Comparison of Ion‐Beam Irradiation Effects in X2YO4 Compounds

Abstract

O 4 ), and phenakite structures have been investigated using in situ and high-resolution transmission electron microscopy (HRTEM) over a wide temperature range (20-873 K). At low temperatures (<200 K), the olivine and phenakite are susceptible to radiation-induced amorphization with a critical amorphization dose of 0.2-0.5 displacement per atom (dpa). The critical amorphization dose increases with increasing irradiation temperature at varying rates for the various phases, resulting in a distinct critical amorphization temperature for each phase. For the Mg-Fe series of olivine, the susceptibility to amorphization at higher temperatures (room temperature or above) increases with increasing Fe content. Although the spinel phases are, in general, much more resistant to amorphization, a highpressure metastable spinel phase, ␥-SiFe 2 O 4 , is easily amorphized at doses <0.2 dpa at temperatures below 723 K. This phase decomposes after irradiation at 873 K. At 20 K, complete amorphization of the FeCr 2 O 4 spinel (chromite) is achieved at ∼4 dpa, but no evidence of amorphization is observed in MgAl 2 O 4 spinel after 5.4 dpa. The distinct differences in the relative susceptibility of these phases to amorphization are discussed in terms of the structural and chemical controls on the amorphization process.K. E. Sickafus-contributing editor Manuscript No. 189659.

The mineralogy and space weathering of a regolith grain from 25143 Itokawa and the possibility of annealed solar wind damage

“…No detectable radiation damage in the apatite suggests a higher resistance against amorphization due to more rapid annealing of the radiation damage. Irradiation experiments with 1.5 MeV Kr + ions conducted by Wang et al (1994) under comparable conditions to the experiments of Wang et al (1999) showed that the critical, upper temperature of amorphization of Fa 12 olivine and fluorapatite are very similar. Increasing Fe contents of olivine shifts this temperature to higher values (i.e., the structure becomes more vulnerable), and therefore, the observed difference between the two phases in our sample is probably due to the olivine?…”

“…The implantation of energetic particles into crystalline structures causes displacements of ions from structural lattice sites into interstitial positions, the sum of many such displacements eventually leads to amorphization. In situ irradiation experiments by Wang et al (1999) using 1.5 MeV Kr + ions directed at approximately 300 nm thick foils of Fa 12 olivine showed that the disordering of the structure starts with the cations being preferentially knocked into interstitial positions, while the anion sublattice remains intact for a longer time at a given ion flux. Similar experiments by Christoffersen and confirmed that amorphization progresses by the formation of amorphous domains within a residual crystalline matrix that shows no crystallographic misorientations.…”

“…Because the displacement of atoms by energetic particles competes with the relaxation and annealing of the disturbed structure via thermally activated atomic movements, temperature is an important factor in governing the accumulation of radiation damage. Wang et al (1999) showed that for Fa 12 olivine, the number of displacements per atom required for amorphization dramatically increased when the sample? s temperature approached approximately 500 K. This is due to thermal annealing beginning to dominate over the rate of radiation-induced displacement.…”

“…Because olivine? s critical amorphization temperature is higher (approximately 500 K; Wang et al 1999), this process might only work at elevated temperatures.…”

Low‐energy helium ion irradiation‐induced amorphization and chemical changes in olivine: Insights for silicate dust evolution in the interstellar medium

“…The average number ofatomic displacement produced per He implantation event can be evaluated with the Monte Carlo computer code TRIM (TRansport of Ions in Matter; Ziegler et al, 1996). Assuming a displacement energy of25 eV for all species (Wang et al, 1999), there were~40 (4 keV), 70 (10 keV) and 120 (50 keY) atomic displacements per He implantation. The completely amorphous state is the consequence ofaccumulation and overlap ofthe individual events.…”