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High-pressure phase behavior and equations of state of ThO2 polymorphs

High-pressure phase behavior and equations of state of ThO2 polymorphs AbstractThO2 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 ~60 GPa and ~2500 K. It undergoes a transition from the cubic fluorite-type structure (thorianite) to the orthorhombic α-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 [V0 = 26.379(7), K0 = 204(2), αKT = 0.0035(3)] and the high-pressure cotunnite-type phase [V0 = 24.75(6), K0 = 190(3), αKT = 0.0037(4)] are reported, holding K0′$\begin{array}{}K_0^{'}\end{array} $ 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. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png American Mineralogist de Gruyter

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Publisher
de Gruyter
Copyright
© 2018 Walter de Gruyter GmbH, Berlin/Boston
ISSN
0003-004X
eISSN
1945-3027
DOI
10.2138/am-2018-6212
Publisher site
See Article on Publisher Site

Abstract

AbstractThO2 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 ~60 GPa and ~2500 K. It undergoes a transition from the cubic fluorite-type structure (thorianite) to the orthorhombic α-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 [V0 = 26.379(7), K0 = 204(2), αKT = 0.0035(3)] and the high-pressure cotunnite-type phase [V0 = 24.75(6), K0 = 190(3), αKT = 0.0037(4)] are reported, holding K0′$\begin{array}{}K_0^{'}\end{array} $ 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.

Journal

American Mineralogistde Gruyter

Published: May 25, 2018

References