Simulation of the Crystal-To-Amorphous Transformation in Irradiated Quartz
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SIMULATION OF THE CRYSTAL-TO-AMORPHOUS TRANSFORMATION IN IRRADIATED QUARTZ
UMA JAIN*, ADAM C. POWELL AND LINN W. HOBBS Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, U.S.A. *Gargi College, Delhi University, New Delhi, India. ABSTRACT Quartz and other crystalline polymorphs of silica transform from the crystalline to an aperiodic state under irradiation. There is a need to understand the structural changes involved during this amorphization process. We have built an engineering model which simulates the growth of amorphous regions within a crystalline matrix during the crystal-to-amorphous transition in irradiated quartz. The resulting crystal structure is displayed on the computer screen or plotted on a printer with the orthogonal coordinates of all the atoms in the cluster and the interatomic distances stored in a file. We find the bond lengths increase by about 3%, which is a reasonable value to expect since quartz expands 14% by volume during the amorphization. The results also show the crystal structure surrounding the strained region to be somewhat disturbed, consistent with what is observed experimentally. DESCRIPTION OF PRESENT WORK Irradiation studies of crystalline quartz have been reviewed by Clinard and Hobbs [1], Laermans [2], Griscom [3,4], and Lell et al. [5]. The primary defects produced in irradiated quartz, such as the E' center and the [02"] peroxy radical have also been well documented [6-11]. In addition, there is a secondary defect response -- a gradual loss of long range order under irradiation leading ultimately to the amorphous state, i.e. a crystal-to-glass (c-g) transition known mineralogically as metamictization. Alpha quartz expands 14% by volume and silica glass compacts 3% by volume on metamictization, approaching a common terminal density. A fair amount of experimental information on c-g transition has been derived from diffraction and electron microscopy methods [12-21]. Based on these observations we have deduced the following model for the formation of strain centers leading to amorphization of the crystal. We assume that, for moderate energy electron irradiation (< 1500 keV), the atomic displacement damage in Si0 2 occurs via radiolysis. The exciton in Si0 2 can be as large as 10eV. Since the energy needed to break a Si-O bond is about 5eV, a sufficiently localized exciton can lead to a severed Si-O bond. This could result in the formation of an oxygen vacancy (the well-known E' center) and its complementary defect, the [02=] peroxy linkage. The mobilities of these defects will be enhanced in the presence of irradiation because they will be in an excited state, and the effective activation energy will be less than what it is otherwise. With this enhanced mobility, it is possible that the oxygen atoms, after breaking away from their bonds, will rebond bridging different silicon atoms. This will result in a different arrangement of the [Si0 4] tetrahedra, giving rise to a small, more randomly-connected region in the cry
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