Iron bcc-hcp transition: Local structure from x-ray-absorption fine structure

The x-ray-absorption fine-structure technique has been utilized to investigate the pressure-induced bcc-hcp transition in iron from a local structural perspective. Our measurements of the associated lattice constants show a structurally distorted hcp phase with an anomalously large $c/a$ ratio during the transition as reported by previous x-ray-diffraction measurements. We have also observed a structurally distorted bcc phase with an anomalously large lattice constant during the transition. These anomalies are attributed to interfacial strain between the bcc and hcp phases. A transition model based on lattice shearing movements and occurrence of an intermediate fcc structure is consistent with our data and previous results.