The structure of six-line ferrihydrite

Nanocrystalline ferrihydrite (bulk formula 5Fe2O3 · 9H2O) synthesized by hydrolysis of Fe(III) nitrate at 348 K was investigated by neutron diffraction at 5, 293 and 323 K. The structure is refined as a sum of a defect-free phase (space group P¯ 31c) and a defective phase (space group P3) based on earlier discussed models from X-ray diffraction re- sults on a six-line ferrihydrite. The size of the nanoparticles was found to be 2.7 (0.8) nm. The material reveals antiferro- magnetic order below TN = 330(20) K with collinear Fe spins (sequence +−+ −) perpendicular to the trigonal axis. The ordered moment is 3.2 µB/Fe at 5 K.