First Principles Study of Negative Thermal Expansion (NTE) in Manganese Nitride materials

We report ab initio study of the electronic, mechanical and structural properties for the negative thermal expansion (NTE) application of antiperovskites manganese nitrides materials Mn3XN (X=Al, Ga, Zn). Using first principle calculations based on generalized gradient approximation (GGA), the electronic, structural, and mechanical properties of the Mn3XN (X = Al, Ga, Zn) phases were investigated. The optimized lattice parameters give the first reference to the upcoming theoretical and experimental studies. THERMO_PW as a post-processing code was used for the computation of elastic constants that were used to derive bulk modulus (B), shear modulus (G), Young's modulus (E), Poisson's ratio (õ), and hardness. The calculated elastic constants are in excellent agreement with the available data. Moreover, obtained elastic constants revealed that both the Mn3ZnN and Mn3GaN phases are brittle. The band structure and density of states analysis showed that these phases are electrical conductors, having strong directional bonding existing near the Fermi level between Mn -N atoms due to Mn-d and N-p hybridization. The Mn3GaN compound has the highest Debye temperature (452.8K), while Mn3ZnN has Debye temperature of (283.4K) indicating that they are thermodynamically and dynamically stable and can be synthesized experimentally. Mn3AlN compound has Debye temperature (0.000K) indicating the system is unstable for compressional deformation.