Chemical bonding analysis in Ti1-x-yAlxTayN solid solutions

A comprehensive study of the evolution of electronic structure and chemical bonding in disordered Ti1-xAlxN and Ti1-x-yAlxTayN systems was performed by means of ab initio density functional theory calculations using crystal orbital Hamilton population technique. Progressive changes in the character of interatomic chemical bonding were revealed when sequentially alloyed TiN with Al and Ta. Alloying TiN with Al leads to the change in the Ti=N bonding character from ionic to covalent, whereas Al=N bonds being strongly ionic. The following alloying of Ti1-xAlxN solid solutions with Ta results in a significant reduction of the ionicity of the Al-N bonds, while retaining the covalency of the Ti-N bonds. In addition, alloying with Ta introduces metallic character of chemical bonding in Ti1-x-yAlxTayN, with the degree of metallicity increasing with growing Ta concentration. The gain in metallicity was found to be provided not only by Ta-Ta bonds, which make the main contribution, but also by Ta-N bonds, which have covalent-metallic character. A strong dependence of bonding energies in Ti1-xAlxN and Ti1-x-yAlxTayN on local atomic surrounding was found.