Anion voidage and the void superlattice in electron irradiated Caf₂

Abstract

Anion voidage and anion void lattice formation in electron irradiated (100 keV–1 MeV) fluorite (CaF₂) has been investigated by in situ transmission electron microscopy. At low fluences (≲1 dpa) the damage consists of randomly distributed anion voids and large bubble-like defects associated with fluorine gas. At higher fluences (≳ 1 dpa) the anion voids form a three-dimensional ordered superlattice with a simple cubic structure, whose axes are commensurate with those of the matrix. The superlattice has a unit cell parameter A₀ in the range 15–30 nm and a ratio A₀/R(superlattice parameter/anion void radius) in the range 3–6. The fully ordered superlattice “crystallizes” from randomly distributed anion voids by an ordering process in which locally ordered areas grow together after a fashion similar to that of epitaxy, leaving distorted superlattice regions between individual, fully ordered blocks.

The basic radiation damage events are radiolytic and confined to the anion sublattice. Damage processes and defect behaviour on the anion sublattice, passively stabilized by the unaffected cation sublattice, leading to anion voidage and anion void lattice formation, can hence be considered analogous to voidage processes in metals. This close correlation between anion voidage in fluorite and voidage in metals is further emphasized by the observation of large faceted anion voids in fluorite irradiated above 330 K.

Theories based on elastic void-void interactions normally used to explain void lattice stabilization in metals are inapplicable to the simple cubic anion void lattice in fluorite, the formation of which can only be explained in terms of a mechanism due to Foreman. In this model crowdion interstitials having an excess of mobility along close-packed rows give rise to a superstructure where the voids are also arranged along close-packed rows, each void shielding its neighbours from interstitial capture. It is suggested that the anisotropic interstitial flux in fluorite is provided by V_K centres, which structurally are F₂ ⁻ molecules extended along the close-packed anion rows. It is envisaged that the linearly diffusing V_K centres in fluorite are capable of approaching very close to the anion voids before decaying into anion Frenkel pairs with enhanced probability of injection of anion interstitials into the anion voids, thus providing the interstitial type defect required by Foreman's model.