Gluing together metallic and covalent layers to form Ru2C under ambient conditions

Ru 2 C has recently been synthesised at high pressure and high temperature, and was assumed to have a structure with space group P 3&cmb.macr; m 1. However, subsequent theoretical work has revealed that this structure is unstable under ambient conditions, which motivated us to look for the stable structure. In this work, we explore the structures of Ru 2 C by using an unbiased swarm structure searching algorithm. The structures with R 3 m and R 3&cmb.macr; m symmetries have been found to be lower in energy than the P 3&cmb.macr; m 1 structure, at the same time being dynamically stable under ambient conditions. These layered structures consist of alternating Ru bilayers and C monolayers in the R 3 m structure, and alternating Ru tetra-layers and C bilayers in the R 3&cmb.macr; m structure. The C layers are more evenly distributed and more covalently bound to the Ru layers in the R 3 m structure than in the R 3&cmb.macr; m structure. Instead, in the R 3&cmb.macr; m structure there exists more Ru-Ru metallic bonding, which has a crucial role in diminishing the hardness of this material. Our findings should stimulate further explorations of the structures and properties of the heavy transition metal carbides and nitrides, potentially leading to industrial applications. We explore the structures of Ru 2 C at ambient conditions by using an unbiased swarm structure searching algorithm. The structures with R 3 m and R 3&cmb.macr; m symmetries have been respectively found to have the lowest energies for one and two formula units.