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Significance Statement
Layered ternary transition-metal carbides and nitrides (commonly known as “MAX phases”) have a very intriguing combination of metal and ceramic-like properties and are therefore strong candidate materials for a myriad of technological and engineering applications ranging from cutting tools (L. Pierre de Rochemont, US Patent 20120012403A1, 2012) and heat exchangers {M. W. Barsoum, Progress in Solid State Chemistry, 28 [1-4] 201-281 (2000)} all the way to spacecraft fuselages [S. Li et al., Applied Physics Letters, 92 221907 (2008)] and coatings for jet engine components {Q. M. Wang et al., Surface and Coatings Technology, 204 [15] 2343-2352 (2010)}. In addition to conventional MAX phases which are composed of three elements, researchers have also created solid solution phases by adding more types of elements. Instead of solid solution phases, our work illustrates the possibility of obtaining a new family of crystalline phases with periodic atomic arrangements. An example phase, (Cr2Hf)2Al3C3, is calculated to be energetically much more favorable than models of allotropic segregation and solid solution phases. Our work also indicates that (Cr2Hf)2Al3C3 has much higher elastic moduli compared with the two competing phases. Facinating questions such as “How many phases can be synthesized?” and “What exotic properties do they possess?” await answers from experimental explorations.
* Corresponding author ([email protected]);
Figure Legend: The crystallographic evolution from Cr2AlC to (Cr2Hf)2Al3C3. (a) The unit cell of Cr2AlC. (b) Cr-C layers separated from (a). (c) A 6×6 expansion of the middle layer in (b). (d) The same structure as that of (c), but with hexagons facilitating the observation of the hexagonal arrangements. (e) The structure from (c), with the central Cr atom in each hexagon replaced by Hf. (f) The same structure as that of (e), but with blue frames indicating the new unit cell. Derived in a similar approach from the top and bottom layers of (b), the left column of (g) shows the other two new layers, and the right column of (g) shows the new Al layers. (h) shows the preliminary unit cell of (Cr2Hf)2Al3C3, assembled with all the cell layers from (g) and (f).
Journal Reference
Journal of the American Ceramic Society, Volume 97, Issue 8, pages 2646–2653, 2014. Yuxiang Mo1,*, Sitaram Aryal2, Paul Rulis3, Wai-Yim Ching3
[expand title=”Show Affiliations”]
- Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA
- Department of Physics and Mathematics, Tennessee State University, Nashville, TN 37209, USA
- Department of Physics and Astronomy, University of Missouri-Kansas City, Kansas City, MO 64110, USA.
Abstract
The term “MAX phase” refers to a very interesting and important class of layered ternary transition-metal carbides and nitrides with a novel combination of both metal and ceramic-like properties that have made these materials highly regarded candidates for numerous technological and engineering applications. Using (Cr2Hf)2Al3C3 as an example, we demonstrate the possibility of incorporating more types of elements into a MAX phase while maintaining the crystallinity, instead of creating solid solution phases. The crystal structure and elastic properties of MAX phase-like (Cr2Hf)2Al3C3 are studied using the Vienna ab initio Simulation Package. Unlike MAX phases with a hexagonal symmetry (P63/mmc, #194), (Cr2Hf)2Al3C3 crystallizes in the monoclinic space group of P21/m (#11) with lattice parameters ofa = 5.1739 Å, b = 5.1974 Å, c = 12.8019 Å; {Alpha} = {Beta} = 90°, {Gamma} = 119.8509°. Its structure is found to be energetically much more favorable with an energy (per formula unit) of −102.11 eV, significantly lower than those of the allotropic segregation (−100.05 eV) and solid solution (−100.13 eV) phases. Calculations using a stress versus strain approach and the VRH approximation for polycrystals also show that (Cr2Hf)2Al3C3 has outstanding elastic moduli.
© 2014 The American Ceramic Society
