Effects of transition metals on physical properties of M2BC (M = V, Nb, Mo and Ta): A DFT calculation

Abstract

In the present study, the effects of transition metals on structural, electronic, elastic, optical and thermodynamic properties of M2BC (M = V, Nb, Mo and Ta) have been investigated using the density functional theory (DFT) based first-principles method. The electronic band structures along with Fermi surface, elastic anisotropy, Vickers hardness, analysis of Mulliken populations, optical and thermodynamic properties are studied for the first time. The optimized unit cell parameters are compared with available theoretical and experimental results and a reasonable agreement is recorded. The mechanical stability of these compounds is confirmed by the calculations of single crystal elastic constants using the Born criteria. The compounds herein exhibit metallic conductivity where major contribution comes from the d-orbital electrons. The total density of states at EF are found to be 9.15, 6.77, 6.37 and 5.83 states/eV/unit cells for M2BC (M = V, Nb, Mo and Ta) compounds, respectively. The hardness values of 10.71, 12.44, 8.52 and 16.80 GPa are noted for the M2BC (M = V, Nb, Mo and Ta) compounds, respectively. The value of bulk modulus, B is found to increase in the sequence of B (V2BC) < B (Nb2BC) < B (Ta2BC) < B (Mo2BC), indicating Mo2BC is highly stiff among the compounds under study. The Mo2BC and Ta2BC compounds might be considered as potential candidates for cutting and forming tools due to the moderately ductile and highly stiff behavior compared to other benchmark hard coating materials such as TiN, TiAlN, Ti0.5Al0.5N and c-BN. Ta2BC compound could also be a promising thermal barrier coating (TBC) material due to its low thermal conductivity, Debye temperature and damage tolerant behavior. Different anisotropy calculations (shear anisotropic factors, percentage anisotropy factors and universal anisotropic index) confirm that the compounds are structurally anisotropic in nature. The bond between C-V/Nb/Mo/Ta shows that the degree of covalency is higher compared to B-V/Nb/Mo/Ta bond. Various optical functions (such as dielectric constants, refractive index, photo-conductivity, absorption, loss function and reflectivity) are calculated and discussed using established formalisms. The amount of reflectivity is always more than ∼50% with no significant change in the near infrared, visible and near ultraviolet region (up to ∼ 6 eV), this result makes the compounds promising coating materials to diminish solar heating.