High-Pressure Synthesis of Bulk Cobalt Cementite, Co₃C

Abstract

Transition metal carbides find use in a wide range of advanced high-resilience applications including high-strength steels, heat shields, and deep-earth drills. However, carbides of the mid-to-late transition metals remain difficult to isolate and characterize on account of their metastability, which precludes the preparation of high-quality bulk single crystal samples using traditional solid-state methods. Herein, we report a combined computational and experimental survey of the cobalt–carbon binary system under high pressures and demonstrate that pressure offers a route toward the bulk synthesis of the metastable cementite-type cobalt carbide, Co3C, which under ambient conditions can only be prepared in low-dimensional thin film or nanoparticle forms. First-principles calculations reveal two competitive low-energy stoichiometric phases under ambient pressuresPnnm-Co2C (Fe2C-type) and Pnma-Co3C (Fe3C-type)consistent with the known low-dimensional phases that have been studied for their promising magnetic properties. However, the calculated formation enthalpy of Pnma-Co3C decreases steadily with the applied pressure, while that of Pnnm-Co2C increases. We pursue these results using high-pressure laser-heated synthesis methods coupled with in situ X-ray diffraction and observe the formation of Pnma-Co3C above 4.8 GPa. We determine the experimental bulk modulus of Co3C to be K 0 = 237 GPa (K p = 4.0). First-principles calculations of the phonon modes in Co3C reveal dynamical instabilities at ambient pressure that are absent under compression. These results offer a promising new route for the synthesis of rare-earth-free magnets.

Combined First-Principles and Experimental Investigation into the Reactivity of Codeposited Chromium–Carbon under Pressure

“…Flakes isolated from the cosputtered samples measuring approximately 90 μm × 90 μm were sandwiched between MgO disks (125 μm in diameter). 31 The MgO disks were loaded into the sample space of rhenium gaskets (originally 250 μm thick) that were preindented to a thickness of 35−45 μm and EDM-drilled to host a 127 μm sample space in the center of the indent. DACs were taken to pressure in our argon-filled glovebox to ensure that the sample space was sealed with an inert environment.…”

“…To address this challenge, we turned to a high-pressure synthesis. This approach has proven to be one of the most productive methods for the bulk synthesis of metastable compounds that cannot be accessed with traditional heating methods, and in fact, it has already had demonstrable success in the discovery of bulk transition metal carbide phases. − …”

Synthesis of large-sized spherical Co–C alloys with soft magnetic properties though a high-pressure solid-state metathesis reaction

“…Consequently, the production of single-phase (Co 3 C) samples will be kinetically favorable in high pressure environments. 28 Under experimental conditions of 7 GPa and 1400-1500 °C, S. Naka et al reported absence of stoichiometric cobalt carbide when cobalt was used as a catalyst for diamond synthesis. 29 Absence of cobalt carbides was shown also by W. Utsumi et al in their in situ X-ray diffraction study of graphite-to-diamond transformation using various solvent-catalysts under high pressure and high-temperature conditions.…”

“…5,14 Co 3 C nanoparticles have been synthesized using methods like wet chemical synthesis, 5,23,24 mechanical alloying, 3,6,25 ion irradiation, 26 and the one-pot template-free solvothermal approach. 27 P. V. Marshall et al utilized a DAC (diamond anvil cell) device with a size of 300 mm to synthesize microscale Co 3 C under conditions of 5-15 GPa and 1200 K. 28 However, stable carbides are not present in the Co-C system. Instead, metastable cobalt carbides are commonly found in mixed forms.…”

“…Consequently, the production of single-phase (Co 3 C) samples will be kinetically favorable in high pressure environments. 28 …”