Work

Protonated water layer in ludlamite lamellar iron phosphate (Fe3(PO4)2·4H2O)

Year: 2026

Type: article

Abstract: The vibrational signature (Raman and IR) of natural ludlamite single crystals is discussed, along with the evolution of the Raman spectrum after heating to 90, 150, and 200°C. The observation of broad "ABC" bands in the stretching mode region—characteristic of a strong, symmetric hydrogen bond—indicates the formation of H 2 O-H + -OH 2 (or H 5 O 2 + ) species. After deuteration, the νOH/νOD wavenumber ratios show values between 1.18 and 1.25, confirming the strong bonding that leads to an (H₅O₂⁺, 2 H₂O) n water layer between the [Fe 3 (PO 4 ) 2 ] n sheets, as well as the high proton mobility. Consequently, the chemical formula of the compound should rather be Fe 2.5 (PO 4 ) 2 H 5 O 2 ·2·H 2 O. An Fe 3 III O(HPO₄) 2 ·nH₂O form is obtained by immersion in molten ammonium nitrate. Ludlamite, like its lamellar counterparts HUP, HUAS, and HFeS, is a reasonably good proton conductor with a thermal stability up to ≈ 150°C.

Source: Journal of Molecular Structure

DOI: 10.1016/j.molstruc.2026.147932

Authors: Philippe Colomban, Ricardo Scholz, Roberto L. Moreira

Institutions: Centre National de la Recherche Scientifique, Sorbonne Université, Molécule aux Nanos-objets : Réactivité, Interactions et Spectroscopies, Universidade Federal de Ouro Preto, Universidade Federal de Minas Gerais

Language: English

Topic: Solid-state spectroscopy and crystallography

Subfield: Materials Chemistry

Field: Materials Science

Domain: Physical Sciences

Open Access status: hybrid

Estimated APC paid: $3,360

Funder: Conselho Nacional de Desenvolvimento Científico e Tecnológico