Prediction of a new high pressure phase in Fe3P

Iron-rich phosphorus minerals, such as schreibersite (Fe,Ni)3P and nickelphosphide (Ni,Fe)3P with space group I-4 and Z=8, often occur in iron meteorites, and are considered as possible components in the earth's core. However, the high pressure phase of Fe3P is indefinite, which makes it difficult to discuss the structures and properties of iron-rich phosphorus alloys under high pressure. Scott et al. (2007, 2008) reported that above 17 GPa after laser heating, Fe3P develops a new structure with a strong diffraction line at a d-spacing around 2.02 Å. Although the structure of the high-pressure polymorph is unidentified, the closest-matched one is from the orthorhombic system. To seek the possible high pressure phase of Fe3P, we carried out ab initio calculations to discuss the stabilities of Fe3P in several structures, i.e., the AuCu3-type structure (Fm-3m), the cementite structure (Pnma) and the high-temperature polymorph of AlCu3 structure (P2mm). The first-principle calculations are based on the density functional theory (DFT), with highly accurate projector augmented wave method (PAW) as implemented in Vienna ab-initio simulation package (VASP). The local spin density approximation (LSDA) plus Hubbard U method was used. Results indicate that both AuCu3-type and cementite structure are unstable under moderate pressure (Fig. 1). And the cementite structure (Pnma) distorted to Cmcm structure automatically during the calculation. In contrast, the P2mm structure shows a stable zone above 16.4 GPa. The P2mm structure has the strongest diffraction line at a d-spacing of 1.89 Å around 20 GPa, which is close to experimental data. No soft phonon of this structure was observed up to 49 GPa which confirms the stability of the new structure under high pressure.