We found a new phase transition in Fe2P from Co2P-type (C23) to Co2Si-type (C37) structure above 42 ± 2 GPa based on in situ X-ray diffraction experiments. While these two structures have identical crystallographic symmetry, the orthorhombic unit cell is shortened in a-axis but elongated in c-axis, the coordination number of phosphorous increases from nine to 10, and the volume reduces by 2% across the phase transition. The new C37-type Fe2P phase has been found to be stable, at least to 83 GPa at high temperature. The Birch-Murnaghan equation of state for C37 Fe2P was also obtained from pressure-volume data, suggesting that phosphorous contributes to 17% of the observed density deficit of the Earth's outer core when it includes the maximum 1.8 wt% P as observed in iron meteorites. In addition, since both Fe2S and Ni2Si are also known to have the C37 structure under high pressure, (Fe,Ni)2(S,Si,P) could have wide solid solution and constitute planetary iron cores, although it is not dense enough to be a main constituent of the Earth's inner core.

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