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

Abstract

First-principles calculations based on density-functional theory in the pseudopotential approach have been performed for the energetics and crystal structure of Y2Si3O3N4 with the melilite-type structure. The calculations 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 experimental results obtained with neutron diffraction. In addition, the calculations 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 crystallographic Y site. All the nitrogen ions exhibit similar electronic structure, in contrast 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 site, while the silicon atoms have almost the same charge distribution.