Influence of ordered carbon‐vacancy networks on the electronic structures and elastic properties of Nb₄AlC_3−x

Abstract

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 influences on bonding characteristics and elastic properties are unraveled by thoroughly comparing Nb12Al3C8 and vacancy‐free Nb4AlC3. In Nb12Al3C8, the carbon vacancies break only relatively weak Nb–C bonds and form ordered equilateral triangular carbon‐vacancy networks (OETCVNs) to maximize the bond strengthening effect. The networks slightly shift partial and total density of states toward the Fermi energy level, and bring about a feature of “de‐metallization”. Meanwhile, the presence of OETCVNs results in the softening of elastic modulus, decreasing of the anisotropy of Young's modulus, yet increasing that of shear modulus. These results shed lights on the carbon‐vacancy ordering behavior of MAX phases, and provide an opportunity to tailor their electronic structures and elastic properties through defect engineering.

Prediction and synthesis of a family of atomic laminate phases with Kagomé-like and in-plane chemical ordering

“…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 – 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.…”

“…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 – 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.…”

Anti-perovskite carbides Ca6CSe4and Sr6CSe4for photovoltaics with similar optoelectronic properties to MAPbI3

“…Moreover, it is noted that several anti-perovskite carbides, such as Mg 3 CNi, Co 3 CZn, Fe 3 CZn, Ni 3 CZn, and Ti 3 CAl, have been synthesized experimentally through the repeated sintering, self-propagating hightemperature synthesis, mechanical alloying method, conventional powder metallurgy, solid-state reaction, one-step thermal reduction, or in situ reaction/hot-pressing route. [53][54][55] These synthesis methods of carbides may provide insight into the synthesis of the predicted M 6 CCh 4 anti-perovskites.…”

“…Halide perovskites are commonly fabricated via feasible room-temperature spin-coating approaches, which are low-cost but lead to poor stability as well as heat resistance. 12,79 Compared with halide perovskites, most synthesis methods for anti-perovskite carbides require high temperature, [53][54][55] indicating their great potential on concentrator cells. Fig.…”