Synthesis and Crystal Structure of Maricite and Sodium Iron(III) Hydroxyphosphate

Maricite and sodium iron hydroxyphosphate (SIHP) are recently discovered iron(II) and iron(III) compounds that play a major role in phosphate “hideout” and corrosion in high-pressure boilers. This paper reports a novel method for synthesizing maricite by thermally decomposing the complex of aqueous iron(III) nitrilotriacetic acid at 200 °C and methods for the hydrothermal synthesis of SIHP from Fe3O4 or FePO4. The crystal structure of maricite is identical to the impure natural mineral (Le Page Y.; Donnay G. Can. Mineral. 1977, 15, 518−521). The X-ray diffraction pattern of SIHP is consistent with an orthorhombic unit cell containing 8 units of formula Na3Fe(PO4)2·(Na2(1-x)H2xO), with x = 0.226 ± 0.025. This is similar, but not identical, to the formula Na4Fe(OH)(PO4)2·1/3NaOH proposed by Ziemniak and Opalka (Proc. Sixth Int. Symp. on Environmental Degradation of Materials in Nuclear Power SystemsWater Reactors; Gold, R. E., Simonen, E. P., Eds.; Mater., Metals, Minerals Soc., 1993; pp 929−935.). The main structural feature is a chain of iron(III) ions linked by bridging oxygens and phosphate bridges. The iron phosphate chains are held together by sodium ions in the ratio 3Na/Fe/2(PO4). Charge balance with the O2- bridge between each iron ion is maintained by Na+ and H+ ions located in a relatively open cage in the phosphate lattice. The structure is significant in that it explains the variable stoichiometry observed in powder diffraction patterns, and it identifies the stoichiometry of H and O, which cannot be determined from solubility studies.