We report a study of the structures and energetics of F- ions in the o
ctadecasil structure. Our calculations find high stability for fluorid
e ions within the double 4-ring (D4R) of this structure; the structure
is, however, stable in the absence of these species. We propose that
fluoride ions are incorporated during the synthesis of the material an
d that the high activation energy for their migration out of the D4R r
esults in their retention in the structure after calcination. (C) Else
vier Science Inc. 1997