The structural, electronic, and elastic properties of Fe-P binary compounds under extreme pressures: First-principles calculations

The structural, electronic and elastic properties of Fe-P binary compounds under the pressure range of 60–360 GPa are investigated using first-principles calculations. Firstly, based on the calculations of formation enthalpy ($∆H$) and mechanical stability criteria, it is proved that FeP and Fe₁₅P do not undergo phase transition, and they are stable within the pressure range of 60–360 GPa. It is noteworthy that Fe₂P and Fe₃P suffer phase transition, and they remain stable within each phase. Secondly, the formation of chemical bonds between P and Fe atoms in Fe₁₅P is demonstrated through Bader charge and Electron Localization Function (ELF). The electronic properties of Fe-P binary compounds are examined using Partial Density of States (PDOS) and band structures. Additionally, the presence of P elements is found to reduce the density ($\rho$) of these compounds, bringing Fe₁₅P and Fe₃P closer to the Preliminary Reference Earth Model (PREM) data. On the one hand, bulk modulus ($B_H$) and shear modulus ($G_H$) of Fe-P compounds increase with increasing pressure, but remain lower than those of pure iron. On the other hand, the compressional sound velocity ($V_P$) of the Fe₃P and Fe₁₅P in the pressure range of 135–360 GPa, which is lower than that of pure iron and closer to the PREM data. Moreover, the shear sound velocity ($V_S$) of the Fe-P binary compounds at 330–360 GPa, which is also lower than that of pure iron and closer to the PREM data. The Poisson's ratio ($\sigma$) of FeP, Fe₂P and Fe₃P is consistently higher than that of pure iron, but aligns more closely with PREM data under pressure of the Earth's inner core. Lastly, the $\rho$, $\sigma$, $V_S$ and $V_P$ of Fe₃P can all match PREM data. Therefore, Fe-P binary compounds may serve as a suitable model for the Earth's core, suggesting that the light elements P could potentially exist within the Earth's core.