Radiolysis and defect structure in electron-irradiated α-quartz

-The defect microstructure of electron-irradiated a-quartz was studied by transmission electron microscopy. Formation of heterogeneously-nucleated disordered strain centers and a homogeneous crystalline -+ amorphous transformation of the surrounding matrix were observed. Both features are shown to be attributable to radiolysis, a mechanism for which is proposed. 1 . Introduction. A persistent observation in many crystalline systems based on [SiO,] tetrahedra is loss of long-range order under irradiation, leading ultimately to the amorphous (known mineralogically as the metamict [I]) state. For the most part, only fast particle irradiation has been considered. For example, it has been shown 12-41 that quartz and other Si02 polymorphs (with the apparent exception of coesite [2]) all ultimately transform under reactorneutron or fast-ion irradiation to a common glass of unique density different from that of fused silica. Spectroscopy [5-61 has confirmed that intrinsic point defects are formed, while thermal conductivity [7] and X-ray diffuse scattering [2, 81 measurements have suggested the presence of individual disordered zones. A number of parenthetical remarks regarding the radiation stability of many mineral silicates examined in the electron beam of electron microscopes has also accumulated in the geological and mineralogical literature [9]. The corollary is that fused silica has been reported to densify by 3 % (compaction [lo]) under neutron [3] or ion [lo] irradiation to the same metamict state as for crystalline Si02. Both transformations are technologically significant for nuclear waste storage [ll] and frequency control applications [12]. Transmission electron microscopy (TEM) is a powerful technique for study of the microstructural alterations accompanying radiation damage [13]. It is particularly convenient for studying irradiation of SO2-based systems in that damage can be produced in situ by the electron beam and evaluated dynamically [14, 151. There have been several previous TEM studies of irradiated a-quartz. Das and Mitchell [16] concluded that two damage processes occur, viz knock-on displacement, which leads to formation of local centers of strain, and radiolysis (from the dominant ionizing component of fast electron irradiation), which leads to metamictization. Baeta and Ashbee [17] proposed that the strain centers formed were prismatic dislocation loops, while MacLaren and Phakey [18] concluded they were loops which coalesced into amorphous regions, and Weissmann and Nakajima [19] presumed they were interstitial silicon clusters. The TEM observations reported here show that both stages of the damage can be attributed to radiolysis, and that the strain centers, which form rapidly and heterogeneously, are definitely associated with amorphous inclusions. These observations, together with existing point defect information (5 4.2), provide the basis for advancing a radiolytic model for the mechanism of the metamict transformation. 2. Specimen preparation and electron microscopy. Synthetic a-quartz was obtained from Sawyer Research Products. Eastlake, Ohio in the form of large hydrothermally-grown single-crystal boules with typical impurity content of 10-100 ppm H, 10'' e m-' s-I) and fluences ( 1 0 ~ ~ 1 0 ' ~ e m-') necessarily involved at this level of resolution [15], particularly in the weak-beam mode, occasioned such rapid specimen degradation that minimal exposure techniques [15, 231 had to be employed to minimize unintentional damage during manipulation and recording. All observations reported here were made at the ambient temperature (290 K) of the microscope. The magnitude of electron-beam heating was negligible (< 1 K [15]). 3. TEM observations. 3.1 DAMAGE MORPHOLOGY. During electron irradiation, crystals underwent a gradual crystalline + amorphous transformation (Fig. l), the transformation occurring first in the most intense portion of the gaussian electron-beam profile and spreading outwards. Loss of crystallographic detail in the image was accompanied by gradual fading of Bragg-diffracted beam intensities, appreciable increase in the diffusely-scattered background and appearance of an amorphous halo. Buckling of the specimen due to an increase in volume of the irradiated material was apparent (Fig. 1) from extinction contours which followed the curvature of the irradiated area. At a dose of order 100 times less than that for metamictization (within seconds for the 10'' e m-' (0001) foil induced by most intense portion of a 40 keV electron s-' dose rate), centers of strain (at A in figure 2) beam. were nucleated at an initial density 1021-1022 m-3 and grew to eventual dimensions 20-50 nm. Stereomicroscopy indicated that these centers formed uniformly throughout the thin section, but were absent in the very thinnest specimen regions and in denuded zones adjacent to boundaries. Upon further irradiation, the associated strain-field contrast in the adjoining matrix disappeared, the centers themselves remaining brighter than background in bright-field conditions (Fig. 3a). Growth of the centers continued, however (Fig. 3b), with occasional coalescence, until they were overtaken by the amorphous transformation front. Very weak contrast from the centers persisted even after the surrounding matrix had become functional1,y