Time-of-flight Rietveld neutron structure refinement and quantum chemistry study of Y-α-sialon

A Y-α-sialon was prepared using a specially designed temperature regime supporting formation of α-sialon and the amount of non-crystalline phase was remarkably reduced by a chemical treatment. Chemical composition of the sample was determined by EDX analysis. Crystal structure of Y-α-sialon was refined using high quality time-of-flight neutron powder diffraction data taken with the General Purpose Powder Diffractometer (GPPD) at the IPNS, Argonne National Laboratory. Unrestrained structure refinement was done in the P31c space group. The list of simultaneously refined parameters included profile parameters, atomic coordinates, individual isotropic temperature parameters and the occupancy parameter of Y. The contents of Y in the cell refined to 0.38(1) and calculated Si–N distances in tetrahedra varied from 1.716(3) to 1.773(3) Å, but their distributions in the respective tetrahedra differ as in the parent structure of α-Si₃N₄. Anisotropic temperature parameters refinement, as well as constrained refinements of the occupancy parameters of Si–Al and N–O pairs, were also attempted, but they all ended with unphysical values. Variation of the background was analyzed using RDF formalism. Three peaks centered around 1.7 Å (with a shoulder at 2.2 Å), 2.8 Å and 3.5 Å proved that the composition of amorphous phase was close to that of crystalline. Ab initio cluster calculations performed at the B3LYP/6-31G** level of theory excluded configuration with O atoms bonded to three Si atoms and proved that the most probable is the configuration where an O atom is shared by three Al atoms.