Preliminary study of some “pyro”-silico-nitrides

Two oxynitrides of the LnSiDN system were studied by IR spectroscopy : Nd₂Si₃O₃N₄, with a melilite-like structure and Nd₄Si₂O₇N₂ with a cuspidine-like structure. The first compound belongs to the space group D³_2d(P4̄2₁m) with two formula units per unit cell. There are 10 vibrations of B₂ species and 18 of E species active in the IR. A preliminary study by neutron diffraction displayed that the oxygen and nitrogen atoms are perfectly ordered, and that two silicon atoms are bridged by an oxygen. In the present work, we compare the (SiON₂)₂O IR vibrations with those of silicon oxynitride previously studied and |Si₂O₇|^6? ion in akermanite. The space group of cuspidine is C⁵_2h (P2₁/c with 4 formula units per unit cell. So, 21 vibrations of A_u species and 21 of B_u species are active in IR absorption for Nd₄Si₂O₇N₂. The absorption bands are not as sharp as the bands in Nd₂Si₃O₃N₄. This fact might be explained by a disordered structure as the nitrogen position has not been exactly determined.