The formation of domain boundaries in the I-w phase of Y-Si-Al-O-N and Er-Si-Al-O-N

It has been shown that domain boundaries may be formed in the monoclinic I- w phase of Y-Si-Al-O-N or Er-Si-Al-O-N as a consequence of its ordered stru cture. Three classes of orientational domain boundaries have been identifie d: the (001) twin, a 60 degrees rotation about [010], and a 60 degrees rota tion about [010] followed by (001) twinning. High-resolution transmission e lectron microscopy images are shown of all these classes of orientational d omain boundaries. It is shown that they arise from two different sources. F irstly, most twin boundaries seem to have arisen from growth accidents duri ng crystallization. In contrast with this, the rotational domain boundaries and disordered boundaries between twin-related domains seem to have arisen from crystal coalescence during the glass crystallization heat treatment. Evidence is presented that suggests that adjacent crystals rotate during th e heat treatment to a similar crystallographic orientation of the hexagonal Y or Er cation sublattice, followed by coalescence. This could then lead t o the formation of these domain boundaries if the orientations of the monoc linic unit cell with respect to the hexagonal sublattice are different in t he two crystals. Finally, it is suggested that van der Waals forces acting across the viscous glass may be responsible for the alignment of adjacent c rystals during heat treatment.