Defect aggregation in anion-excess fluorites II. Clusters containing more than two impurity atoms

A theoretical study has been made of defect clustering in fluorite crystals. The emphasis is on the aggregation processes that can occur beyond the dimer stage which was discussed previously (J. Corish, C. R. A. Catlow, P. W. M. Jacobs, and S. H. Ong,Phys. Rev. B25, 6425 (1982)). The systems studied comprise CaF2, SrF2, and BaF2 doped withMF3whereM =Y, Lu, Yb, Er, Tb, Gd, Eu, or La, and also SrCl2 doped with GdCl3, LaCl3, or PrCl3. Two tetramers, one containing a single anion vacancy and two 〈111〉 relaxed F− ions and the other with a planar arrangement of the dopant ions, are stable for certain systems while a trimer and hexamers, with an octahedral arrangement of substitutionalM3+, also show considerable stability. A notable feature is the increase in the relative stability of larger complexes as the ratio of the radius of the dopant ion to that of the host lattice cation is decreased. The results are discussed in relation to recent EXAFS and neutron scattering work on these systems and should provide a useful guide in future experimental studies.