Ordering of nitrogen and oxygen in nitrogen-containing melilites Y 2Si 3O 3N 4 and Nd 2Si 2.5Al 0.5O 3.5N 3.5

Abstract

Ordering of N and O atoms in nitrogen-containing melilite Y₂Si₃O₃N₄ (Y-M) was investigated by Rietveld refinement in the space group P2₁m using neutron powder diffraction data. The results show that, contrary to previous work using X-ray data, the occupancies by N and O atoms can be refined to show that about 1.7 N and 0.3 O atoms occupy 2c (in Wyckoff notation, space group no. 113) sites in the Y-M unit cell, while the remaining N atoms in the unit cell are disordered at 8f (in Wyckoff notation) sites. The similar ordering of N and O atoms in Nd₂Si_2.5Al_0.5O_3.5N_3.5 (Nd-M′) was also confirmed, by refinement from neutron data, which means that similar distributions of N and O atoms occurred at 2c sites in spite of the fact that the total numbers of N atoms per unit cell were eight and seven in Y-M and Nd-M′ respectively. The present results give a more detailed picture of O,N ordering in these structures than was provided by the previous work of Wang and Werner based on X-ray data alone.