Filling the gaps of uranium oxide hydrates with magnesium(II) ions: unique layered structures and the role of additional sodium(I) ions

Alkaline earth metal ions play an important role in the formation of secondary uranium minerals due to their abundance in the Earth's crust. Although uranium oxide hydrate (UOH) minerals and synthetic phases with calcium, strontium and barium ions have been investigated, their counterparts with magnesium ions are much less studied. In this work, synthetic UOH materials with magnesium ions have been investigated with three new compounds being synthesised and characterised. Compound Mg-2(H3O)(2)(H2O)(6)[(UO2)(3)O-4(OH)](2) (U-Mg1 with a U : Mg ratio of 3 : 1) crystallises in the monoclinic P2(1)/c space group having a layered crystal structure, constructed by beta-U3O8 layers with 6-fold coordinated Mg2+ ions as interlayer cations. Compound Na2Mg(H2O)(4)[(UO2)(3)O-3(OH)(2)](2) (U-Mg2p with U : Mg : Na ratios of 6 : 1 : 2) crystallises in the triclinic P space group having a layered structure, constructed by a unique type of uranium oxide hydroxide layer containing both alpha-U3O8 and beta-U3O8 features, with alternating layers of 6-fold coordinated Mg2+ and 6-/8-fold coordinated Na+ interlayer cations. Compound Na2Mg(H2O)(4)[(UO2)(4)O-3(OH)(4)](2) (U-Mg2n with U : Mg : Na ratios of 8 : 1 : 2) crystallises in the triclinic P space group having a corrugated layer structure, constructed by a unique type of uranium oxide hydroxide layer with mixed 6-fold coordinated Mg2+ and 7-fold coordinated Na+ interlayer cations. The structural diversity in the UOH-Mg system was achieved by adjusting the solution pH using NaOH, highlighting the importance of solution pH control and the additional Na+ ions in the formation of UOH phases. The extra structural flexibility offered by the Na+ ions emphasizes the opportunity for synthesising UOHs with dual-cations to further improve our understanding of the alteration products of spent nuclear fuel under geological disposal.