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Crystal Structure and Stability in Aqueous Solutions of Na0.5[NpO2(OH)1.5]·0.5H2O and Na[NpO2(OH)2]

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journal contribution
posted on 19.05.2022, 21:10 authored by David Fellhauer, Jun-Yeop Lee, Nicole A. DiBlasi, Olaf Walter, Xavier Gaona, Dieter Schild, Marcus Altmaier
The ternary neptunium­(V) (Np­(V)) hydroxides Na0.5[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) Å3 for (I) and orthorhombic, P212121, 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.