Hydrogen Bonding in Parascorodite and Relative Stability of Fe(AsO4)⋅2H2O Polymorphs

Abstract

Density functional theory (DFT) has been used with CRYSTAL14 program to optimize hydrogen atom positions in the crystal structure of parascorodite Fe(AsO₄)⋅2H₂O. It has been shown that taking into account hydrogen bonding, the structure forms six-membered asymmetrical cycles –Fe1–O1–H1^…O3–As–O2– and eight-membered cycles –Fe1–O1–H2^…O2–Fe1–O2^…H2–O1– with lateral topological symmetry. Calculation of the structural complexity parameters for scorodite and parascorodite and their comparison with thermodynamic characteristics shows that the Fe(AsO₄)⋅2H₂O polymorph modifications correspond well to Goldsmith’s rule that states that metastable transitional polymorphs are structurally simpler than thermodynamically stable phases. Scorodite is a stable phase, whereas parascorodite is metastable, which agrees well with the conclusions made in the previous works. Crystallization of parascorodite under natural conditions of oxidation zones of ore-mineral deposits may occur due to the high speed of its nucleation as a metastable phase, i.e., due to the kinetic stabilization of its structure.