Synthesis of Metastable Transition Metal Carbides Using High Pressure*

The synthetic phase space opened up by high pressure is vast, and likely harbors countless undiscovered metastable materials with the potential to propel next-generation technologies. However, this phase space remains only partially explored, being limited by: (1) the difficulties involved in precisely targeting specific chemical compositions under the constraints of extreme pressure methods, which makes doped and higher-order phases difficult to access; and by (2) the high costs required to search for low-symmetry and/or high-order phases using theory-based approaches, which inherently biases searches toward well-ordered phases with small unit cells. Here, we will present results from our studies into the use of high pressures to access novel transition metal carbides (TMCs), including new approaches that aim to move us beyond the current frontiers in high-pressure materials discovery.

We will first present results from our work on the laser-heated high-pressure synthesis of metastable TMCs in the diamond anvil cell, including our discovery of bulk cobalt cementite, Co₃C. We will introduce a novel approach to precursor preparation that allows us to reliably and repeatably target specific chemical compositions in systems than contain multiple competing phases on the convex hull. We will then present results from our recent study on zirconium carbide, which under normal synthesis conditions exhibits a remarkably high degree of sub-stoichiometry at the carbon site. The use of cluster expansion methods allows us to survey a broad range of phase space at a much lower cost than would be permitted with first-principles approaches alone, revealing details on the effect that pressure has on the range of sub-stoichiometry that can be synthetically accessed.