Synthesis and Characterization of Layered Actinide (U, Np, Pu) Oxide and Hydroxide Phases

Systematic structural comparisons across the early actinides remain limited by the scarcity of well-defined transuranic layered oxide and oxyhydroxide phases. Here, we report the synthesis and single-crystal characterization of new layered actinide compounds spanning U, Np, and Pu obtained under mild hydrothermal conditions in concentrated alkali hydroxide media. These include hydrated oxides α-Cs₂U₂O₇^•0.5H₂O and Rb₂An₂O₇^•0.5H₂O (An = Np, Pu), oxy-hydroxides Rb₆[(AnO₂)₆O₈(OH)₂]^•xH₂O x = 0, 0.5 (An = U, Np) and Rb₄[(UO₂)₅O₆(OH)₂]^•2H₂O, as well as nitrate-intercalated compounds Cs₆[(AnO₂)₃O₄(OH)₂](NO₃)₂ (An = Np, Pu). Single-crystal X-ray diffraction studies reveal extended two-dimensional architectures constructed from edge- and vertex-sharing actinyl polyhedra, with systematic evolution in equatorial coordination, hydration, and anionic sheet topology across the U-Np-Pu series. Incorporation of nitrate anions within the interlayer region of Cs₆[(AnO₂)₃O₄(OH)₂](NO₃)₂ establishes a previously unobserved structural motif in layered transuranic oxyhydroxides, demonstrating an additional pathway for anion-mediated framework stabilization. Correlation of crystallographic metrics with single-crystal Raman spectroscopy provides new vibrational benchmarks linking differences in An═O_yl bond lengths to the equatorial coordination and the interstitial cations. These findings expand the structural hierarchy of layered actinide materials and address clarifying periodic trends governing topology, bonding, and vibrational signatures in high-valent 5f oxide systems.