<p>Iron-rich phosphides are functional materials for the hydrogen evolution reaction and also analogues of Fe-Ni-P minerals found in meteorites and pyrometamorphic rocks on Earth. They are also potential components in solid metallic cores of the Earth, Mars, and other terrestrial planets. The thermal expansion coefficients (<i>α</i>) of three iron-rich phosphides (FeP, Fe₂P, and Fe₃P) were investigated by in-situ synchrotron X-ray powder diffraction from room temperature to 1073&#xa0;K. By fitting the obtained volume-temperature (<i>V</i>-<i>T</i>) data using a function of α(<i>T</i>) = α₀ + α₁⋅<i>T</i> (Fei model), the parameters α₀ and α₁ for FeP’s volume are 2.60(22) ⋅10^− 5 K^− 1 and 3.57(36) ⋅10^− 8 K^− 2, for Fe₂P’s volume are 3.26(37) ⋅10^− 5 K^− 1 and 3.70(64) ⋅10^− 8 K^− 2, and for Fe₃P’s volume are 1.96(20) ⋅10^− 5 K^− 1 and 4.01(32) ⋅10^− 8 K^− 2, respectively. Axial thermal expansions are different in three iron phosphides. Combined with previous results, the density profiles of solid Fe₃P and FeP in the Martian core conditions were calculated and compared with those of Fe₃S, FeS and fcc Fe. Fe₃P has a similar density to Fe₃S under Martian core conditions, suggesting that phosphorus can exist in the Martian solid inner core if the core comprises iron sulfide.</p>