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Differential charge density maps of (a) Nb 12 Al 3 C 8 on (10 10) and (b) Nb 4 AlC 3 on (11 20). For the sake of clarity, only the negative density is shown here. The shrinkage of the regions with negative density around Nb3 in Nb 12 Al 3 C 8 compared to those around Nb2 in Nb 4 AlC 3 indicates an enrichment of electrons around Nb3 in Nb 12 Al 3 C 8 compared to Nb2 in Nb 4 AlC 3 .
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![Figure 1: Projections of (a) Nb4AlC3 along [], and (b) Nb12Al3C8 along...](profile/Xiaohui-Wang-5/publication/309148234/figure/fig3/AS:417103959085058@1476456741592/Projections-of-a-Nb4AlC3-along-and-b-Nb12Al3C8-along-The-octahedral_Q320.jpg)
Carbon-vacancy-bearing Nb4AlC3−x has the best high-temperature mechanical robustness among MAX phases. The existing form of the vacancies has been long overlooked. Recently, the vacancies in Nb4AlC3−x have been identified to be ordered, establishing an ordered compound Nb12Al3C8. Here, the spatial distribution of the ordered vacancies and their inf...
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... atoms results in the redistribution of excess valence electrons: Nb atoms of the carbon-vacant octahedrons, e.g., Nb3, bear more electrons than the corre- sponding Nb atoms (Nb2) in vacancy-free Nb 4 AlC 3 , as evi- denced by the shrinkage of the regions with negative density around Nb3 in Nb 12 Al 3 C 8 compared to those around Nb2 in Nb 4 AlC 3 (Fig. 3) [Figs. 2(h)-(m)] on the whole, by and large signifying that OETCVNs weaken the chemical bonds. Whereas, several peaks marked by arrows move to lower energy levels, which implies the strengthening of some Nb-C bonds due to the redistribution of valence electrons. Virtually, one third of Nb3-C2 are significantly strengthened (Table ...
Citations
... This suggestion is well supported by several known facts about MAX phases and MXenes. In fact, carbon vacancies had been observed in MAX phases, 12 and interstitial oxygen is known to occur in these materials. 13,14 Furthermore, the formation energy of carbon vacancies in Ti2C, when its surface is bare or functionalized with F or OH groups is as high as 2.8 eV, 15,16 the same value found on the corresponding parent MAX phase, Ti2AlC. ...
... [3,[16][17][18][19][20] The recent experimental identification of oxygen content (up to 40 %, at an average around 25 %) amid the X layers of some MXenes, using ultralow-energy secondary-ion mass spectrometry, with atomic resolution, further added to their tuneability. [21,22] This property was found to be inherited from the parent MAX phases, which have long been known to contain X vacancies [23,24] and some oxygen, both in interstitial and substitutional positions. [25][26][27][28] The MAX phases in which oxygen has been found include Ti 2 AlC and V 2 AlC, whose corresponding MXenes, Ti 2 C and V 2 C, are included in the ones that were discovered to be in fact oxycarbides when synthesised using HF etching. ...
... This suggestion is well supported by several known facts about MAX phases and MXenes. In fact, carbon vacancies had been observed in MAX phases, 12 and interstitial oxygen is known to occur in these materials. 13,14 Furthermore, the formation energy of carbon vacancies in Ti2C when its surface is bare or functionalized with F or OH groups is as high as 2.8 eV, 15,16 the same value found on the corresponding parent MAX phase, Ti2AlC, 17 while when surrounded by oxygen, carbon vacancies are much more likely to appear, with formation energies lower by around 2 eV, making them statistically more common than vacancies on other 2D materials like graphene or MoS2. ...
... In general, MXenes are synthesized from the transition metal ternary layered compounds with the formula of +1 (MAX), where is an early transition metal, is mainly a group IIIA or IVA elements, and is C and/or with = 1, 2, or 3. Since the -bond is weaker than the --bond in MAX phase, chemical etching is a common modification method to remove the elements from MAX phase. During the etching process, some chemical functional groups such as -F, -O, or -OH are often bound to the exfoliated layers to synthesize functionalized MXenes [27][28][29]. This fabrication process makes it possible to manipulate the characteristics of MXenes through different functional groups saturating the exfoliated layers. ...
MXenes can exhibit unique physical properties by saturating the exfoliated layers with different functional groups, which have been designed for extreme conditions due to their superior resistance to high-temperature and radiation damage. In this study, we focus on the effects of possible hole defects caused by radiation on electronic structure and optical properties for three functionalized MXenes Ti3C2T2 (T = F, O, OH). The structure–activity relationship between different defect types and optical responses is established. Hole defects have a significant effect on the electronic structure of Ti3C2T2, which mainly modify the electronic distributions at deep energy level. In Ti3C2F2 and Ti3C2O2, the hole defects will break the localization of F p-orbital and O p-orbital and release more delocalized electrons. While, defects will bring more isolated peaks corresponding to the O-H bonds in the deep energy level of Ti3C2(OH)2. The optical absorption regions for these systems are mainly concentrated in the ultraviolet (UV) region with absorption onsets in the infrared range. For Ti3C2F2 and Ti3C2O2, the hole defects in triangle, diamond and hexagonal forms can significantly improve the light absorption intensity in the UV region. While for Ti3C2(OH)2, hole defects will strengthen the light response in the visible and infrared regions.
... The formation of Mo vacancy results in a significant decrease in the density of states around −12 eV, which is mainly due to the decrease in Mo-4d. 11,34 It is noted that the hybridization energy peak around −5 eV has reduced because of vacancy defects, especially for C vacancy. The reason is that the number of Mo-C bonds is reduced. ...
As one of the MAX phases, Mo2GeC can also be considered as a potential material for use in next generation fission and fusion program reactors. We used first-principles calculations to investigate the formation energies, stable configuration, and interatomic bonding of intrinsic defects (mono-vacancy, self-interstitials, antisites, and Frenkel pairs). For all intrinsic defects, only the value of the formation energy for the C vacancy defect is negative, and the biggest formation energy occurs for GeFP. The existence of mono-vacancy shrinks the Mo2GeC structure, while the existence of interstitials, antisites, and Frenkel pair defects expands the Mo2GeC structure. In order to further illustrate the stability of defects, we calculated the DOS and PDOS of defects. We can find that defects have a certain effect on the density of states of Mo2GeC. When mono-vacancy and antisite defects are generated, the DOS at the Fermi level decreased, while the production of self-interstitials and Frenkel defects caused the DOS at the Fermi level to increase. We also found that the C vacancy, Ci1, and Mo–Ge antisite pair caused a small pseudo-gap energy at the Fermi level, indicating that their structure is relatively stable, which is consistent with the result of low formation energy. In addition, a small isolated peak at the point of −13.5 eV for Ci1 appeared, which is attributed to the C-2s orbital. We hope that our results could provide theoretical guidance for future experiments and applications of Mo2GeC.
... A minor amount of oxygen appears to be incorporated into the coatings during growth, with a fairly higher quantity for Ti-based coatings. Since experimentally synthesized MAX phase compounds routinely comprise (accommodate) substantial carbon and/or nitrogen vacancies and interstitial oxygen can substitute for carbon and/or nitrogen in MAX phase structures, 16,42,43 these phenomena (C-deficiencies and oxygen impurities) should not adversely affect the crystallization behavior of MAX phases upon annealing of the multilayers. Figure 2 illustrates the typical multilayered design of an as-deposited Ti/C/Al (2:1:1) coating on the Si substrate as an example. ...
Mn + 1AXn (MAX; n = 1–3) phases are ternary layered nitride and carbide compounds featuring a combination of metallic and ceramic properties. Highly basal-plane textured and polycrystalline Cr2AlC, Ti2AlC, and Ti3AlC2 single-phase coatings have been synthesized on both amorphous and polycrystalline substrates via controlled thermal annealing of magnetron-sputtered nanoscale multilayers built by individual transition metal, carbon, and aluminum layers. Formation of substitutional solid solution carbide phases was triggered via solid-state diffusion reactions during annealing. Lower ordered Ti2AlC initially crystallized at an intermediate temperature range and was recognized as an intermediate reactant in the case of synthesizing the Ti3AlC2 312 MAX phase via annealing corresponding stoichiometric multilayers. The crystallization onset temperatures identified via in-situ high-temperature x-ray diffraction measurements were approximately 480, 660, and 820 °C for Cr2AlC, Ti2AlC, and Ti3AlC2, respectively. Contrary to the usually observed columnar structure representative of magnetron-sputtered coatings, the coatings synthesized via the current approach are composed of plateletlike, elongated crystallites. The nanoscale multilayered design stimulates the textured growth of MAX structures during thermal annealing. More specifically, the preferred crystallographic orientation relationships among the as-deposited transition metal layers, the intermediate solid solution phases, and the end-product MAX phases facilitate the growth of textured MAX phase films.
... Notably, chemically ordered MAX phase structures have been reported for 312 (n = 2) and 413 (n = 3) phase structures only, having multiple M sites, each with different symmetry. In addition, ordered carbon vacancies have been reported for 413 (n = 3) phases Nb 4 AlC 3 and V 4 AlC 3 , having multiple C sites (38)(39)(40). However, for the most common MAX phase structure, that is, 211 (n = 1), only chemically disordered alloys with M site solid solution (15-17, 20, 41) have until recently been realized. ...
The enigma of MAX phases and their hybrids prevails. We probe transition metal (M) alloying in MAX phases for metal size, electronegativity, and electron configuration, and discover ordering in these MAX hybrids, namely, (V 2/3 Zr 1/3) 2 AlC and (Mo 2/3 Y 1/3) 2 AlC. Predictive theory and verifying materials synthesis, including a judicious choice of alloying M from groups III to VI and periods 4 and 5, indicate a potentially large family of thermo-dynamically stable phases, with Kagomé-like and in-plane chemical ordering, and with incorporation of elements previously not known for MAX phases, including the common Y. We propose the structure to be monoclinic C2/c. As an extension of the work, we suggest a matching set of novel MXenes, from selective etching of the A-element. The demonstrated structural design on simultaneous two-dimensional (2D) and 3D atomic levels expands the property tuning potential of functional materials.
The MAX phases are a class of nanolaminated materials composed of an early transition-metal (M), an A-group element (A) and C, N, B and/or P (X). Progress in MAX phase research in recent years has increased their number from the original 50 or so, to more than 300 phases. Since half of the 342 MAX phases have been discovered after 2018, an overview of the progress made in the field is timely. Currently, 28 M elements, 28 A elements, and 6 X elements have been incorporated in the MAX phases, alloys included. We further categorize MAX phases based on the synthesis route used to make them; if made via a one-step approach in bottom-up synthesis or formed through elemental replacement reactions in top-down synthesis. This classification is also correlated to theoretical phase stability predictions, that in turn, can be used to identify novel synthesizable MAX phase compositions as well as to suggest suitable synthesis routes. Furthermore, using phase stability predictions we identify 182 new theoretically stable MAX phases awaiting experimental confirmation. Notably, as MAX phases are precursors for MXenes, the dramatically increased interest in the latter for a large host of potential applications renders the former even more valuable.
