The amorphization of complex silicates by ion-beam irradiation
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Robert C. Birtcher MSD 212, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439 (Received 27 March 1992; accepted 14 July 1992)
Twenty-five silicates were irradiated at ambient temperature conditions with 1.5 MeV Kr + . Critical doses of amorphization were monitored in situ with transmission electron microscopy. The doses required for amorphization are compared with the structures, bond-types, compositions, and physical properties of the silicates using simple correlation methods and more complex multivariate statistical analysis. These analyses were made in order to determine which properties most affect the critical amorphization dose. Simple two-variable correlations indicate that melting point, efficiency of atomic packing, the dimensionality of SiO4 polymerization (DOSP), and bond ionicity have a relationship with critical amorphization dose. However, these relationships are evident only in selected portions of the data set; that is, for silicate phases with a common structure type. A clearer relationship between the silicate properties and critical amorphization dose was determined for the entire data set with multiple linear regression. Several regression models are proposed which describe the variation in amorphization dose. All regression models contain the following properties: (i) melting point; (ii) a structural variable (DOSP, elastic modulus, and/or atomic packing); and (iii) the proportion of S i - 0 bonding (instead of bond ionicity). The regression models are equivalent, because they represent combinations of similar properties. Notably, density and atomic mass are not controlling properties for the critical amorphization dose. Melting and amorphization by ion irradiation are apparently related processes. Neither melting point nor critical amorphization dose can be predicted by considering only the structure, composition, or bonding of a particular phase. The S i - 0 bond is the most covalent bond in silicates, and is the "weak link" in the structure with respect to amorphization. Thus, DOSP is also an important property, as the topology of these "weak links" influences a structure's ability to accumulate amorphous regions. The efficiency of atomic packing is related to the process of defect self-recombination during amorphization. The bulk modulus and shear modulus are important variables within the regression models because of their direct relationship to atomic packing.
I. INTRODUCTION The crystalline-to-amorphous transition (C-A) is important for understanding the structure of solid phases. The C-A transition is readily induced in many ceramic phases by ion-bombardment,1"3 and can also result from the a-decay of radionuclides (U and Th series) to produce the metamict state in minerals.4 Furthermore, the recent discovery that many ceramics are rendered noncrystalline by ultrahigh pressures (13-35 GPa)5"7 adds to the general importance of the amorphous state. There is much current interest in aperiodic and radiationdamaged ceramics, because they have relevance to the ev
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