Elucidating the Chemical Order and Disorder in High-Entropy MXenes: A High-Throughput Survey of the Atomic Configurations in TiVNbMoC₃ and TiVCrMoC₃

Abstract

Expanding the MXene design space from ordered and random double-transition-metal (DTM) MXenes to include high-entropy (HE) MXenes with four or more principal elements enables a powerful approach for enhancing MXene properties. While many DTM MXenes possess unique structures that strongly influence material properties, HE MXenes are largely unknown because they are only recently synthesized. Since certain combinations of transition metals (TMs), e.g., Mo-Ti and Cr-Ti, lead to ordered DTM MXene phases, where Mo/Cr atoms occupy the outer TM layers and Ti atoms occupy the inner layers, it is critical to investigate any possibilities of TM segregation in the atomic layers of HE MXenes. Therefore, we present a high-throughput first-principles study of the atomic configurations of two recently synthesized HE M4C3 MXenes: TiVNbMoC3 and TiVCrMoC3. Combining density functional theory, cluster expansion, and Monte Carlo simulations, we predict a unique preferential occupancy of the TM atoms in the four layers within the single-phase HE MXenes, even at temperatures as high as 2900 K. Across a wide compositional range, the outer (inner) layers are predominantly occupied by two of the four TM elements, with Cr most preferentially occupying the outer layers, followed by Mo, V, Nb, and Ti. The strong compositional dependence of the interlayer segregation highlights the HE MXenes’ tunability. Within each TM layer, the atoms largely form a solid solution, with a tendency for Nb-V separation at lower temperatures. Our results elucidate the chemical order and disorder in HE MXenes, guiding experiments in designing MXenes with enhanced properties within the huge compositional space.

First-Principles Studies on High-Entropy Ti0.75V0.75Cr0.75Mo0.75C2 MXene Nanosheets as Anode Materials in Zinc-Ion Batteries

“…It is revealed that the interlayer spacing is similarly large and it varies very little with placements of the four transition metal atoms. The energy differences are also small; it is worth mentioning that the first configuration (Figure S1a) with Mo and V (Ti) atoms in the outer (inner) layer has the lowest energy among them, which is consistent with the reported elemental preference, , where the Mo and V atoms prefer to be in the outer layer, while Ti atoms prefer the inner layer. Also, the energy of the configuration with Ti atoms in the outer layer and Mo/V atoms in the inner layer (Figure S1b,c) is relatively higher.…”

Applications of MXene‐Based Single‐Atom Catalysts

“…Leong et al calculated the thermostatically stable configurations of TiVNbMoC 3 and TiVCrMoC 3 , and found that the metals showed obvious preferential occupation at high temperatures, with Cr most inclined to the outer layer, followed by Mo. [23] In addition, the metals in high-entropy MAX precursor were in a solid solution state above 1873 K, while Nb and V atoms showed significant separation in the inner layer below 928 K. x observed along the a/b axis. Reproduced with permission.…”

Titanium–Tantalum Double-Ordered MXene Nanosheets as Supercapacitor Electrodes

“…Our previous works on double-ordered MXene nanosheets displayed promising results both computationally and experimentally, , wherein a double transition metal provided more space for Li-ion intercalation, leading to high performance with cyclic stability. Nemani et al, Hong et al, and Leong et al explored the synthetic aspects of double-ordered, high-entropy MXene (HEM) via computational and experimental methods. − Du et al have successfully synthesized the HEM containing the transition metals Ti, V, Zr, Nb, and Ta using the classical technique of the MAX phase followed by HCL and LiF etching and used as a Li-ion anode. The high molar concentration and lattice distortions resulted in a high mechanical strain, further aiding the dendrite-free Li on HEM layers.…”