Electronic structure of Ba-Si-N compounds with different Ba/Si ratios from first principles calculations

Ternary barium silicon nitrides have been reported with intriguing crystal chemistry. First-principles calculations were applied to study the electronic properties of the ternary barium silicon nitrides, as well as those of the recently discovered silicon nitride with spinel structure (ã-Si3N4). The calculations show that the Ba/Si ratio, i.e. the degree of cross-linking between the SiN4 tetrahedra, has a strong influence on the electronic structure. The local electronic stucture of the nitrogen atoms is influences strongly by its coordination number with Si (NSix), as well as the way the SiN4 tetrahedra are connected (corner- vs. edge-sharing). The valence band is composed of predominantly the N 2p states. The calculations also show that the top of the valence band consists mainly of the 2p states of the N atoms with less Si neighbours. With the exceptions of ã-Si3N4 in which also SiN6 coordination is present, the calculated energy gap of the remaining compounds becomes larger with increasing degree of cross-linking between the SiN4 tetrahedra, in agreement with experimental results.