Structure analysis and stability field of β-iron at high P and T

New synchrotron X-ray diffraction data confirm our previous report of the transformation of thr hexagonal close-packed (hcp) phase of iron on to the Pbcm orthorhombic lattice (beta-iron) at high P and ir. The volume differences between the epsilon and beta, and the beta and gamma polymorphs are determined as 1.4 and 1.8%, respectively, indicating positive Clapeyron slopes between these polymorphs in the P-T phase diagram. All three polymorphs have a similar bulk modulus between 30 and 60 GPa. The Pbcm-polymorph can be observed in a metastable state as quenched from high T at high P and also at high T for P lower than 35 GPa where beta-iron is not a stable phase. Metastability is possible because the gliding of the same dense atomic layers is involved in both T-induced F-hcp to gamma-fce and epsilon-hcp to beta-Pbcm transformations. These observations explain why a controversy exists on the structure and P-T stability field of beta-iron. From our set of experiments, we estimate that Pbcm-iron is stable above 35 GPa and 1500 K, and that the (gamma, beta, liquid-iron) triple point is located at about 55 GPa and 2400 K.