Solvent-driven morphology control of plutonium oxalate predicted by molecular dynamics simulation

Abstract

Plutonium(IV) oxalate, the industrial precursor to PuO₂, requires precise habit control as its morphology strongly influences the subsequent nuclear fuel processing. However, its morphology is still rarely studied due to actinide radiological constraints that obscure experimental study on the specific growth mechanisms of its crystal faces. In this work, a well-established modified attachment energy (MAE) model, which incorporates solvent–crystal interfacial interactions, was used to investigate the solvent–surface interactions that govern habit formation under 13 different solution environments. The results show that in vacuum, the intrinsic lattice structure leads to a rod-like morphology dominated by the {0 1 1}. Upon introducing HNO₃, NaNO₃ or HCl, NO₃⁻ and H₂O establish strong Pu(cry)–O(sol-NO₃⁻) and O(cry-C₂O₄²⁻)–H(sol-H⁺) contacts on the {1 1 0} face. This interaction creates an adsorption barrier that effectively lowers the attachment energy, causing the {1 1 0} face to expand from 3.9% to 48% as acid concentration rises. Conversely, the {0 2 0} surface is fully terminated by oxalate ions. Here, transient O(cry-C₂O₄²⁻)–H(sol-H⁺) bonding and O(cry-C₂O₄²⁻)–O(sol-H₂O) repulsive interactions facilitate rapid advancement and eventual disappearance of this face. As a result, the aspect ratio of the crystal increases predictably from 1.7 to 2.4, offering a reliable and low-risk method for tailoring the morphology of plutonium oxalate through solvent selection.