O/N Ordering in Y2Si3O3N4 with the Melilite-type Structure from First-Principles Calculations

First-principles calcns. based on d.-functional theory in the pseudopotential approach were performed for the energetics and crystal structure of Y2Si3O3N4 with the melilite-type structure. The calcns. show the following ordering of the O/N atoms in the crystal: N atoms fully occupy 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 good agreement with the exptl. results obtained with neutron diffraction. The calcns. show that there is a preferential distribution 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 crystallog. Y site. All the N ions exhibit similar electronic structure, in contrast to the O ions. However, there are slightly more electrons (.apprx.0.1 electrons per ion) for N at the 2c site than those at the 8f site, while the Si atoms have almost the same charge distribution. [on SciFinder (R)]