The incorporation of large Ce ions into the interstitial sites in an alpha-
( Si-Al-O- N) crystal structure is found to have produced a high density of
structural defects. The defects are of planar nature and connect to form d
omain boundaries. Analysis via transmission electron microscopy has reveale
d that only a single lattice translation of 1/3 [10 (1) over bar0] type is
involved in the formation of domains which are enveloped by some specific f
aces in this strongly bonded covalent compound, that is (0001), {10 (1) ove
r bar1} and the common surface with the matrix crystal on {10 (1) over bar0
}. Development of the domain-boundary structures is discussed via the appli
cation of image simulation based on different structural models of defects.