Electrical Resistivity of Iron Phosphides at High-Pressure and High-Temperature Conditions With Implications for Lunar Core's Thermal Conductivity

Based on cosmochemistry evidence and element partitioning experiments, phosphorus is thought to be present in the iron-rich cores of Earth and Moon. Phosphorus has a similar effect as silicon and sulfur on the electrical and thermal transport properties of iron at core conditions. However, the magnitude of the impurity scattering caused by phosphorus, the temperature dependence of iron phosphorus compounds, and the change across melting all have not been intensively investigated. We measured the electrical resistivity of Fe3P, Fe2P, and FeP using a four-wire method at 1.3 to 3.2 GPa and temperatures up to 1800 K. We also identify the melting temperatures of FeP, Fe2P, and Fe3P by sudden changes in resistivity upon heating. The present experimental results demonstrate that phosphorus can enhance the electrical resistivity of iron more effectively than silicon. The resistivity of iron phosphides decreases with increasing pressures and decreasing phosphorus content. The resistivity of Fe-P alloys obeys the Matthiessen's rule, which describes the positive linear correlation between resistivity and phosphorus content. This finding is comparable to previously observed atomic order-disorder in Fe-Si and Fe-C systems. Furthermore, the resistivity of liquid Fe2P and Fe3P shows a negative linear correlation with temperatures. Different from pure iron, the calculated thermal conductivity of Fe3P increases by 33% upon melting. It is speculated that the thermal conductivity of the lunar solid inner core may be much lower than that of the liquid outer core when ordered iron light element compounds (e.g., Fe3C and Fe3P) are present in the solid core.