Enhancing Properties with Distortion: A Comparative Study of Two Iron Phosphide Fe 2 P Polymorphs

Kong, Seongyoung; Singh, Prashant; Sarkar, Arka; Viswanathan, Gayatri; Kolen’ko, Yury V.; Mudryk, Yaroslav; Johnson, Duane D.; Kovnir, Kirill

doi:10.1021/acs.chemmater.3c03003

DOE OSTI · 2309758

Enhancing Properties with Distortion: A Comparative Study of Two Iron Phosphide Fe 2 P Polymorphs

Kong, Seongyoung·Singh, Prashant·Sarkar, Arka·Viswanathan, Gayatri·Kolen’ko, Yury V.·Mudryk, Yaroslav·Johnson, Duane D.·Kovnir, Kirill

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Abstract

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. Further, 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.

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY Crystal structure Diffraction Elements Granular materials Phase transitions

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Kong, Seongyoung, Singh, Prashant, Sarkar, Arka, Viswanathan, Gayatri, Kolen’ko, Yury V., Mudryk, Yaroslav, Johnson, Duane D., Kovnir, Kirill. 2024-01-24. Enhancing Properties with Distortion: A Comparative Study of Two Iron Phosphide Fe 2 P Polymorphs. https://doi.org/10.1021/acs.chemmater.3c03003

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