High-pressure phase transformations of fluorite-type dioxides |
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| Authors: | Lin-Gun Liu |
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| Institution: | Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 2600 Australia |
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| Abstract: | Phase transformations in six fluorite-type dioxides (“TbO2”, PbO2, “PrO2”, CeO2, UO2 and ThO2 in the order of increasing cation size, where the quotation marks indicate non-stoichiometric materials) have been investigated in the diamond-anvil press coupled with laser heating. Together with earlier work, the results show that the post-fluorite phase transformations of these dioxides fall into two groups. The smaller cation group (HfO2, ZrO2 and “TbO2”) transforms to a cotunnite or a distorted cotunnite-type structure at pressures in the vicinity of 100 kbar and at about 1000°C. The larger cation group (from PbO2 to ThO2) is believed to transform to a different type of orthorhombic modification at high pressures. It is plausible that this high-pressure phase may possess a Ni2Si-related structure, as was observed in ThO2 and “PrO2” at pressures greater than 150 and 200 kbar, respectively. The dioxides with Ni2Si-related structure are the most closely packed dioxides known to exist. It is suggested that all the polymorphic structures of ZrO2 (baddeleyite, tetragonal fluorite, cubic fluorite and cotunnite) plus the new phase found in the large cation group dioxides are possible high-pressure models for the post-rutile and/or post-Fe2N phases of SiO2. The polyhedral coordination in these model structures varies from 7 to 10, compared with 6 for the rutile-and Fe2N-type SiO2. The fact that the larger cation group adopts a very closely packed structure at high pressures substantiates the earlier hypothesis that the radioactive elements U and Th in the earth's deep mantle are most likely to occur as dioxide rather than as silicate. |
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