Crystal Structure and Stability in Aqueous Solutions of Na_0.5[NpO₂(OH)_1.5]·0.5H₂O and Na[NpO₂(OH)₂]

The ternary neptunium­(V) (Np­(V)) hydroxides Na_0.5[NpO₂(OH)_1.5]·0.5H₂O (I) and Na­[NpO₂(OH)₂] (II) were synthesized in aqueous NaOH solutions at T = 80 °C, and their crystal structures were determined to be monoclinic, P2₁, Z = 2, a = 5.9859(2), b = 10.1932(3), c = 12.1524(4) Å, β = 98.864(1)°, V = 732.63(4) ų for (I) and orthorhombic, P2₁2₁2₁, Z = 4, a = 5.856(7), b = 7.621(9), c = 8.174(9) Å, V = 364.8(7) ų for (II). By combining the detailed structural information with results from systematic solubility investigations, a comprehensive chemical and thermodynamic model of the Np­(V) behavior in NaCl–NaOH solutions was evaluated. The results reveal a great stability of the ternary Na–Np­(V)–OH solid phases that significantly enhances the predominance field of the entire Np­(V) redox state to high alkalinity.