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 approximately 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) A, and c=13.101(4) A. V4AlC3-x (x approximately 0.31) represents a 413-phase with space group P63/mmc, Z=2, a=2.9302(4) A, and c=22.745(5) A. 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) A, and c=22.983(5) A) 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 approximately 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'.
Single crystals of the new ternary carbides Ta4AlC3 and Ta3AlC2 were synthesized from molten aluminum and characterized XRD, EDX, and WDX measurements. Crystal structures were refined for the first time on the basis of single-crystal data. Both compounds crystallize in a hexagonal structure with space group P63/mmc and Z = 2. The lattice constants are a = 3.1131(3) A and c = 24.122(3) A for Ta4AlC3 and a = 3.0930(6) A and c = 19.159(4) A for Ta3AlC2. The crystal structures can be explained with a building block system consisting of two types of partial structures. The intermetallic part with a composition TaAl is a two layer cutting of a hexagonal closest packing. The carbide partial structure is a fragment of the binary carbide TaC (NaCl type). It consists of three (Ta4AlC3) or two layers (Ta3AlC2) of CTa6-octahedra linked via common corners and edges. Both compounds are members of the series (TaC)nTaAl. The crystal quality of Ta3AlC2 is improved by using a Al/Sn melt for crystal growth leading to small quantities of Sn in the crystal: Ta3Al1-xSnxC2, x approximately 0.04. On the basis of reliable data a detailed discussion of structural parameters is possible. According to the building principle structure models can be developed for the whole series (MX)nMM' including coordinates for all atoms.
In materials with low-dimensional structures, the instability of the weakly bonded layered slabs against folding, and the saturation of dangling bonds in finite layers, leads to the formation of a variety of nanostructures with spherical or cylindrical shapes. In analogy to nested carbon fullerenes [1] and nanotubes, [2] Tenne and co-workers [3,4] discovered related MQ 2 -based (M = early transition metal, Q = S, Se) concentric closed-shell materials, which laid the foundation for the entire field of nanostructures based on inorganic materials.[5] Substantial progress has been made in the application of MoS 2 and WS 2 nanoparticles as lubricants [6±9] and heterogeneous catalysts, [10,11] and MoS 2 nanotubes as electrochemical hydrogen-storage [12] and lithium-intercalation materials. [13,14] During the past few years a huge body of experimental evidence has been accumulated that suggests the phase diagram of the elements that form layered compounds includes new phases of hollow and closed nanostructures. Provided that the crystal growth is limited and the crystallites remain smaller than a critical size (ca. 100 nm), these nanostructures are the thermodynamically preferred phases. In other words, a key step in the preparation of nanostructured layered materials is the restricted growth of the newly formed two-dimensional (2D) crystallites. [15,16] Various strategies have been developed to prepare nanostructured metal chalcogenides, including sulfides and selenides, through different growth mechanisms. A characteristic feature of all closed-shell structures is that high reaction temperatures (> 800 C) or large activation energies are needed to overcome the activation barrier associated with the bending of the otherwise flat 2D layers.[ can provide large activation energies in a very short period, such as electron irradiation, [19] laser ablation, [20] microwave plasma, [21] arc discharge, [22] and pulsed-laser vaporization, [23,24] have been used to synthesize a variety of inorganic fullerene (IF)-like nanostructures. Schuffenhauer et al. [25] have synthesized nested fullerene-like NbS 2 by the reaction between NbCl 5 vapor and H 2 S gas at 400 C followed by further heat treatment at 550 C. Similarly, Coleman et al. [26] have synthesized ReS 2 fullerene-like nanoparticles by sulfidization of ReO 2 obtained from ReO 3 decomposition. As the above methods have to rely mostly on high temperatures and/or complicated processes they may not be optimally suited for the large-scale preparation of nanostructured metal chalcogenides with a minimum amount of side products. On the other hand, as the mechanical, physical, and catalytic properties of nanostructured chalcogenides strongly depend on their size and shape, it is desirable to devise synthetic procedures that enable significant control of the particle size and morphology. From an applications point of view, the focus on nanoparticle growth will be on high-purity, high-yield, and, therefore, low-cost products.In this contribution we report on the facile, large-scale s...
Structure D 2000V2AlC, V4AlC3-x (x ≈ 0.31), and V12Al3C8: Synthesis, Crystal Growth, Structure, and Superstructure. -Single crystals of V 2 AlC (I) are prepared by reaction of the elements using excess Al as a flux (V:C:Al = 2:1:10, pressed pellets, corundum crucible, argon, 1500°C, 50 h). V4AlC2.7 (II) is prepared under similar conditions but with the addition of cobalt to the melt and V12Al3C8 (III) is obtained from a V/C/Al/Co melt at lower temperature (Tmax = 1300°C). The three compounds crystallize in the hexagonal space group P6 3 /mmc with Z = 2 (single crystal XRD). The crystal structures can be divided in two types of partial structures. The VAl intermetallic part is a two-layer cutting of the hexagonal closest packing and the carbide partial structure is a fragment of the binary carbide VC1-x containing one or three layers. (I) is a H-phase (211-phase) and (II) represents a 413-phase. The C-deficit is limited to the carbon site of the central layer. In the structure of (III) (413-phase) the vacancies on the carbon sites are ordered. -(ETZKORN, J.; ADE, M.; HILLEBRECHT*, H.; Inorg. Chem. 46 (2007) 18, 7646-7653; Inst. Anorg. Anal. Chem., Albert-Ludwigs-Univ., D-79104 Freiburg/Br., Germany; Eng.) -Schramke 46-005
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