Journal
AMERICAN MINERALOGIST
Volume 103, Issue 5, Pages 749-756Publisher
MINERALOGICAL SOC AMER
DOI: 10.2138/am-2018-6212
Keywords
XRD data; ThO2; Raman spectroscopy; ThO2; phase transition; high-pressure studies; diamond-anvil cell; high-temperature studies; laser-heating
Categories
Funding
- NSF [DGE-1144082, EAR-1427123, EAR-0944298]
- Carnegie/DOE Alliance Center (CDAC)
- National Science Foundation-Earth Sciences [EAR-1634415]
- Department of Energy, GeoSciences [DE-FG02-94ER14466]
- DOE Office of Science [DE-AC02-06CH11357]
- Consortium for Materials Properties Research in Earth Sciences (COMPRES) under NSF Cooperative Agreement [EAR 10-43050]
- Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy [DE-AC02-05CH11231]
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ThO2 is an important material for understanding the heat budget of Earth's mantle, as well as the stability of nuclear fuels at extreme conditions. We measured the in situ high-pressure, high-temperature phase behavior of ThO2 to similar to 60 GPa and similar to 2500 K. It undergoes a transition from the cubic fluorite-type structure (thorianite) to the orthorhombic alpha-PbCl2 cotunnite-type structure between 20 and 30 GPa at room temperature. Prior to the transition at room temperature, an increase in unit-cell volume is observed, which we interpret as anion sub-lattice disorder or pre-transformation melting (Boulfelfel et al. 2006). The thermal equation of state parameters for both thorianite [V-0 = 26.379(7), K-0 = 204(2), alpha K-T = 0.0035(3)] and the high-pressure cotunnite-type phase [V-0 = 24.75(6), K-0 = 190(3), alpha K-T = 0.0037(4)] are reported, holding K-0(') fixed at 4. The similarity of these parameters suggests that the two phases behave similarly within the deep Earth. The lattice parameter ratios for the cotunnite-type phase change significantly with pressure, suggesting a different structure is stable at higher pressure.
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