Radiation effects in the orthophosphates
Abstract
Monazite-structure orthophosphates, including LaPOsb4,\ PrPOsb4,\ NdPOsb4,\ SmPOsb4,\ EuPOsb4,\ GdPOsb4, and natural monazite, and their zircon-structure analogues, including, ScPOsb4,\ YPOsb4,\ TbPOsb4,\ TmPOsb4,\ YbPOsb4, and LuPOsb4 were irradiated by 800 KeV Kr ions over the temperature range of 20 to 600 K. The critical amorphization dose was determined in-situ in the electron microscope using selected-area electron diffraction. The monazite-structure compounds, in general, had lower critical amorphization temperatures. Within each structure, the critical amorphization temperature, Tsb{c}, increased with the atomic number of the A-site cation. Structural topology models were insufficient to explain all of the observed differences, however, the ratio of the electronic-to-nuclear stopping powers showed good correlation with the susceptibility to amorphization at elevated temperatures. The variation in the Tsb{c} because of thermal annealing in geologic samples was calculated for the case of uranium in zircon. Equations used to calculate the critical amorphization temperatures were combined with the standard radioactive decay equation to calculate the alpha-decay event dose required for amorphization in terms of a "critical radioelement concentration", Nsb{c}. This allows the prediction of whether or not a mineral will be metamict given the age, radioelement concentration, and ambient temperature. Calculated values of Nsb{c} show good agreement with the previously estimated values for a suite of zircons from Sri Lanka. Ion-beam-amorphized orthophosphates, including fluorapatite, were irradiated by an 80 to 300 keV electron beam in the transmission electron microscope at temperatures between 100 and 800 K. Bright field imaging and selected-area electron diffraction were used to monitor the microstructural evolution. Amorphous LaPOsb4 and ScPOsb4 crystallized to form a randomly oriented polycrystalline assemblage of the same composition as the original material, but amorphous ZrSiOsb4 crystallized to ZrOsb2 (zirconia) + amorphous SiOsb2. Submicron tracks of crystallites could be "drawn" on the amorphous substrate by the electron beam. The microstructral evolution in amorphous Casb5(POsb4)sb3F under electron-irradiation is more complicated and depends on the temperature and dose rate.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1997
- Bibcode:
- 1997PhDT.......209M