Single crystals of V2AlC and the new carbides V4AlC3-x and V12Al3C8 were synthesized from metallic melts. V2AlC was formed with an excess of Al, while V4AlC3-x (x ≈ 0.31) and V12Al3C8 require the addition of cobalt to the melt. All compounds were characterized by XRD, EDX, and WDX measurements. Crystal structures were refined on the basis of single-crystal data. The crystal structures can be explained with a building-block system consisting of two types of partial structures. The intermetallic part with a composition VAl is a two-layer cutting of the hexagonal closest packing. The carbide partial structure is a fragment of the binary carbide VC1-x containing one or three layers. V2AlC is a H-phase (211-phase) with space group P63/mmc, Z = 2, and lattice parameters of a = 2.9107(6) Å, and c = 13.101(4) Å. V4AlC3-x (x ≈ 0.31) represents a 413-phase with space group P63/mmc, Z = 2, a = 2.9302(4) Å, and c = 22.745(5) Å. The C-deficit is limited to the carbon site of the central layer. V12Al3C8 is obtained at lower temperatures. In the superstructure (P63/mcm, Z = 2, a = 5.0882(7) Å, and c = 22.983(5) Å) the vacancies on the carbon sites are ordered. The ordering is combined to a small shift of the V atoms. This ordered structure can serve as a structure model for the binary carbides TMC1-x as well. V4AlC3-x (x ≈ 0.31) and V12Al3C8 are the first examples of the so-called MAX-phases (MX)nMM‘ (n = 1, 2, 3), where a deficit of X and its ordered distribution in a superstructure is proven, (MX1-x)nMM‘.
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Etzkorn et al. (2007) studied this question.