High-Throughput Survey of Ordering Configurations in MXene Alloys Across Compositions and Temperatures

Abstract

2D transition metal carbides and nitrides known as MXenes are gaining increasing attention. About 20 of them have been synthesized (more predicted) and their applications in fields ranging from energy storage and electromagnetic shielding to medicine are being explored. To facilitate the search for double-transition-metal MXenes, we explore the structure-stability relationship for 8 MXene alloy systems, namely, (VMo)C, (NbMo)C, (TaMo)C, (TiMo)C, (TiNb)C, (TiTa)C, (TiV)C, and (NbV)C with 0 ≤ x ≤ 1, using high-throughput computations. Starting from density-functional theory calculated formation energies, we used the cluster expansion method to build quick-to-compute interactions, enabling us to scan through the formation energies of millions of alloying configurations. For the Mo-rich MXenes, (M1Mo)C (where M1: Ti, V, Nb, Ta) Mo atoms prefer to occupy the surface layers, and ordering persists to high temperatures, based on our Monte Carlo simulations. When Ti is alloyed with Nb or Ta, in the Ti-rich MXenes, Ti atoms prefer the surface layers (e.g., Ti-C-Nb-C-Ti sequence), and in the Nb- or Ta-rich MXenes, Ti occupies only one surface layer and the other two layers are Nb or Ta (e.g., Ti-C-Nb-C-Nb), exhibiting asymmetric ordering. However, alloying Ti with V results in solid solutions across all compositions. (NbV)C phase separates at lower temperatures but forms solid solutions at synthesis temperatures. Postsynthesis annealing at moderate temperatures (800 to 1000 K) increases the ordering for all the compositions. Lastly, by investigating the stability of their precursor MAX phases and surface-terminated MXenes, we discuss the synthesis possibilities of highly ordered MXenes.

Electronic and Optical Properties of 2D Transition Metal Carbides and Nitrides (MXenes)

“…Due to a large variety of transition metals and surface functionalities, MXenes' properties can be tuned by selecting combinations of transition metals, X elements, and controlling their surface chemistries. Close to 30 MXenes have been experimentally reported (marked with green color in Figure ), whereas many more are predicted to exist and have been studied theoretically (marked with black color in Figure ) . Given that close to 100 MAX phases have been made, not counting unlimited solid solutions and a number of ordered double transition metal structures, there are endless and quickly expanding opportunities for producing MXenes with desirable properties.…”

Recent Advances in Layered Ti3C2Tx MXene for Electrochemical Energy Storage

“…More than a dozen MXenes have been reported to date, launching experimental and theoretical activity worldwide. [49][50][51][52][53][54][55][56][57][58] The distinguishing characteristics of MXenes, including unique morphologies, high electronic conductivities, and rich chemistries, have rendered them attractive in many applications, especially for energy storage. [59][60][61][62][63][64][65][66][67][68] As one typical representative among the MXene family, Ti 3 C 2 T x is a primary focus of many researchers because of its well-known etching conditions and the detailed theoretical and experimental studies on its physical and electrochemical properties, as clearly shown by the publication record ( Figure 1d).…”

Robust cluster expansion of multicomponent systems using structured sparsity

“…As mentioned in Sec. II B, similar hierarchical cluster selection rules have been used for CE 3,4,41,[43][44][45]56,57 . Compared to previous works, the combination of these rules with sparsity-driven regularization in our work leads to more robust ECIs.…”

“…Initially, CE was largely applied to binary alloys 2, [13][14][15][16][17][18][19][20][21][22][23][24][25][26] . Thereafter, it has been applied to more complex systems, including ternary to quinary alloys 5,6,12,[27][28][29] , semiconductors 7,30 , battery materials 31,32 , clathrates 33,34 , magnetic alloys [35][36][37] , and nanoscale alloys 3,4,11,[38][39][40][41][42] . In complex systems, the reduced symmetry increases the number of symmetrically distinct clusters, exacerbating the cluster selection problem.…”