The sialons MLn[Si4-xAlxOxN7-x] with M = Eu, Sr, Ba and Ln = Ho-Yb -: Twelve substitution variants with the MYb[Si4N7] structure type

The oxonitridoalumosilicates (so-called sialons) MLn[Si4-xAlxOxN7-x] with M = Eu, Sr, Ba and Ln = Ho, Er, Tm, Yb were obtained by the reaction of the respective lanthanoid metal, the alkaline earth carbonates or europium carbonate, resp., AlN, "Si(NH)(2)" and MCl2 as a flux in a radiofrequency furnace at temperatures around 2100 degrees C. The compounds MLn[Si4-xAlxOxN7-x] are relevant for the investigation of substitutional effects on the materials properties due to their ability of tolerating a comparatively large phase width up to x approximate to 2.0(5). The crystal structures of the twelve compounds were refined from X-ray single crystal data and X-ray powder data and are found to be isotypic to the MYb[Si4N7] structure type. The compounds crystallize in space group P6(3)mc (no. 186, hexagonal) and are made up of chains of so-called starlike units [N-[4](SiN3)(4)] or [N-[4]]((Si,Al)-(O,N)(3))(4)], respectively. These units are formed by four (Si,Al)(N/O)(4) tetrahedra sharing a common central nitrogen atom. The structure refinement was performed utilizing an O/N-distribution model according to Paulings rules, i.e. nitrogen was positioned on the fourfold bridging site and nitrogen and oxygen were distributed equally on both of the two-fold bridging sites, resulting in charge neutrality of the compound. The Si and Al atoms were distributed equally on their two crystallographic sites, referring to their elemental proportion in the compound, due to being poorly distinguishable by X-ray methods. The chemical compositions of the compounds were derived from electron probe micro analyses (EPMA).