Étude du type structural de γ-FeO(OH)(s) et comparaison avec la structure de Cu(OH)2(s)

Abstract

Comparison of the crystal structures of Cu(OH)_2(s) and γ-FeO(OH)_(s) shows a structural analogy between both solid phases, despite the difference of formulas. This analogy is related to similar connection modes of the polyhedra, which give rise to two-dimensional structures composed of wavy sheets. Lepidocrocite, γ-FeO(OH)_(s), is obtained from divalent iron hydroxide Fe(OH)_2(s), which exhibits also a two-dimensional framework of brucite type. We propose a mechanism for this reaction, which is probably topotactic. The resulting lepidocrocite structural type allows strong deformations of the octahedral cationic coordination, contrarily to the brucite type. Deformations can be due to the loss of hydrogen atoms in γ-FeO(OH)_(s) or to the strong Jahn–Teller effect that affects divalent copper in Cu(OH)_2(s). If copper hydroxide cannot be isostructural with brucite as it is for Fe(OH)_2(s), an analogy exists with lepidocrocite, even if copper has a pentahedral surrounding, different of that of trivalent iron, which is octahedral. In both crystal structures, one oxygen atom has a peculiar surrounding composed of only two cations and one hydrogen atom. This feature explains probably the low stability of this structural type.