Electronic structure and bonding in the Y-Si-O-N quaternary crystals

The Y-Si-O-N quaternary crystals are an important part of structural ceramics. Their crystal structures are complex and not precisely determined. Little is known about their electronic structure and bonding which are indispensable for a fundamental understanding of structural ceramics. Within the equilibrium phase diagram of the SiO2-Y2O3-Si3N4 system, there are only four known crystalline phases: Y2Si3N4O3 (M-melilite), Y4Si2O7N2 (J phase or N-YAM), YSiO2N (wallastonite), and Y-10[SiO4](6)N-2 (N-apatite). With the possible exception of YSiO2N, these crystals have O/N disorder in that the exact positions of the anions cannot be uniquely determined. Using accurate ab initio total energy relaxation, the atomic positions of the lowest energy configurations in these crystals are determined. Based on the theoretically modeled structures, the electronic structure and bonding are investigated using the ab initio orthogonalized linear combination of atomic orbitals method, and are related to a variety of local cation-anion bonding configurations. These results are presented in the form of atom-resolved partial density of states, Mulliken effective charges and bond order values. Although the strong Si-N and Si-O bonding dominates in these crystals, it is also shown that Y-O and Y-N bonding are not negligible and should be a part of discussion of the overall bonding scheme in these crystals. It is concluded that Y2Si3N4O3 has the strongest crystal bonding among the four crystals. In addition, the optical properties of these crystals are calculated using the ab initio wave functions. All four crystals are insulators with optical band gaps of 3.40, 3.19, 4.40, and 3.70 eV, respectively. These results are further discussed in the context of specific bonding configurations of the cations (Si and Y) with the anions (O and N) and their implications on the metal-containing intergranular glassy films in polycrystalline Si3N4.