- journal_title：American Mineralogist
- Contributor：Denis Andrault ; Guillaume Fiquet ; Thomas Charpin ; Tristan le Bihan
- Publisher：Mineralogical Society of America
- Date：2000-
- Format：text/html
- Language：en
- journal_abbrev：American Mineralogist
- issn：0003-004X
- volume：85
- issue：2
- firstpage：364
- section：Articles
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 ε 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 β-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 Pbcm-iron 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.