Enhancing Properties with Distortion: A Comparative Study of Two Iron Phosphide Fe₂P Polymorphs

Iron phosphide (Fe 2 P) crystallizes in its own hexagonal crystal structure type ( h -Fe 2 P). As found in meteorites, orthorhombic polymorph ( o -Fe 2 P) was originally reported as a high-temperature and high-pressure phase. Recently, o -Fe 2 P was described as being stable at ambient pressure, yet no synthetic methods were developed for single-crystal growth or single-phase bulk powder synthesis. Here, we report a successful method for growing o -Fe 2 P single crystals and synthesizing phase-pure polycrystalline samples using tin-flux. In situ powder X-ray diffraction studies showed that the phase transition from o -Fe 2 P to h -Fe 2 P occurs at about 873 K, and below that temperature, the formation of the o -Fe 2 P phase is favored thermodynamically rather than kinetically. Systematic comparison of transport, magnetic, and electrocatalytic properties of both h -Fe 2 P and o -Fe 2 P phases showed a substantial impact of the crystal structure on properties. The orthorhombic structural distortion resulted in considerable changes in magnetic properties, with the o -Fe 2 P phase exhibiting a 60% lower Fe magnetic moment and a substantially higher ferromagnetic Curie temperature than h -Fe 2 P. Electrochemical measurements toward the hydrogen evolution reaction in acidic media showed that the o -Fe 2 P phase requires an 80 mV lower overpotential than the h -Fe 2 P phase to generate a current density of −10 mA/cm 2, and their electronic structures suggest that the higher density of states at the Fermi energy is the origin of superior catalytic activity in o -Fe 2 P.