Laser-Heated Synthesis of Novel High-Pressure Carbide Phases Under Extreme Pressures

Transition metal carbides have proven to be an industrially important class of materials with outstanding resilience properties arising from their unique mixture of covalent, ionic, and metallic bonding. These properties are found in the high tensile strength of pearlitic steel, which derives from the small amounts of iron cementite (Fe3C) at the grain boundaries; the extreme hardness and chemical resistance of tungsten carbide (WC) used in deep-earth drills; and found in the heat resistance of silicon carbide (SiC) fabric woven around NASA’s inflatable reentry systems. Here we report on a combined computational and experimental approach to the high-pressure syntheses of novel transition metal carbides. In particular, we showcase improved methods for the precise control of precursor stoichiometry, which enables selective synthesis within a rich (crowded) phase space. We used first-principles calculations to determine that cobalt cementite Co3C should become synthetically accessible under high pressures, and then we experimentally pursued this metastable phase using laser-heated diamond anvil cell synthesis. The formation of Co3C was observed, and its bulk modulus was measured to be 273 GPa [1]. We then targeted metastable phases in more complicated binary systems that already contain multiple low enthalpy phases, beginning with the Cr-C system. First-principles calculations revealed that metastable Cr3C would join the three known ambient phases on the convex hull at high pressures. To target the experimental synthesis of this phase, we implemented a novel technique for the preparation of sample precursors that gave us greater stoichiometric control over our samples. This high precision of chemical composition allowed us to carefully target individual phases within a complex landscape containing many competing phases, and thereby more accurately map out the phase diagram.

References

[1] Marshall, P. V.; Alptekin, Z.; Thiel, S. D.; Smith, D.; Meng, Y.; Walsh, J. P. S. “High-Pressure Synthesis of Bulk Cobalt Cementite, Co3C”. Chem. Mater., 2021, 33, 9601–9607.