Browsing by Subject "Density functional theory"

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    2D rare-earth metal carbides (MXenes) Mo2NdC2T2 electronic structure and magnetic properties: A DFT + U study
    (AIP, 2022) Yao, Shukai; Anasori, Babak; Strachan, Alejandro; Mechanical and Energy Engineering, Purdue School of Engineering and Technology
    2D rare-earth metal carbides (MXenes) are attractive due to their novel electronic and magnetic properties and their potential as scalable 2D magnets. In this study, we used density functional theory with the Hubbard U correction to characterize the structure, termination, and magnetism in an out-of-plane ordered rare-earth containing M3C2Tx MXene, Mo2NdC2T2 (T = O or OH). We investigated the effect of the U parameter on the stability and magnetism of two possible termination sites: the hollow sites aligned with the inner Nd atoms (Nd-hollow sites) and those aligned with the closest C atoms (C-hollow sites). We found that increasing UMo stabilized the Nd hollow sites, which minimized electrostatic repulsion between C and O atoms. Using UMo = 3.0 eV and UNd = 5.6 eV, obtained via the linear response method, we found that the energetically preferred termination site was C-hollow in Mo2NdC2O2 and Nd-hollow in Mo2NdC2(OH)2. Regardless of termination and the Hubbard U value, we found Mo2NdC2O2 and Mo2NdC2(OH)2 to be magnetic. The C-hollow termination resulted in ferromagnetic states for all Hubbard U tested with no magnetic moment in Mo. In the case of Nd-hollow, Mo became magnetic for UMo ≥ 4 eV. The difference of Mo magnetism in Nd-hollow and C-hollow was explained by crystal field splitting of the Mo d orbital caused by a distorted ligand.
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    Elucidating the Chemical Order and Disorder in High-Entropy MXenes: A High-Throughput Survey of the Atomic Configurations in TiVNbMoC3 and TiVCrMoC3
    (American Chemical Society, 2022) Leong, Zhidong; Jin, Hongmei; Wong, Zicong Marvin; Nemani, Kartik; Anasori, Babak; Tan, Teck Leong; Mechanical and Energy Engineering, Purdue School of Engineering and Technology
    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.
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    Enzymatic Stetter Reaction: Computational Study of the Reaction Mechanism of MenD
    (American Chemical Society, 2021) Planas, Ferran; McLeish, Michael J.; Himo, Fahmi; Chemistry and Chemical Biology, School of Science
    Quantum chemical calculations are used to investigate the detailed reaction mechanism of 2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexene-1-carboxylic-acid (SEPHCHC) synthase (also known as MenD), a thiamin diphosphate-dependent decarboxylase that catalyzes the formation of SEPHCHC from 2-ketoglutarate and isochorismate. This enzyme is involved in the menaquinone biosynthesis pathway in M. tuberculosis and is thought of as a potential drug target for anti-tuberculosis therapeutics. In addition, MenD shows promise as a biocatalyst for the synthesis of 1,4-functionalized compounds. Models of the active site are constructed on the basis of available X-ray structures, and the intermediates and transition states involved in the reaction mechanism are optimized and characterized. The calculated mechanism is in good agreement with prior kinetic studies and gives new insights into the mode of action of the enzyme. In particular, the structure and role of the tetrahedral post-decarboxylation intermediate observed in X-ray structures are discussed.