ORDERING OF NITROGEN AND OXYGEN IN NITROGEN-CONTAINING MELILITES Y2SI3O3N4 AND ND2SI2.5AL0.5O3.5N3.5

Ordering of N and O atoms in nitrogen-containing melilite Y2Si3O3N4 (Y -M) was investigated by Rietveld refinement in the space group P42(1)m using neutron powder diffraction data. The results show that, contrar y 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, whi le the remaining N atoms in the unit cell are disordered at 8f (in Wyc koff notation) sites. The similar ordering of N and O atoms in Nd2Si2. 5Al0.5O3.5N3.5 (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 pe r unit cell were eight and seven in Y-M and Nd-M' respectively. The pr esent results give a more detailed picture of O,N ordering in these st ructures than was provided by the previous work of Wang and Werner bas ed on X-ray data alone.