Metal to semiconductor transition and phase stability of Ti1-xMgxNy alloys investigated by first-principles calculations

Titanium nitride based materials are applied in several technological applications owing to their stability at high temperatures, mechanical and optical properties, as well as good electrical conductivity. Here, I use theoretical first-principles methods to investigate the possibilities for a semiconducting state as well as the phase stability of Ti1-x MgxNy ternary alloys and compounds. I demonstrate that B1 (NaCl) solid solutions of Ti1-xMgxN with x <= 0.5 are thermodynamically stable with respect to all previously reported phases in the Ti-Mg-N system. For the composition x = 0.5, an ordered TiMgN2 phase with an L1(1)-type order of Ti and Mg on the metal sublattice is predicted to be the configurational ground state at low temperatures. Other ordered phases present in neighboring materials systems are considered but are found to be unstable. The electronic origin of the stability of the B1 structure solid solutions is identified. A metal to semiconductor transition is observed as the Mg content is increased to x = 0.5. TiMgN2 as well as disordered Ti0.5Mg0.5N solid solution are investigated with hybrid functional calculations and predicted to be semiconductors with band gaps of 1.1 eV and around 1.3 eV, respectively, the latter depending on the details of the Ti and Mg configuration.