X-ray Structure Refinement and Vibrational Spectroscopy of Metavauxite FeAl₂(PO₄)₂(OH)₂·8H₂O

• Main Authors: Giancarlo Della Ventura, Francesco Capitelli, Giancarlo Capitani, Gennaro Ventruti, Alessandro Monno
• Format: Article
• Language: English
• Published: MDPI AG 2019-06-01
• Series: Crystals
• Subjects: metavauxite; phosphate; single-crystal X-ray structure refinement; hydrogen bonding network; FTIR and Raman spectroscopy
• Online Access: https://www.mdpi.com/2073-4352/9/6/297

In this paper, we provide a crystal-chemical investigation of metavauxite, ideally FeAl₂(PO₄)₂(OH)₂&#183;8H₂O, from Llallagua (Bolivia) by using a multi-methodological approach based on EDS microchemical analysis, single crystal X-ray diffraction, and Raman and Fourier transform infrared (FTIR) spectroscopy. Our new diffraction results allowed us to locate all hydrogen atoms from the structure refinements in the monoclinic <i>P</i>2₁<i>/c</i> space group. Metavauxite structure displays a complex framework consisting of a stacking of [Al(PO₄)₃(OH)(H₂O)₂]^7&#8722; layers linked to isolated [Fe(H₂O)₆]²⁺ cationic octahedral complex solely by hydrogen bonding. The hydrogen-bonding scheme was inferred from bond-valence calculations and donor-acceptor distances. Accordingly, strong hydrogen bonds, due to four coordinated H₂O molecules, bridge the [Fe(H₂O)₆]²⁺ units to the Al/P octahedral/tetrahedral layer. The hydroxyl group, coordinated by two Al atoms, contributes to the intra-layer linkage. FTIR and Raman spectra in the high-frequency region (3700&#8722;3200 cm^&#8722;1) are very similar, and show a complex broad band consisting of several overlapping components due to the H₂O molecules connecting the isolated Fe(H₂O)₆ and the adjacent Al/P octahedral/tetrahedral layers. A sharp peak at 3540 cm^&#8722;1 is assigned to the stretching mode of the OH group. The patterns collected in the low-frequency region are dominated by the stretching and bending modes of the PO₄^3&#8722; group and the metal-oxygen polyhedra.