Structure analysis and stability field of β-iron at highPandT

Abstract

New synchrotron X-ray diffraction data confirm our previous report of the transformation of the hexagonal close-packed (hcp) phase of iron to the Pbcm orthorhombic lattice (β-iron) at high P and T. The volume differences between the e and β, and the β and γ 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 b-iron is not a stable phase. Metastability is possible because the gliding of the same dense atomic layers is involved in both T-induced ε-hcp to γ-fcc and ε-hcp to β-Pbcm transformations. These observations explain why a controversy exists on the structure and P-T stability field of β-iron. From our set of experiments, we estimate that Pbcmiron is stable above 35 GPa and 1500 K, and that the (γ, β, liquid-iron) triple point is located at about 55 GPa and 2400 K