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 structure. Three classes of orientational domain boundaries have been identified: the (001) twin, a 60° rotation about [010], and a 60° rotation about [010] followed by (001) twinning. High-resolution transmission electron microscopy images are shown of all these classes of orientational domain boundaries. It is shown that they arise from two different sources. Firstly, most twin boundaries seem to have arisen from growth accidents during 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 the heat treatment to a similar crystallographic orientation of the hexagonal Y or Er cation sublattice, followed by coalescence. This could then lead to the formation of these domain boundaries if the orientations of the monoclinic unit cell with respect to the hexagonal sublattice are different in the two crystals. Finally, it is suggested that van der Waals forces acting across the viscous glass may be responsible for the alignment of adjacent crystals during heat treatment.