Structure of synthetic 6-line ferrihydrite by electron nanodiffraction

Single-crystal electron diffraction patterns of nanocrystals in two samples of synthetic six-line ferrihydrite (6LFh) were obtained using electron nan odiffraction to produce diffraction patterns from areas between similar to0 .7 and 5 nm across. One of the samples was synthesized at 75 degreesC by a conventional method; the other was synthesized by a recently developed room -temperature technique. Structures of individual nanocrystals were investig ated by comparing experimental and simulated electron-diffraction patterns. The most common structure in each sample is based on double-hexagonal (ABA C) stacking of close-packed oxygen layers, and is similar to the "defect-fr ee ferrihydrite" structure proposed by Drits and co-workers. Nanocrystals w ith maghemite- or magnetite-like structures and structures similar to those in two-line ferrihydrite (2LFh) occur less commonly in both samples. The s ample synthesized at room temperature has a small amount of hematite, which was not found in nanodiffraction patterns from the 75 degreesC sample. The most common structure in 6LFh is distinct from those previously determined by nanodiffraction of 2LFh, indicating that 6LFh is not simply a more crys talline form of 2LFh.