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.
NaCl‐type carbides of the early transition metals can exhibit a substantial sub‐stoichiometry at the carbon site, impacting a host of bulk properties that depend upon carbon concentration including melting points, mechanical and elastic properties, and superconducting transition temperatures. Unfortunately, control over vacancies remains challenging with current preparation methods, motivating the search for new synthetic approaches that will allow for the prescription of specific vacancy configurations. Here, density functional theory is augmented with alloy cluster expansion to examine the structure and zero kelvin enthalpy of millions of structures across composition and pressure space. The results are used to examine how extreme pressures might be used to access novel ordered and disordered phases of ZrC, many of which have been calculated to be thermodynamically stable yet remain synthetically elusive. High pressure is shown to significantly reduce sub‐stoichiometry and drive the system toward fully stoichiometric ZrC. They examine the root of these changes and find that pressure exerts an influence over the distribution and abundance of specific nearest‐neighbor vacancy pairs. These results suggest that pressure is a powerful tool for the control of vacancies, and can offer a new synthetic handle on the bulk properties exhibited in this industrially important class of materials.
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