Search

Graphical abstract Abstract Interstitial transition metal nitrides and carbides based on simple crystal structures with partial occupation of certain sites by N and C frequently show long-range ordering. In this work the focus is on the important group of superstructures based on a face-centered cubic (fcc)-based arrangement of the metal atoms and octahedral site occupation by N or C. The ordering tendencies are explored for various Ni 4 N and Ni 4 C superstructures using density-functional-theory. The ordering tendencies differ strikingly as revealed by the low-energy superstructures encountered for Ni 4 N and for Ni 4 C: in the case of Ni 4 N 2nd-next-neighbor N-N pairs appear favorable whereas in Ni 4 C 1st-next neighbor C-C pairs are preferred. This can be explained by an interplay between elastic interactions favoring 1st-next neighbor pairs of N and C atoms in an fcc-based arrangement of metal atoms and chemical/Coulomb interactions favoring 2nd-next neighbor (i.e. more distant) pairs. While the elastic interactions are expected to be similar for N and C, the chemical/Coulomb interactions are stronger for N-N as compared to C-C and are thus able to explain the different ordering tendencies. [ABSTRACT FROM AUTHOR]

We investigated the effect of shock impact on some transition metal carbides and nitrides {MC/N (M = Zr, Nb, Ta, Ti)} within the Debye-Gruneisen theory. Parameters required for the model were obtained from first principles by fitting the total free energy vs. volume data obtained from DFT to the equation of state (EOS) proposed by Li. The EOS model was used to predict shock Hugoniot curves for these compounds to verify the effect of increasing pressures and temperatures on the EOS. Reasonably good agreement with experimental data was noted for NbC, ZrC, TaC and ZrN and TiN up to ∼ 200 GPa considering the metals to exist in B 1 (NaCl) phase. However, in case of ZrC, TaC and ZrN, single Hugoniot data points were available at higher pressures ( ∼ 400 GPa). In all three cases, the predicted Hugoniot corresponding to B 1 phase did not match with the experimental data point at that pressure. This was attributed to a structural phase transition occurring in the metals. We found all the compounds to be stable in the rock salt structure ( B 1) and transform to CsCl ( B 2) structure at high pressures (beyond ∼ 200 GPa) and 0 K. Since experiments are performed at finite temperatures, we investigated this transition at finite temperatures. From our analysis, we concluded that B 1 → B 2 transition does not occur at high pressures under shock loading. Experimental evidence of the transition has also not been reported for these compounds as per our knowledge. The only other possibility of a structural phase transition was solid-liquid transition i.e. melting. We found that all the compounds melt under shock loading at high pressures beyond ∼ 200 GPa. [ABSTRACT FROM AUTHOR]

Using density functional theory and the Peierls–Nabarro model, we calculated the shear strength and generalized stacking fault energy of TiN and TiC as prototypes of transition metal nitrides and carbides. In spite of its higher elastic moduli, TiN was found to have a lower plastic resistance as compared to TiC due to the higher shear strength and stacking fault energy of TiC. A mechanism based on the development of a deformed electronic structure is proposed to explain the origin of this difference in plastic resistance. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ag and Au electrocatalysts supported on transition metal carbides (TMCs) and nitrides (TMNs) were synthesized and evaluated for CO 2 RR. • TaC and TaN were identified as promising supports for Ag and Au catalysts for electroreduction of CO 2 to synthesis gas. • ΔG (*HOCO) was identified as a key descriptor for CO 2 RR based on a linear correlation between ΔG (*HOCO) and J ECSA (CO). • Electron transfer from TMC/TMN substrates to Ag/Au played a dominant role in controlling the CO 2 RR activity. Synthesis gas produced from the electrochemical carbon dioxide reduction reaction (CO 2 RR) using renewable energy represents a promising solution toward carbon neutrality. Both high activity and controlled CO/H 2 ratios are crucial for coupling CO 2 RR with downstream thermochemical reactions. In the current study, transition metal carbides (TMCs) and nitrides (TMNs) of Group VB elements (V, Nb and Ta) were used as supports to tune the CO 2 RR activity and CO/H 2 ratios over Ag- and Au-based electrocatalysts. Combined electrochemical measurements and density functional theory (DFT) calculations identified the Gibbs free energy for adsorption of the *HOCO intermediate (ΔG (*HOCO)) as a potential descriptor for CO 2 RR, which could be adjusted by using different TMC and TMN supports. As opposed to structural strain effects, the electronic modification was found to be the key controlling factor in the interactions between TMC/TMN and supported Ag/Au. TaC and TaN have been identified as promising supports for Ag- and Au-based electrocatalysts for CO 2 RR to synthesis gas production. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, have gained notable attention recently because these super-strong materials have many promising applications. In this study, computational analysis is conducted to explore the in-plane elastic constants, 2D stiffness and shear modulus of oxygen terminated M 4 X 3 MXenes by means of the density functional theory (DFT) calculations. Our results reveal that the binding site of the oxygen atoms can greatly affect the mechanical properties. The crystal orbital Hamilton population (COHP) analysis suggests that the impact of the oxygen-binding site is because the M − O bonding strength can significantly influence their mechanical stiffness. Our outcomes may, therefore, provide the theoretical foundation for the advance of MXene-based applications in mechanical engineering. [Display omitted] • 2D M 4 X 3 O 2 materials are super-strong materials and possess outstanding mechanical properties. • Introducing the oxygen termination groups to the surface of MXenes improve their mechanical strength. • The influence of transition metal on the mechanical properties of MXenes has been investigated. • The COHP analysis indicates that the M–O bonding strength mainly determines the mechanical strength of M 4 X 3 O 2. [ABSTRACT FROM AUTHOR]

MXenes, transition metal carbides and nitrides, are a bourgeoning class of two-dimensional (2D) materials due to their tunable electronic and magnetic structures, rich surface chemistry and thermal stability. Here, we perform structural, electronic, vibrational and thermal properties of six transition-metal carbides (Re 2 C, Tc 2 C, Mo 2 C, W 2 C, Os 2 C, Ru 2 C) both 2H and 1T phases by first-principle calculations within density functional theory. Although physical and thermal properties of some pristine MXenes put forward to understanding tunable electronic and magnetic properties, many of 1T-phase and almost never of 2H-phase are not investigated in detailed. Firstly, the atomic structure of six MXenes both 1T and 2H phases are optimized, and their respective dynamical stabilities are discussed. Secondly, electronic band structure calculations reveal that the pristine MXenes are metallic. Finally, phonon calculations of pristine MXenes are computed with the density functional perturbation theory and reported. Raman-active modes are predicted and to assign them to specific atomic motions. Ab-initio molecular dynamic simulations (AIMD) are also performed to check the thermal stability of these MXenes. Our results clearly proved that while 2H-W 2 C, 2H-Mo 2 C, 2H-Re 2 C and 1T-W 2 C can keep the stabilities at very high temperature (1500 K). The results suggest that these pristine MXenes combining with outstanding properties such as non-magnetic metallic state and high-temperature thermal stability would provide promising candidates for using from energy storage to spintronic applications. • The new 2D MXenes have been predicted by first-principle calculations. • Structural, electronic, vibrational and thermal properties are performed. • AIMD are proved thermal stabilities at the different temperature. • MXenes would provide promising candidates for future applications. [ABSTRACT FROM AUTHOR]