Crystal Structure and Stability in Aqueous Solutions of Na$_{0.5}$[NpO$_{2}$(OH)$_{1.5}$]·0.5H$_{2}$O and Na[NpO$_{2}$(OH)$_{2}$]

The ternary neptunium(V) (Np(V)) hydroxides Na$_{0.5}$[NpO$_{2}$(OH)$_{1.5}$]·0.5H$_{2}$O (I) and Na[NpO$_{2}$(OH)$_{2}$] (II) were synthesized in aqueous NaOH solutions at T = 80 °C, and their crystal structures were determined to be monoclinic, P2$_{1}$, Z = 2, a = 5.9859(2), b = 10.1932(3), c = 12.1524(4) Å, β = 98.864(1)°, V = 732.63(4) Å3 for (I) and orthorhombic, P2$_{1}$2$_{1}$2$_{1}$, Z = 4, a = 5.856(7), b = 7.621(9), c = 8.174(9) Å, V = 364.8(7) Å3 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.