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

¹ ESA-7046, Géomateriaux, Institut de Physique du Globe, Paris 75252, France.
² UMR-5570, Géologie, Ecole Normale Supérieure de Lyon, Lyon 69364, France.
³ European Synchrotron Radiation Facility, ID30, Grenoble 38043, France.

Correspondence: ^* E-mail: andrault{at}ipgp.jussieu.fr

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 (ß-iron) at high P and T. The volume differences between the and ß, and the ß and 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 ß-iron is not a stable phase. Metastability is possible because the gliding of the same dense atomic layers is involved in both T-induced -hcp to -fcc and -hcp to ß-Pbcm transformations. These observations explain why a controversy exists on the structure and P-T stability field of ß-iron. From our set of experiments, we estimate that Pbcm-iron is stable above 35 GPa and 1500 K, and that the (, ß, liquid-iron) triple point is located at about 55 GPa and 2400 K.

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