Discovery and Characterization of a Metastable Cubic Interstitial Nickel-Carbon System with an Expanded Lattice

Metastable, i.e., kinetically favored but thermodynamically not stable, interstitial solid solutions of carbon in iron are well-understood. Carbon can occupy the interstitial atoms of the host metal, altering its properties. Alloying of the host metal results in the stabilization of the FeC phases, widening its application. Pure nickel finds niche applications, mainly focusing on catalysis, while nickel alloys are widely applied, e.g., in gas turbines, reactors, and seawater piping. Nickel carbide (Ni₃C) is the well-known stable Ni-C system displaying a rhombohedral (R3̅c) crystal structure. Some reports describe an elusive cubic Ni-C system, observed during certain catalytic reactions occurring on nickel and formed by the occupation of the interstitials of the metal with carbon: to date, the stabilization and characterization of this phase have not been accomplished. Hereby, we report on the synthesis of a cubic metastable NiC phase using chemical vapor deposition of methane on supported nickel nanoparticles. The structure was predicted by DFT/ReaxFF, synthesized and monitored with in situ time-resolved synchrotron XRD, and experimentally confirmed by Rietveld refinement and (S)TEM-EELS under ambient conditions. The results show an Fm3̅m phase with a lattice parameter of a = 3.749 ± 0.037 Å at room temperature, with the highest ever reported atomic percentage of carbon occupying the octahedral interstices of 23.1%, resulting in a NiC_0.3 phase. The degree of occupation of the interstitial voids by carbon can be controlled, enabling the tuning of the host metal's d-spacing and composition, highlighting the applicability of this synthesis route for catalytic nanoparticle preparation.