Capturing ammonium nitrate in a synthetic uranium oxide hydrate phase: revealing the role of ammonium ions and anion inclusions

Abstract

Although uranium oxide hydrate (UOH) minerals and synthetic phases have been extensively studied, the role of ammonium ions in the formation of UOH materials is not well understood. In this work, the stabilization of a synthetic UOH phase with ammonium ions and the inclusion of ammonium nitrate were investigated using a range of structural and spectroscopic techniques. Compound (NH₄)₂(NO₃)[(UO₂)₃O₂(OH)₃] (U-N1) crystallises in the orthorhombic Pmn2₁ space group, having a layered structure with typical α-U₃O₈ type layers and interlayer (NH₄)⁺ cations as well as (NO₃)⁻ anions. The presence of uranyl, (NH₄)⁺ cations and (NO₃)⁻ anions were further confirmed with a combination of FTIR and Raman spectroscopies through characteristic vibrational modes. The roles of the (NH₄)⁺ cations for charge compensation and facilitating the inclusion of (NO₃)⁻ anions via hydrogen bonding were revealed and discussed. The findings have implications for uranium geochemistry, reprocessing of spent nuclear fuel and possible spent nuclear fuel alteration pathways under geological disposal.