Ordering of oxygen and nitrogen in J-phase Lu₄Si₂O₇N₂

The crystal structure of cuspidine-type Lu₄Si₂O₇N₂, the so-called J-phase, has been refined by Rietveld analysis from time-of-flight neutron powder diffraction data in order to confirm the ordering of oxygen and nitrogen atoms. This compound crystallizes in a monoclinic cell, space group P2₁/c (No. 14-1) with a = 7.4243(1), b = 10.2728(1), c = 10.6628(1) Å, and β= 109.773(1)°. The diffraction peaks are also indexable with monoclinic lattice parameters of a = 7.4243(1), b = 10.2728(1), c = 10.7356(1) Å, and β = 110.828(1)°, space group P2₁/n (No. 14-2). The nitrogen atoms in Lu₄Si₂O₇N₂ are connected to Si atoms and form Si₂(O,N)₇ silicon oxynitride unit of composition Si₂O₅N₂. One N atom occupies the bridging site between Si atoms, and the other N atom is statistically situated at the terminal sites. It is estimated that this unit comprises SiO₃N and SiO₂N₂ tetrahedra. The four Lu sites correspond to 6-fold, 7-fold (x2) and 8-fold coordination with Lu-(O,N) interatomic distances of less than 2.9 Å.