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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]