O/N ordering in Y2Si3O3N4 with the melilite-type structure from first-principles calculations

First-principles calculations based on density-functional theory in the pse udopotential approach have been performed for the energetics and crystal st ructure of Y2Si3O3N4 With the melilite-type structure. The calculations sho w the following ordering of the O/N atoms in the crystal: N atoms fully occ upy the bridging site (2c) and O atoms fully occupy the terminal site (4e) with 2 O and 6 N atoms at the bridging 8f site. These conclusions are in go od agreement with the experimental results obtained with neutron diffractio n. In addition, the calculations show that; there is a preferential distrib ution of the O and N atoms at the 8f site, resulting in two different local coordinations of Y, as compared to only a single averaged crystallographic Y site. All the nitrogen ions exhibit similar electronic structure, in con trast to the oxygen ions. However, there are slightly more electrons (about 0.1 electrons per ion) for nitrogen at the 2c site than those at the 8f si te, while the silicon atoms have almost the same charge distribution.