Rietveld refinement of the crystal structures of the yttrium silicon oxynitrides Y2Si3N4O3 (N-melilite) and Y4Si2O7N2 (J-phase)

Rietveld crystal structure refinements have been carried out on the X-ray powder patterns of synthetic N-melilite, Y2Si3N4O3, and J-phase, Y4Si2O7N2. The melilite structure of Y2Si3N4O3 is confirmed, in which sheets containing Si2O7 double tetrahedra are linked by Y polyhedra in which two of the eight Y-O contacts are significantly longer (2.78 Angstrom). The present data were unable to furnish unambiguous details of O/N ordering in either N-melilite or J-phase. The structure of J-phase contains either Si2O5N2 or Si2O6N ditetrahedral units containing N in position and arranged lengthwise along the a-axis. These units are linked by Y polyhedra. Of the four distinguishable Y sites, three have seven contacts with O and N atoms within 2.8 Angstrom. One of these sites also has one longer O/N contact at 2.92 Angstrom. The fourth Y site is in 6-fold coordination.