure sintering (HPS) process of a stoichiometric Al 7 Cr mixture, the ternary phase Al 45 Cr 7 C 0.32 (pentatetracontaaluminium heptachromium carbide) was obtained serendipitously. The C atom (occupancy 0.32) is irregularly surrounded by six Al atoms. Structure descriptionPhase formation of a quasicrystalline phase in an Al-Cr alloy by ion irradiation of an amorphous film was reported by Lilienfeld et al. (1986). The structure of the binary phase Al 45 Cr 7 (-Al 7 Cr) phase was determined by Cooper (1960) . During a project aimed at formation of quasicrystalline material in the Al-Cr system under high-pressure conditions, we obtained the title compound and learned that light atoms can be incorporated into the Al 45 Cr 7 host structure. We attribute the slight increase in the unit-cell lengths and the decrease in the monoclinic angle in comparison with the values of the binary phase reported by He et al. ) to the presence of partially occupied light atoms that must have been diffused into the crystal structure during the HPS process, or were present as impurities in the employed metals. In principle, the light atoms could be boron, nitrogen (from the BN crucible), or carbon (from the graphite liner). In fact, we found that the educts were contaminated by small amounts of carbon (see Supplementary information).The asymmetric unit of Al 45 Cr 7 C 0.32 comprises of sixteen Al, three Cr and one partially occupied C sites (occupancy 0.32). Since the host structure has been described in detail (Cooper, 1960; He et al., 2006), we discuss here only the role of the additional carbon atom in Al 45 Cr 7 C 0.32 . The position of the C1 atom in the crystal structure can be seen in Fig. 1. It is located on a site with symmetry m (Wyckoff position 4i) and is connected to six