E. JOHNSON et al.: Radiolytic Radiation Damage of Sodalite 585

phys. stat. sol. (a) 49, 585 (1978)

Subject classification: 11; 10.2; 22.8

Physics Laboratory 11, II . C . Qrsted Institute, University of Copenhagen1)

Radiolytic Radiation Damage of Sodalite BY

E. SOHNSON, J. PERRER~), and L. T. CHADDERTON

“In sitn” electron irradiation of sodalite in the transmission electron microscope (TEM) results in forniation of radiolytic damage in the form of chlorine ion voids and chlorine gas bubbles. The chlorine ion voids are identical with metallic sodium particles. The damaging process is discussed in terms of point defects and radiation induced voidage. The latter is confined to the chlorine ion sub lattice.

,,In situ“-Bestrahlung von Sodalith im Transmissions-Elelrtronenmikroskop erzeugt radio- lytisch Clilorionen-Locher und Chlorgasblasen. Die Chlorionen-Locher sind identisch mit Partikeln von metallischem Katrium. Der Schadigungsprozel3 wird diskutiert auf der Grundlage von Gitter- leerst’ellen und strahlungsinduzierter Locherbildung, die auf das Chlorionen-Untergitter be- schriinkt ist.

I . Introduction The aggregation of F centres in a range of ionic crystals [ l ] following exposure to

ionizing radiation is well dociiniented. Using transniission electron microscopy (TEM) such aggrcgat’es were first observed to form during high flrience “in situ” electron irradiat.ions of PbI, [ 2 ] and lat,er in alkali halides [:3, 41 and alkali earth fluorides [ 5 to 71. I n these inaterials t’he process of niet’al colloid forniat’ion is best discussed in terms of anion vacancy clustering [8] . For exaniple, the siniple cubic inet,al colloid snperlatt’ice observed in CaF, [9] can as well be regarded as an anion void superlat’t’ice

Siniilar ionizat,ion damage effects are known to occur in struct.urally milch more complicated minerals. Fink coloration of sodalite ( D;a,Al,Si,O,,Cl,) exposed to ionizing radiation has been shown to originate at radiolytically generated P centres - elec- trons trapped on chlorine vacancies [ 12, 131. Bleaching with visible light retiloves the coloration, whilst repeated colour-bleach cycling gives sodalite a blue coloration showing increased persistence against bleaching [14]. This has recently been attrihiitcd t o the foriiiat,ion of alkali metal colloids [15].

T n this note we rcport on electron irradiation induced formation of defects in soda- lite during “in sitir” irradiation in t’he TEM. Sinall defects which appear to be metal rolloids are chlorine ion voids. A damaging scheme is proposed which is closely ana- logous to that for anion void forniation in ionic crystals, and to that for void form a t’ ion in metals.

2. Experimental 1 he naturally occurring sodalite cryst’als used in this investigation originate in the

agyait ic Iliniaiissaq intrusion forming part of the Garder magmatic province of southern (ireenland. The formation has been discussed by Engell [ 161. The yellowish- green crystals have been described by B0ggild [17]. Sodalite is a framework silicate with a cwhic structure and hclonging to the P45n space group [18]. Positions of atoins in the unit cell and X-ray struct~iire factors have been determined [18]. X-ray diffrac-

[lo, 111.

,,

I ) Vniversitetsparkcn 5, DK-2100 Copenhagen, Denmark. ?) Ciiiest scientist from the University of Barcelona, Spain.

586 E. JOHNSON, J. F E R X E R , and I,. T. CHADDERTOX

tion iiieasurenients of the sodalite used in this investigation yielded a lattice paranieter of a, = (0.887 + 0.001) nni.

The chemical composition of the sodalite crystals in wty’, was deteniiined froni niicroprobe analysis, yielding: SiO, = 37.8 0.7; AI,O, = 30.6 1 0.3; Sa,O =

Samples for electron niicroscopy were prepared by crushing followed by sediinenta- tion of the powder in an alcohol-water suspension onto carbon-coated copper grids. This yielded crystal flakes with edges thin enough for transmission electron niicro- scopy.

The “in sita” radiation dairiage experiments were carried out in a 3EOL lO0U transnlission electron microscope operated a t 100 kV. The electron flux was iiionitoretl using a high sensitivity Faraday cup placed above the fluorescent screen of the niicroscope (Mundt, unpublished). Prior to irradiation, suitable flakes were exaiiiined using the lowest possible beam flux adequate for observation (s 1021 electrons/ni2 s). Irradiations were carried out using heain fluxes larger than electron/ni2 s, which is roughly equivalent to an energy deposition of the order of 1 O 1 O Had/s. Radiation damage was investigated and photographed under dynaniical diffraction conditions, and also under defocusing phase contrast conditions with the crystal oriented for kinematical diffraction. The beam flux used for exnniination and photography was kept less than loz1 electrons/ni2 s.

== 23.7 0.1; K,O = 0.03; CaO = 0.02; FeO = 0.15; C1 = 6.15 3- 0.1.

3. Results and Discussion “Unirradiated” crystals - crystals irradiated to fluences less than = loz2 elec-

trons/ni2 - appear homogeneous and show only the presence of conchoidal fracture

a b Fig. 1. Sodalite crystal irradiated with 100 keV electrons to a dose of 2 x loz3 electrons/ni2. Kinematical phase contrast imaging: a) overfocus, b) underfocus. Two types of objects are distin-

guished: (A) dilute inclusions, (B) extended bubble-like defects

Kadio1yt.k Radiation Damage of Sodalite 587

a b

Fig. 2. Sodalite crystal irradiated with 100 keV electrons to a dose of lo2' electrons/m2. a ) Strain contrast, S,,, = 0; b) underfocus phase contrast, S,,, S O . The dilute inclusions in b) are a t the

positions of the loop-like defects in a)

lines formed during specimen preparation. This kind of crystal perfection - an ahscence of grown-in defects and initially excellent Kikuchi patterns - is in accord with the general classification of sodalite as a n antistress mineral [lU], and with its formation under low stress conditions [It;].

Tno types of radiation induced defect are shown in Fig. 1 in a crystal irradiated to a fluence of z 5 x loz4 electrons/m2. Both niicrographs are taken under senii- kinenlatical diffraction conditions using phase contrast imaging. The dot-like defects labelled A appear in underfocus as bright spots surrounded by a dark ring (Fig. 1 b). In overfocus the contrast is reversed to a dark spot Surrounded by a brighter ring (Fig. l a ) . Such phase contrast reversal indicates that the defects can best he inter- preted as dilute inclusions surronnded by a comparatively weak strain field [ d o ] . Ilcfects labelled I3 appear extended and are revealed as sets of w&vy lines or ringq, qualitatively similar to the bubble-like aggregates observed in electron irradiated CaF2 110, 111.

Fig. 2 shows micrographs of a sodalite crystal irradiated to a fluence of = lo2' elec- trons/ni2. Fig. 2a is taken with the crystal tilted into the exact Bragg condition for the (4401 reflection. Under such diffraction conditions, where strain contrast IS 01)- tiinized, only R type defects are observed, appearing as dislocation-like l o o p with the line of no contrast nearly perpendicular to the reciprocal lattice vector for the excited reflection involved. The saiiie area is shown in Fig. 2b, taken in underfocris phase contrast under highly kineniatical diffraction conditions. Here the loop-like defects are invisible. Instead dot-like defects are observed a t the positions of the loop-like defects of Fig. 2a.

The development of the dot-like defects as function of irradiation dose is shown in Fig. 3. Here the crystal is imaged in phase contrast, and the defocus (under-focus) is in each micrograph adjusted to give optiinuni visible defect contrast. The nuinher of

588

a

E. JOHNSON, J. FERRER, and L. T. CHlDDERTON

b

c d

Fig. 3. Dosc dependence of chlorinc ion void growth in sodalite irradiated with 100 keV electrons. The niirrographs are taken in underfociis phase contrast. a ) I W ; b) 2 x 10”; c) 3 x 10”; d)

5 x 1oY4 ele&oris/ni2. The diffraction pntt.erns are t.aken for S,,, = 0

defects can be seen to he nearly independent of dose, whilst individiml defects prow in size. Inserted in Fig. 3 are diffraction patterns taken hoth prior to and after the irradiation. The orlentation of the crystal in these pattern3 ([440] strongly excited) is different from the orientation inaintained during the irradiation itself (no reflection strongly excited). The disappcarance of Kikuchi lines after irradiation and the even distribution of intensity hetveen all the (110) systeinatic reflections indiczttes a de- creased perfection although a prevailing crystallinity in the high dose irradiated

Radiolytic Radiation Damage of Sodalite 589

Fig. 4 Fig. 5

Fig. 4. Sodalite structure. The cubic structure is based on an aluniinosilicate framework represent- ed as a body centred cubic space-filling packing of trunca,ted ochhedra enclosing the chlorine and sodium ions. The chlorine ion sublattice is body centred cubic and each chlorine ion is snr-

rounded by four sodium ions. CI-, o Na+, A-Si framework

Fig. 6 . Chlorine ion void size as function of irradiation dose for sodalite irradiated with 100 keV electrons

crystals [ 2 2 ] . Bleaching of the irradiated crystals for several days with white light was found to have no visible effect on the radiation induced niicrostructure.

From the high degree of similarity bet'ween the defects in sodalite - dilute inclu- sions and loops - and t'hose ohserved in irradiated alkali halides [a, 41 and in CaF, LO, 211, we interpret t,he dot-like defect's (t,ype A) as sodium nietal colloids formed by aggregation of chlorine vacancies (I? centres). The loop-like defects (type B) would then most naturally be assigned to a dislocation loop-like aggregation of int'erstitial chlorine ions displaced during the radiolytic damaging processes. The displacement nlechanisni leading to anion Frenkel pair formation in sodalite is as yet unknown. However, in the light, of our experimental ok)servat,ions, and bearing in mind tha t the sodiiin-chlorine substructure of sodalite (Fig. 4) essentially forms a kind of simple ionic snbstructiire, we propose that the Frenkel pair format,ion niechanisni in sodalite niay well be similar t,o t,hat, operating in more simple ionic structures, based on a nieeha- nisni of the Pooley-Hersh t,ype [ 2 3 to 251. The covalently bonded aluniinosilicate Imsket-like franiework should then be regarded as a passive niediuni, stahilizirig point defect foriiiation and aggloiiieration.

\Tithin the framework of this iiiodel the radiation daniagc processes are tied to processes taking place on t,he chlorine ion sublattice. Sodinin nietal colloid forniat,ion shonld accordingly he regarded as equivalent to the formation of chlorine ions voids. I7oid formation can take place in irradiated metals if interstitials are prefercntially trapped lati]. Therefore, and by analogy with t,he cases of anion void forination in the alkali halides and the alkali earth fluorides [ll], we consider the loop-like defects in sodttlite to he chlorine interstitial t,raps, permitting the clust,ering of chlorine vacancies and hence the forination of chlorine ion voids (i.e. sodiuni inetal). Sricleation of the anion voids appears to he strongly heterogenorrs (Fig. 2 ) ; the nucleation centres are clearly associated wit,h the loop-like defects. This effect is well known for metals. where the presence of inert gczs stabilizes void nuclei [ 2 7 ] , and where voids are fre- quently observed to nncleate on irradiation induced dislocations or on preexisting defects [%I. It, should be emphasized here that. growth of vacancy aggregates at the centres of interstitial dislocation loops is by no means self-contradictory. The nnclea- tion arid growth of vacancy aggregates a t the centres of large interstitial loops have

590 E. JOHNSOR. J. FERRER, and L. T. CHADDERTOK

been observed in irradiated metals, and ascribed to the very particular flow pattern of migrating point defects in the neighhourhood of the loops [29].

After the early nricleation stage chlorine anion voids are seen to grow slowly as the irradiation proceeds (Fig. 3). The average void size is plotted in Fig. 5 as a function of irradiation dose. There is a n increase froiii = 10 to = 25 nni, the size agreeing well with the calculated size of alkali metal colloids giving rise to the permanent blue coloration studied by Hassib et al. [15]. The observed persistence of this coloration against bleaching agrees with our observation that bleaching with white light has no observable effect iipon the void structure. Siniultaneously the chlorine loops grow, occasionally escaping froin the surface. The observed deterioration of the perfection of the diffraction patterns with increasing radiation dose (Fig. 3) parallels that seen in metals irradiated a t low teiiiperatures, and is explained by the introduction of massive strains originating froiii the presence of large numbers of defects [."'I. I n sodalite these strains originate partly a t the chlorine ion voids - presumably acting as incoherent sodiiiin inclusions - and partly a t dispersed interstitial chlorine gas, siniilar to observations of irradiated CaF, [SO]. Provided sublimation of the crystal is avoided, however, the saniple crystallinity is retained. Loss of interstitial chlorine by the escape of loops a t the surface does not alter this scheme, contrary to what is oh- served for several other silicate ininerals [Yl], even at lower electron fluences. It is believed that this is due to the passive role played in the damaging processes by the silicate framework in sodalite, which remains stable j u s t as long a s chlorine anion voids can form and grow. When this is not the case, a s for example when beani- induced sublimation gives rise to a loss of both sodium and chlorine, the silicate frame- work structure collapses and the crystal is aniorphized.

4. Conclusion Radiolytic damaging processes in sodalite are considered to take place primarily

on the sodiun-chlorine substructure, leaving a silicate frame work as the passive mediuai. Frenkel pair formation on the chlorine sublattice results in the growth of extended interstitial chlorine defects and sodium inclusions.

If, as we propose, chlorine ion voids in sodalite are identical with the inclusions, just a s fluorine ion voids are identical with iiietallic calcium in CaF2 I l l ] , then the damaging process in sodalite is hest discussed within the franiework of what we refer to as radiation induced voidage - already niuch studied for a large range of nietals and for a nuniber of structurally siinple ionic crystals. Conversely, it is equally reason- able to consider work on damage in complicated ionic structures - hitherto classified under colour centre and colloid science research - in teriiis of radiation induced voidage niodels.

-4 rl~)iorc.ledge,,ieiits

The authors are grateful to the Ilnnish Satural Science Research Council (Porsk- ningsrgd) for their support and to the S A T 0 research grants prograiimie for the award of a research grant. The sodalite was snpplied by Dr. 0. V. Pedersen, Museuiii for Geology, Copenhagen. X-ray diffraction analysis was perfomled by Dr. E. Leo- nartlsen, and niicroprobe analysis by Ur. J. (:. Ronsbo, Department of Mineralogy, University of Copenhagen.

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