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

Iron phosphide (Fe2P) crystallizes in its own hexagonal crystal structure type (h-Fe2P). As found in meteorites, orthorhombic polymorph (o-Fe2P) was originally reported as a high-temperature and high-pressure phase. Recently, o-Fe2P 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-Fe2P 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-Fe2P to h-Fe2P occurs at about 873 K, and below that temperature, the formation of the o-Fe2P phase is favored thermodynamically rather than kinetically. Systematic comparison of transport, magnetic, and electrocatalytic properties of both h-Fe2P and o-Fe2P 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-Fe2P phase exhibiting a 60% lower Fe magnetic moment and a substantially higher ferromagnetic Curie temperature than h-Fe2P. Electrochemical measurements toward the hydrogen evolution reaction in acidic media showed that the o-Fe2P phase requires an 80 mV lower overpotential than the h-Fe2P phase to generate a current density of −10 mA/cm2, and their electronic structures suggest that the higher density of states at the Fermi energy is the origin of superior catalytic activity in o-Fe2P.