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

Abstract

Metastable, <i>i.e.</i>, 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_<i>x</i> phases, widening its application. Pure nickel finds niche applications, mainly focusing on catalysis, while nickel alloys are widely applied, <i>e.g.</i>, in gas turbines, reactors, and seawater piping. Nickel carbide (Ni₃C) is the well-known stable Ni-C system displaying a rhombohedral (<i>R</i>3̅<i>c</i>) 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_<i>x</i> phase using chemical vapor deposition of methane on supported nickel nanoparticles. The structure was predicted by DFT/ReaxFF, synthesized and monitored with <i>in situ</i> time-resolved synchrotron XRD, and experimentally confirmed by Rietveld refinement and (S)TEM-EELS under ambient conditions. The results show an <i>Fm</i>3̅<i>m</i> phase with a lattice parameter of <i>a</i> = 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 <i>d</i>-spacing and composition, highlighting the applicability of this synthesis route for catalytic nanoparticle preparation.