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

Abstract

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

Synthesis and Microstructural Characterization of Substoichiometric Ti2Al(CxNy) Solid Solutions and Related Ti2AlCx and Ti2AlN End‐Members

“…As the binary carbides and nitrides (TiC and TiN) constituting the MX block in the MAX phase structure have a wide composition range, substoichiometry of the X component may be expected. Indeed, vacancies on the C or N sites have been evidenced for Ti 2 AlN, Ti 4 AlN 3 , and also for Ti 3 AlC 2 and V 4 AlC 3 systems.…”

Influence of ordered carbon‐vacancy networks on the electronic structures and elastic properties of Nb4AlC3−x

“…As a result, the carbon vacancies in Nb 12 Al 3 C 8 form OETCVNs parallel to (0001) in the central carbon layer (Figure C), and the translation vector of the OETCVNs is 1/2[0001] of Nb 12 Al 3 C 8 unit cell (Figure D). These features also apply for another experimentally identified ordered phase, V 12 Al 3 C 8 . To minimize the energy rise caused by carbon vacancies, the following strategies are involved: (i) distributing the carbon vacancies in the OCT‐2 a type octahedrons with breaking only the relatively weak Nb–C bonds; and (ii) forming 3NN planar OETCVNs to maximize the bond strengthening effect.…”

“…There are two distinct types of Nb 6 C octahedrons in Nb 4 AlC 3 ( P 6 3 / mmc ), involving the carbon atoms located at the 4 f sites with Nb–C bond lengths of 2.173 and 2.253 Å, and the 2 a sites with a Nb–C bond length of 2.277 Å (OCT‐2 a ). Similar to the carbon vacancies in Ta 4 AlC 3− x and V 4 AlC 3− x , those in Nb 12 Al 3 C 8 are located in the OCT‐2 a type octahedrons . Table presents the crystal structure information of Nb 12 Al 3 C 8 and Nb 4 AlC 3 .…”

“…Owing to the predominant structural units of M 6 X octahedron layers, ternary MAX phases inherit some features of binary carbides/nitrides. For instance, MAX phases are capable of bearing certain metalloid vacancies …”

“…Metalloid vacancies in “413” phases have been experimentally and theoretically suggested to be most likely located at the 2 a sites of P 6 3 / mmc space group. Resembling the binary analogs, carbon vacancies at the 2 a sites in V 4 AlC 3− x and Nb 4 AlC 3− x are prone to be ordered, giving rise to ordered phases V 12 Al 3 C 8 and Nb 12 Al 3 C 8 , respectively. These phases are featured with ordered equilateral triangular carbon‐vacancy networks (OETCVNs) as discussed latter.…”