Structure of synthetic 2-line ferrihydrite by electron nanodiffraction

Comparison of experimental single-crystal electron diffraction patterns of synthetic two-line ferrihydrite (2LFh) with simulated single-crystal electr on-diffraction patterns indicates that a synthetic 21,Fh sample contains hi ghly disordered material and nanocrystals with structures based on hexagona l (ABAB) and cubic (ABC) stacking of close-packed layers of O2- and OH- ion s. An apparently continuous variation in ordering exists between the highly disordered material and each of the crystalline structures, suggesting tha t both nanocrystalline structures represent local extremes of three-dimensi onal ordering. Experimental diffraction patterns were obtained using electr on nanodiffraction, a technique in which the finely focused beam from a fie ld-emission gun in an electron microscope can be used to produce diffractio n patterns from areas <1 nm across. Nanodiffraction patterns from the highl y disordered material have diffuse streaks rather than distinct reflections , and are consistent with a two-dimensional structure that consists of clos e-packed anionic layers with essentially complete stacking disorder and nea rly random distribution of Fe atoms. The structure with cubic stacking is s imilar to maghemite and has similar to 25% of the Fe in tetrahedral sites. The structure with hexagonal stacking consists of double chains of face-sha ring Fe octahedra; each octahedron shares one face, two edges, and three co rners with adjacent octahedra. Previous results from transmission electron microscopy, powder X-ray and electron diffraction, and synchrotron-based te chniques reflect the overall high degree of structural disorder rather than the characteristics of the maghemite-like and double-chain structures.