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 pressuresPnnm-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.
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