Sublattice specificity of magnetization and enthalpy of mixing (ΔH_mix) in (Fe, Ni)₃P solid solutions revealed by first-principles calculations

As a transition-metal phosphide and source of reactive phosphorus, the Fe-phosphide schreibersite (Fe, Ni)₃P is critical to the Earth, planetary, and materials-science communities. However, the thermodynamic properties of schreibersite as a function of composition remain unconstrained. To investigate the sublattice-specific thermochemistry of discrete solid solutions of schreibersite along the Fe₃P–Ni₃P binary, we employed first-principles density functional theory (DFT) calculations. This work examines the schreibersite structure using special quasirandom structures and a four-sublattice model to explore the connection between the enthalpy of mixing (ΔH_mix ) and the magnetization of Fe and Ni on each metal sublattice. DFT and coordination analysis suggest that schreibersite shows sublattice-specific trends in magnetization and ΔH_mix due to the interaction of Fe and Ni. Analysis of the pair distribution functions of schreibersite suggests that Fe–Ni interactions control the structural symmetry of the sublattice, with lower ΔH_mix associated with a higher number of split Fe–Ni peaks. The ΔH_mix of Fe-rich compositions is controlled primarily by Fe magnetization, where larger total magnetization on a sublattice is associated with higher ΔH_mix. In comparison, the ΔH_mix of Ni-rich compositions is controlled by the packing density of the first and second nearest neighbor cages, where a larger cage is associated with a lower ΔH_mix. Examination of the magnetic exchange energies suggests that order–disorder transition temperatures are also sublattice specific.