High-pressure synthesis of U2[CO3]3 and U[CO3]2 as potential host phases for uranium in the Earth’s mantle

It is well established that a significant amount of heat produced in the Earth's mantle is due to the decay of uranium. However, uranium cannot be incorporated in large amounts into the most common mantle minerals. Here, we suggest that carbonates could be host phases for uranium in carbon-rich mantle lithologies. Two anhydrous uranium carbonates, U₂[CO₃]₃ and U[CO₃]₂, were simultaneously synthesized by a reaction of UO₂ with CO₂ in a laser-heated diamond anvil cell at 20(1) GPa and 1800(200) K. Their crystal structures were obtained from synchrotron-based single crystal diffraction data and reproduced by density functional theory-based calculations. In U₂[CO₃]₃ trivalent uranium cations are present, while uranium is four-valent in U[CO₃]₂. The synthesis of U₂[CO₃]₃ and U[CO₃]₂ is a significant extension of the chemistry of uranium compounds and we provide a straightforward synthesis route for a U^III-containing compound.