Determination and refinement of the crystal structure of M2SiAlO5N “B-phase” (M=Y, Er, Yb)

The crystal structure of the compound M2SiAlO5N “B-phase” (with M=Y, Yb, Er) is determined by Rietveld analysis of X-ray powder diffraction patterns. The pseudo α-wollastonite derived structure, often proposed in the literature [D.P. Thompson, The crystal chemistry of nitrogen ceramics, Materials Science Forum 47 (1989) 21–42], is used as a model for simulation of an X-ray diffraction pattern. The simulated pattern obtained with this model exhibits several peaks that are not observed on the experimental pattern. Moreover, when refining this model by Rietveld analysis, the final atom co-ordinates are strongly shifted with respect to their initial values and lead to aberrant bond lengths. A new model of the structure of B-phase is proposed: after refinement, the final reliability factors show that this new model is in very good agreement with experiments. According to this new model, the alternative layers of yttrium cations and (Si,Al)(O,N)4 tetrahedra proposed in the pseudo α-wollastonite model is confirmed but the tetrahedra do not form rings as was initially suggested, but instead are randomly linked to each other.