FePd crystalline alloy with chemical stoichiometry of 1:1 is important permanent magnets and ultrahigh magnetic recording material, however there is few reports on its stable crystal structures under normal and high pressures. Thus, the stable crystal structures of FePd alloy were explored here by using particle swarm optimization algorithm and the soft-mode phase transition theory. Four new phases of FePd were discovered, namely, Imma, Cmmm-I, Cmmm-П, and P4/nmm. The structure, magnetisim, elasticity and lattice dynamics of the system were systematically investigated by first-principles calculations. It was found that the ferromagnetic state of these structures is more energetically favorable than the non-magnetic state below their respective critical pressure of ferromagnetic collapse. The phonon dispersion curves have no imaginary frequency and elastic constants fulfill Born’s criteria demonstrated that these four structures are dynamically and mechanically stable at pressure range of 0 to 59.1 GPa. These four new phases were found have negative formation enthalpy and the three orthorhombic structures were identified to be meta-stable in the pressure range of 0 to 59.1 GPa since they possess very small enthalpy difference which could not be firmly distinguished by our calculations relative to the experimentally known L10 phase. After 59.1 GPa, P4/nmm structure became the energetically most stable structure accompanied by simultaneous collapse of magnetic moments and lattice volume. The underlying mechanism was analyzed and attributed to be the simultaneously collapse of magnetoelastic force of Fe-Fe.
