Heavy ion irradiation studies of columbite, brannerite, and pyrochlore structure types

Several natural and synthetic samples of pyrochlore, columbite, and brannerite were irradiated in situ in the high voltage electron microscope at Argonne National Laboratory using 1.5 MeV Kr+ ions at room temperature. The columbite samples were selected because they have the same stoichiometry as brannerite and they exhibit a range of cation order prior to irradiation. The results of this study demonstrate that all the samples pass through the crystalline–amorphous transformation at a relatively low ion dose. For the AB2O6 oxides, Dc ranges from 0.8 to 1.5×1014 ions cm−2, with little difference between the columbite and brannerite structure types. Within the group of columbite samples, there appears to be a systematic trend of increasing critical dose with increasing order parameter, although the critical doses are essentially within error of one another. Ordered columbites do not appear to revert to the subcell upon irradiation. The brannerite data set indicates a possible correlation between Dc and the mean atomic number of the A-site cation. The results for the two natural pyrochlores gave a slightly higher range of doses compared with the AB2O6 oxides. The pyrochlore data set indicates a possible correlation between Dc and the mean atomic number of the B-site cation. Electron diffraction results also suggest that the pyrochlore compositions irradiated in this work may not transform to the fluorite subcell until just before Dc is reached, if at all. For comparison, the samples investigated in this work have Dc values at room temperature that are generally on the low side of the range of values previously reported for synthetic zirconolite and pyrochlore samples (2–6×1014 ions cm−2) under similar irradiation conditions.