High resolution electron microscopy has been used to examine the nanoscale structure of the related B and I-w phases in Y and Er SiAlONs. B-phase was found to consist of nanoscale domains with a lower symmetry than the average hexagonal symmetry. The monoclinic cell of I-w can be clearly imaged by HREM and work is in progress to use these images in the determination of atomic positions in the unit cell. One consequence of the ordered structure of I-w is the possibility of forming domain boundaries, and two of the three possible types of domain boundaries have now been imaged using HREM
