Exploring the Real Ground-State Structures of Molybdenum Dinitride

Abstract

Molybdenum dinitride (MoN 2 ) was recently synthesized at a moderate pressure of 3.5 GPa, and a layered MoS 2 -type structure has been proposed. However, our firstprinciples calculations of thermodynamic, mechanical and dynamical properties suggest that this layered R3m structure is unstable. Therefore, stable structures of MoN 2 at pressures from atmospheric pressure to 100 GPa have been further examined by utilizing a widely adopted evolutionary methodology USPEX for crystal structure prediction. We find that the ground state of the MoN 2 system is a pernitride structure with space group P6 3 /mmc which transforms to a P4/mbm phase above 82 GPa. Chemical bonding analysis shows that one could assign MoN 2 as Mo 4+ (N 2 4− ); i.e., Mo is formally a d 2 metal, in agreement with the experimental results of Wang et al. The presence of covalent N 2 dumbbells and strong bonding between Mo 4+ and N 2 4− is the source of the superior mechanical properties of these predicted ultra-incompressible MoN 2 pernitrides.

Effect of mixed surface terminations on the structural and electrochemical properties of two-dimensional Ti3C2T2 and V2CT2 MXenes multilayers

“…As a result, we assume that after Na intercalation, each -OH group is replaced with a -O termination. Finally, in agreement with experiment 9 and previous calculations, 12,32 the Na atoms are located directly on top of the carbon atoms for both Ti 3 C 2 T 2 and V 2 CT 2 .…”

“…Furthermore, ion intercalation typically causes the number of -OH groups on the surface to be reduced or even eliminated. 6,32,46,47 This instability of the H atoms in -OH terminations is also found to be the case here. As a result, we assume that after Na intercalation, each -OH group is replaced with a -O termination.…”

“…They are in good agreement with previous calculations. 32,33 Fig. 2(b) and (d) then compare the averaged DOS of the structures with randomly populated terminations (solid black line) with those calculated as a weighted average of the uniformly terminated structures, shown with a red dotted line.…”

“…The vertical distance between the two Na layers is 3.1Å, which is similar to that calculated for a Na bilayer adsorbed on a Ti 3 C 2 O 2 monolayer (3.08Å). 32 The average vertical distance between the Na atoms and the terminating atoms is 1.5Å, while the average distance between Na and the carbon atoms is 3.93Å. The increase in the c lattice parameter found here for a Na double layer in V 2 CT 2 is considerably higher than the experimentally reported increase of 4.6Å, 48 suggesting that this value does not indicate the presence of a Na double layer, but rather other intercalants such as H 2 O, or a large solvation shell around the intercalated Na atoms.…”

Thermodynamic Routes to Novel Metastable Nitrogen-Rich Nitrides

“…In agreement with a recent computational structure prediction study, we also calculated a pernitride in the P4/mbm symmetry to be the lowest-energy structure, stabilizable under 1.23 eV/N. 79 It will be interesting to determine the mechanisms that yield the formation of MoN 2 in the R3m-MoS 2 structure, and to understand the factors that give preference to this structure over the lower-energy MoN 2 structures. We also predicted Mo 3 N 5 and Mo 2 N 3 phases in the Mo−N system, with critical Δμ N 2 of +0.97 and +1.37 eV/N, respectively.…”