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

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.