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Actinides such as uranium (U) and plutonium (Pu) are essential materials for nuclear power plants and nuclear weapons, but their long-term storage safety has always been a serious concern. The U.S. alone currently has an inventory of over 10 9 pounds of depleted uranium waste materials from the manufacturing of nuclear weapons and from the nuclear power industry. Even simple issues such as oxidation become major problems in the storage of radioactive materials. For example, the oxidation of UO 2 generates a powder-like U 3 O 8 that can cause the splitting of the storage sheath, [1][2][3] which significantly complicates the handling and storage of UO 2 fuel and impacts the safety of long term storage of actinides. A better control of the surface chemistry and the phases of oxides (UO 2 , U 3 O 7 , U 3 O 8 , and UO 3 ) is, therefore, crucial for interpreting the surface chemistry of the oxides and for developing new applications of depleted uranium. So far, almost all research on the oxidation states and surface chemistry of uranium oxides has been carried out using sintered polycrystalline materials. As data obtained from single crystal substrates are, without exception, easier to interpret than data obtained from polycrystalline materials, it is essential that high quality single-crystal-like uranium oxides be prepared for our attempts to better understand the intrinsic physical properties of the materials. Herein we report the first such attempt to date to control the oxidation states in uranium oxides through epitaxial stablization to grow single-crystal-like uranium oxide films. These epitaxial uranium oxides are stable in air because their oxidation states are crystallographically pinned. The difficulty in preparing single crystal uranium oxides with controllable phases lies in the coexistence of many oxides with different oxidation states, where conversions between the oxides are relatively facile. An additional challenge in the growth of single crystal and single phase uranium oxides stems from the abundance of polymorphic structures in these oxides. For example, there exist both orthorhombic (Amm2) and hexagonal (P6 2m) phases of U 3 O 8 . Using polycrystalline materials in experiments to extract intrinsic properties of the materials can create tremendous ambiguity. For instance, experimental results indicate that the kinetic rate of U 3 O 7 formation on UO 2 depends heavily on the initial form of materials studied: powders, polycrystalline pellets, or crystals.[2] It has also been shown that intra-granular oxidation proceeds more slowly than oxidation along grain boundaries. [4] This is, however, not unexpected considering that grain boundaries and/or crystallographic imperfections often have a profound effect on chemical reactions. Neglecting the impact of the preferential diffusion of oxygen along grain boundaries can eventually lead to inappropriate interpretations of the surface chemistry of the materials. Epitaxial films, whose crystallographic alignment is mainly controlled by that of the substrate,...

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