Mixed-Valence Iron Dumortierite Fe_13.5^2.22+(As⁵⁺O_4–x)₈(OH)₆ and Its Intricate Topotactic Exsolution at Mild Temperatures

Abstract

Fe 13.5 2.22+ (AsO 4−x ) 8 (OH) 6 , x = 0.25, was prepared using the reaction of iron metal with arsenate in aqueous solution and autogenous pressure. Its crystal structure reveals a dumortierite-like framework with mixed-valent Fe 2+/ Fe 3+ in double chains creating channel walls. Remarkably, hexagonal channels consist of chains of face-sharing Fe 2+ O 6 octahedra, 3/4th occupied, whereas AsO 4 tetrahedra occupy triangular ones with a single "up" orientation according to the polar P6 3 mc symmetry. We have analyzed the transformation of this phase upon heating, in which several chemical processes interact, including dehydroxylation, arsenate to arsenite reduction, and oxidative exsolution of a significant part of iron (ca. 15%) found at the surface as hematite and amorphous Fe-rich surficial layer. It leaves a strongly disordered composite structure between several Fe 3+ -based subunits, in which ∼80% of them is ordered in a complex supercell. Because of the high degree of disorder, the crystal chemistry of the individual subunits and their plausible imbrication were considered to unravel the most plausible ideal 3D model.

The hidden story in BaNiO3to BaNiO2transformation: adaptive structural series and NiO exsolution

“…1 More rarely, structures based on oxo-anion frameworks can exsolve metal oxide upon oxidation. This results in complex depleted lattices after partial metal oxide removal, observed for instance in olivine-like LiFe(PO4), layered BaFe2(PO4)2 and more recently in the Dumortierite Fe13.5(AsO4)8 (OH)6 compounds, [2][3][4] the Fe 2+ /Fe 3+ redox couple leaves Fe-deficient compounds with α-Fe2O3 nano-particles decorating the surface. The possibility to exsolve other transition metal oxides seems appealing.…”

Topotactic transformation of gadolinite‐ to melilite‐type structure revisited

“…The supplementary study of the annealed crystal demonstrated that the last (No 3) reaction is correct (see above). A similar process with the topotactic oxidative exsolution of Fe 2 O 3 (hematite) from a Fe‐containing dumortierite‐like compound upon heating has been recently described 99 …”

Magnetic properties of a new cobalt hydrogen vanadate with a dumortierite-like structure: Co13.5(OH)6(H0.5VO3.5)2(VO4)6

“…The novel cobalt hydrogen vanadate Co 13.5 (OH) 6 (H 0.5-VO 3.5 ) 2 (VO 4 ) 6 is isostructural with Mg 13.4 (OH) 6 (HVO 4 ) 2 -(H 0.2 VO 4 ) 6 (Ðordević et al, 2008) and to the mineral phosphoellenbergerite, Mg 12 (Mg,Fe,&) 2 (PO 4 ,PO 3 OH, AsO 4 ) 6 -(PO 3 OH,CO 3 ) 2 (OH) 6 (Raade et al, 1998). The phosphoellenbergerite mineral and other related structures (Che et al, 2005;Gu et al, 2007;Ni et al, 2009;Amoró s et al, 1996;Mentre et al, 2018) were reported as dumortierite-like materials (Evans & Groat, 2012) due to their similar topology to the mineral dumortierite, i.e. (Al,&)Al 6 (BO 3 )Si 3 O 13 (O,OH) 2 .…”