Amorphization and Dynamic Recovery of A 2 BO 4 Structure Types During 1.5 MeV Krypton Ion-Beam Irradiation
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were: forsterite (Mg2SiO4), fayalite (Fe 2 SiO 4 ), phenakite (Be2SiO 4 ) and synthetic Mg 2GeO4. The (Mg,Fe) 2 SiO4 compositions are the end-members of a complete solid-solution series with the olivine structure. This structure (Pbnm) is a derivative of hcp of the oxygens with Mg and Fe in octahedral coordination and Si in tetrahedral coordination. The SiO 4 tetrahedral monomers are isolated from one another (nesosilicates), but are joined along three edges and one apex to the edge-sharing chains (parallel to c) of A-site octahedra [8]. Mg2GeO 4 has the olivine structure (Ge substitutes for Si in tetrahedral sites), but the increased covalency of the Ge-O bond leads to lengthening of the shared edges. Recent studies of olivine have shown that nuclear interactions between energetic particles and target atoms (displacement damage), rather than ionization effects, are mainly responsible for ion beam-induced amorphization [2], and amorphization occurs directly within the displacement cascade under Kr÷ ion irradiations [9]. Phenakite (Be 2 SiO 4 ) has a structure (R3) in which the A- and B-site cations are all in tetrahedral coordination. The tetrahedra form corner-sharing 6- and 4-membered rings (perpendicular to c) in which the Be and Si atoms alternate. The rings are stacked (parallel to c) to form a cornersharing three dimensional network [10]. EXPERIMENTAL PROCEDURES The fayalite (Fe2 SiO 4 ), phenakite (Be2SiO4) and Mg2GeO4, are synthetic end-member compositions. The forsterite (Mg2SiO 4 ) is natural and has an actual composition of (Mg0.88Feo. 12)2SiO4 as determined by analytical electron microscopy. TEM samples prepared by Ar ion milling were irradiated with 1.5 MeV Kr+ ions in the HVEM-Tandem Facility at Argonne National Laboratory [11] at a dose rate of -3.4x10 11 ions/cm 2s. The facility consists of a high-voltage electron microscope (HVEM) connected to a tandem ion accelerator; thus, the selected area electron diffraction (SAD) pattern can be monitored in situ during ion irradiations to determine the Dc. The HVEM was used at an accelerating voltage of 300 keV. The SAD patterns were observed at the maximum observable sample thickness, as thinner regions become amorphous at lower doses due to surface effects. The irradiations were performed between 15 to 700 K, using a liquid helium-cooled cold stage or a hot stage. The maximum temperature increase due to beam heating was 60 'C. At 1.5 MeV, most of the Kr+ ions completely penetrate the electron transparent thickness (- 250 nm) of the samples, and the Kr concentration introduced into the sample is negligible. Because of differences in the density and displacement energy of the different targets, the per ion energy loss and damage production in the observable sample thickness are different. TRIM [12] calculations were performed to convert the critical ion dose to a damage dose in displacements per atom (dpa). However, this conversion may not be accurate, as the displacement energy (Ed) of 15 eV was assumed for all the materials. Nevertheless, the variations in
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