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$\bar\Box3\Box$1c) and a defective phase (space group P3) based on earlier discussed models from X-ray diffraction results on a six-line ferrihydrite. The size of the nanoparticles was found to be 2.7 (0.8) nm. The material reveals antiferromagnetic order below T_N=330(20) K with collinear Fe spins (sequence + - + -) perpendicular to the trigonal axis. The ordered moment is 3.2 μ_B/Fe at 5 K.