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[NpO(OH)]·0.5HO and Na[NpO(OH)] were synthesized in aqueous NaOH solutions at = 80 °C, and their crystal structures were determined to be monoclinic, 2, = 2, = 5.9859(2), = 10.1932(3), = 12.1524(4) Å, β = 98.864(1)°, = 732.63(4) Å for and orthorhombic, 222, = 4, = 5.856(7), = 7.621(9), = 8.174(9) Å, = 364.8(7) Å for . 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.