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

Abstract

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 Mg₂(H₃O)₂(H₂O)₆[(UO₂)₃O₄(OH)]₂ (U-Mg1 with a U : Mg ratio of 3 : 1) crystallises in the monoclinic P2₁/c space group having a layered crystal structure, constructed by β-U₃O₈ layers with 6-fold coordinated Mg²⁺ ions as interlayer cations. Compound Na₂Mg(H₂O)₄[(UO₂)₃O₃(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 α-U₃O₈ and β-U₃O₈ features, with alternating layers of 6-fold coordinated Mg²⁺ and 6-/8-fold coordinated Na⁺ interlayer cations. Compound Na₂Mg(H₂O)₄[(UO₂)₄O₃(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.