Ordering of nitrogen and oxygen in nitrogen-containing melilites Y2Si3O3N4 and Nd2Si2.5Al0.5O3.5N3.5

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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 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 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.

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