Micron, 1980. Vol.: I I , pp. 247-250 Pergamon Press Ltd. Printed in Great Britain

THE VOID SUPERLAI-I'ICE IN FLUORITE

E. JOHNSON and L. T. CHADDERTON*

Physics Laboratory II, H. Co (~rsted Institute, Universitetsparken 5, DK-2100 Copenhagen ~, Denmark

*Present Address: Division of Chemical Physics, CSIRO, P.O. Box 160, Clayton, Victoria 3168, Australia

Introduction. Formation of radiolyt ic radiation damage severely l imi ts transmission electron microscopy (TEM) investigations of ionic materials, in particular alkal i halides I and alka- l ine-earth halides 2-~. On the other hand, this effect can be advantagously ut i l ized in stu- dies of radiation damage behaviour under ' in s i tu ' TEM irradiat ion conditions. Such studies have lead to the discovery of radiation induced formation of a superlattice structure in CaF 2 and SrF 2 2,5,6, which in CaF 2 was subsequently ident i f ied as a simple cubic anion void superlattice confined and orientated parallel to the f luorine sublattice of the matrixT, 8. This superlattice structure cannot be explained from current void la t t ice theories developed for metal sg, I° , and i ts formation was tentat ively suggested to be due to the Foreman mecha- nism, an i n te rs t i t i a l void screening effect originating from linear diffusion of anion inter- s t i t i a l s along <lO0> close-packed anion rows.

Results. Samples of f luor i te suitable for TEM investigations were made by crushing crystals to po-6wBer and collecting flakes on thin carbon films supported on copper grids.

The beam f lux used for irradiations was measured using a low-noise Faraday cup positioned above the fluorescent screen of the microscope. Displacement rates were estimated using electron stopping powers for fast electrons in f luor i te 12 combined with estimated an;on Frenkel pair formation eff iciencies for f luor i te 13

At low electron doses (< l dpa) the damage consists of random distr ibutions of small defects. Figure l shows the defects imaged under dynamical two-beam conditions as well as under kine- matical phase contrast conditions. The contrast is consistent with that expected from di- luted strain-free inclusions I~-16 Primary radiolyt ic radiation damage in f luor i te is con- fined to the anion sublattice 17, and the defects are accordingly interpreted as anion voids - three dimensional anion vacancy aggregates confined to the anion sublattice - or equivalently as coherent calcium particles.

Approaching an electron dose corresponding to = I dpa, the random anion void distr ibut ion gradually transforms into a regular three dimensional superlattice (Figs. 2 and 3) which re- mains stable for electron.doses at least as high as 50 dpa. The structure of the anion void la t t ice thus formed is simple cubic (Fig. 3) with axes commensurate with the axes of the matrix ? . Hence, the superlattice structure is similar and parallel to the simple cubic anion sublattice to which i t is confined.

TEM contrast calculations from a column of regulary spaced strain-free voids in a crystal, taking into account phase contrast under kinematical di f f ract ion conditions agree with the observed contrast from the anion void superlattice 16 in f luo r i te , thereby confirming the anion void nature of the defects forming the superlattice.

Most of the irradiat ion experiments on CaF 2 were performed at IOO keV electron energy - below the threshold energy for direct displacement of f luorine ions 12 - to avoid possible disturb- ing influences from direct displacement damage on the superlattice formation. At 200 keV electron energy the f luorine displacement rate due to ionization is an order of magnitude larger than the direct displacement rate, and at l MeV electron energy the two displacement rates are comparable 12. Furthermore, direct displacement of calcium ions is possible at

247

248 E. Johnson and L. T. Chadderton

I MeV electron energy. Nevertheless, experiments show that anion void la t t ice formation oc- curs for 200 keV as well as l MeV electron irradiations (Fig. 4), although the time rate of formation decreases with increasing electron energy, corresponding to the decrease in the stopping power due to electronic col l is ions 12.

Discussion. The occurence of anion voidage in f luor i te at room temperature is understandable from a consideration of the energies of migration for anion point defects. Anion vacancies (F centres) diffuse freely at room temperature whilst anion in te rs t i t i a l s (H centres) f i r s t become freely mobile above room temperature 17. This yields ideal conditions for anion void formation which are even further improved by the fact that anion in te rs t i t i a l s , when mobile, tend to form extended f luorine bubble-like defects2,3, 6.

The formation of the anion void la t t ice, on the other hand, must be provided for by the pro- perties of the f luor i te matrix. The mechanism must be able to sustain formation of the sim- ple cubic anion void la t t ice not only under conditions of radiolysis, but even under condi- tions where direct ly displaced anions as well as cations are formed. An analysis of current void la t t ice theories, based on mutual void-void interactions, and hitherto applied to void latt ices in metals I° predicts that a simple cubic superlattice is never stable. Only one superlattice theory applicable to coherent inclusions 19 and therefore appropriate to the anion void lat t ice in f luor i te , predicts s tab i l i t y of a simple cubic superlattice. When ap- plied to CaF 2, however, the theory fa i ls due to a wrong sign in the elastic anisotropy Ic. The only model which can sat is factor i ly explain formation of a simple cubic superlattice in f luor i te is given by Foreman 11 Foreman's model operates through an i n te rs t i t i a l void screening effect originating from linear diffusion of i n te rs t i t i a l s along close-packed direc- tions. When applied to f luor i te , i t implies the existence of anion in te rs t i t i a l s diffusing l inear ly along close-packed <lO0> anion rows. However, the anion i n te rs t i t i a l (H centre) which consists of an F~ molecule is not aligned along <lO0> 2°. On the other hand, the V K centre in f luor i te is aligned along <]00> and diffuses l inear ly along <lO0> at room tempera- ture 2I. The V K centre is a self-tra~ped hole which on recombining with an electron may decay via a self-trapped exciton into an anion Frenkel pair. In this respect the V K centre can be regarded as precursor of the anion i n t e r s t i t i a l . I t may therefore provide the defect neces- sary for operation of the Foreman mechanism. Preferential injection of anion in te rs t i t i a l s into the anion voids necessary for the screening effect is the thought to occur when the l in - early diffusing V K centres decay close to the boundaries of existing anion voids. V K centre decay further away from the anion voids w i l l , as the self-trapped exciton is not aligned along <100>22, 23, lead to formation of mobile anion vacancies and immobile anion in te rs t i - t ia ls . The anion voids in CaF 2 and SrF~ can be regarded as coherent matallic inclusions with

• Z

very small misf i t (2 and 4% respectlvely) 12 and an associated negligible strain f ie ld which allows the V K centre to approach to the anion void boundaries. In BaF 2 and in al l the alkal i halides the metallic inclusions are incoherent, and the associated larger strain f ields pre- vent the V~ centres in BaF 2 and the l inear ly diffusing <llO> anion in te rs t i t i a l s in the alka- l i halides-from reaching the anion voids, in agreement with the non-observation of anion void latt ices in these materials. Direct displacement damage of anions and cations leads to dis- persed distr ibutions of anion and cation point defects, of which only the anion vacancies are mobile at room temperature Is. This is in agreement with observations that occurrence of di- rect displacement damage does not restr ic t anion void lat t ice formation. Radiolytic irradia- tions of f luor i te above room temperature, where anion in te rs t i t i a l s are freely mobile, leads, however, to formation of random distr ibutions of {lO0} faceted anion voids so large that they can be regarded as incoherent calcium inclusions 2". Therefore, the existence of a large ran- dom in te rs t i t i a l f lux in the crystals seems to upset the delicate anion in te rs t i t i a l void screening effect of the Foreman mechanism to the extent that anion void latt ices no longer form, whilst anion voidage s t i l l occurs due to preferential formation of f luorine bubble-like defects.

Conclusion. The superlattice formed in f luor i te under radiolyt ic irradiation conditions in ~ n from i ts contrast behaviour be identi f ied as an anion void superlattice. Its structure is simple cubic with axes commensurate with the axes of the matrix. Its formation is ascribed to the operation of the Foreman mechanism 11 and i t is suggested that the V K cen- tre difussing l inear ly along the close-packed <lO0> anion rows, plays the role of the re- quired anion i n t e r s t i t i a l .

Acknowledgement. The authors grateful ly acknowledge Professor Sir Peter Hirsch, F.R.S. for permission to use the high voltage microscope at Oxford. The work was supported by grants from The Danish Natural Sciences Research Counsil.

The Void Supedattice in Fluorite 249

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23. M.N. Kabler & R.T. Williams, Vacancy-interstitial pair production via electron-hole recombination in halide crystals, Phys. ffev. B, 18, 1948 (]978).

24. E. Johnson, {100} faceted anion voids in electron irradiated f luor i te, Rad. Effects Lett.. 43, 43 (1979).

250 E. Johnson and L. T. Chadderton

Fi 9. ] Contrast from anion voids. a) dynamical d i f f rac t i on conditions, b) underfocus phase contrast, c) overfocus phase contrast.

Fi 9 . 2 { I l l } anion void l a t t i ce y ie ld ing superlat t ice d i f f rac t ion .

2 ° °

SC BCC FCC

Fi 9 . 3 { I f2} anion void l a t t i c e and projections simple cubic, body centred cubic and face centred cubic l~ t t i ces .

o_

{lOO} anion void l a t t i c e in f l uo r i t e i r radiated with l MeV electrons in the HVEM.