Electronic structure, lattice constant, optical and mechanical properties for NaCl-structured Ti-Al-N by density functional theory

The electronic structure, lattice constant, optical and mechanical properties of NaCl-structured Ti-Al-N compounds were studied by density functional theory with the plane wave pseudopotential method. The calculation results indicated that in a NaCl-structured TiN containing dilute solvent Al, the calculated forming energy of the paired defects of substitute Al for Ti+N vacancy is −1.7325 eV per pair of defects, while that of the paired defects of substitute Al for N+Ti vacancy is −10.62 eV per pair of defects. In other words, the energetically preferred occupation sites of Al atoms are Ti sites. In relation to the reference system of TiN, the substitute Al atoms for Ti increased the charge transfer to N atoms, while Al for N decreased it. On the other hand, the substitute Al for Ti exhibits cationic while those for N exhibit anionic. The addition of Al into the cubic TiN resulted in much universe feature of the refractory index in the visible light regime. The calculated reflectivity indicates that TiN is colorful while (Ti3Al)N4 is gray. The smaller lattice constant, lower elastic properties (the elastic tensor, bulk modulus and Young modulus) and higher hardness of NaCl-structure (Ti3Al)N4, than those of TiN, were discussed in terms of effects of the substitute Al for Ti on its electronic structure.