Insights into the physical properties of a new 211 MAX phase Nb2CuC

A systematic density functional theory study with two functionals –generalized gradient approximation (GGA) and local density approximation (LDA)–is carried out to explore the structural, electronic, elastic, thermal, vibrational and optical properties of a new 211 MAX phase Nb2CuC. To facilitate comparison we also study Nb2AlC, the precursor of Nb2CuC. The calculated band structures reveal the metallic conductivity of both compounds. The replacement of Al with Cu modifies the band profiles of Nb2CuC and consequently leads to its improved physical properties. Considering the position of the Fermi level on the total density of states (DOS), the new compound Nb2CuC is structurally less stable than Nb2AlC. The total DOS at the Fermi level obtained with GGA is slightly larger than those obtained with LDA. The Nb–C and Nb–A (A = Cu/Al) are covalent bonds, and Nb–Nb bonds lead to antibonding states in both MAX phases. The charge transfer among constituent atoms indicates some ionic character in the chemical bonds of Nb2CuC and Nb2AlC. Both MAX phases are mechanically and dynamically stable. The Nb2CuC is ductile and consequently damage tolerant, whereas Nb2AlC is brittle. However, Nb2CuC is relatively soft and machinable. In most cases Nb2CuC is more elastically anisotropic than Nb2AlC, and Nb2CuC is expected to be a promising thermal barrier coating material. We propose that Nb2CuC is a better coating material for preventing solar heating than Nb2AlC, and Nb2CuC is expected to be superconductive because its Fermi surface has a nesting nature.