V₂AlC, V₄AlC_3-x (x ≈ 0.31), and V₁₂Al₃C₈:  Synthesis, Crystal Growth, Structure, and Superstructure

Single crystals of V₂AlC and the new carbides V₄AlC_3-x and V₁₂Al₃C₈ were synthesized from metallic melts. V₂AlC was formed with an excess of Al, while V₄AlC_3-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 VC_1-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. V₄AlC_3-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 TMC_1-x as well. V₄AlC_3-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, (MX_1-x)_nMM‘.