First‐principles study of structural, elastic, and electronic properties of M₂₃C₆ and MC carbides (M = Ru, Rh, Pd, Os, Ir, and Pt)

Abstract

Using first-principles calculations within the density functional theory we studied the electronic structure, elastic properties, and stability of M 23 C 6 carbides, where M are the platinum-metals: Ru, Rh, Pd, Os, Ir, or Pt. The lattice constants, elastic parameters, formation energies, and densities of states of M 23 C 6 were compared with those for mono-carbides MC. We demonstrated that these carbides have the positive formation energies and predicted the mechanically stable phases. We found that M 23 C 6 carbides are energetically more favorable than the corresponding MC carbides due to the stronger M-M interactions in M 23 C 6 .Mechanically stable and unstable M 23 C 6 and MC carbides.One of the central issues for the platinum-metal carbides concerns the interplay between their stoichiometry, structure, stability, and properties. For mono-carbides MC, the competing crystal structures such as rocksalt (B1), cesium chloride (B2), ZB (B3), wurtzite (B4), nickel-arsenide (B8), tungsten carbide (WC) were discussed [4]. Attempts were made to understand the influence of stoichiometry and the M/C ratio on the stability of these carbides. The carbon-rich phases (M/C < 1) such as M 2 C 3 and MC 2 (M ¼ Os, Ir) were simulated theoretically [17,18], but as far as we know, no experimental evidences for such phases are available now.The formation of metal-rich phases (M/C > 1) seems to be more realistic, since the majority of the known monocarbides of 3d-5d metals from Groups IVa-Va have a wide homogeneity range from MC to MC 0.50 owing to carbon vacancies, whereas metal-rich carbides, such as M 23 C 6 , M 7 C 3 , M 3 C 2 , M 2 C, or M 3 C may be formed in the related systems consisting of carbon and 3d metals from Groups VIa-VIIIa [2,19,20]. Recently, the properties of substoichiometric carbides M 4 C 3 , M 2 C, M 3 C, and M 4 C (M ¼ Rh, Ir, Pd, and Pt) were simulated in [13,18,21], where the model structures (Ir 4 Ge 3 , anti-CaF 2 , anti-ReO 3 ,

Single-walled carbon nanotube growth from Ni catalyst particles under conventional growth conditions by alcohol catalytic chemical vapor deposition: in situ X-ray absorption fine structure study

“…33) Furthermore, at 800 °C, the Gibbs energy of formation of Ni carbide is ∼28 kJ/mole of carbon, which is much higher than those of Fe carbide and Co carbide, 34) and formation energies of Ir carbides are larger than 0.6 eV, which means that they are in meta-stable states. 35) Such low carbon solubilities of Ni and Ir and the high formation energies of the carbides would result in the metallic state for Ir catalysts and low carbon solubility for Ni catalysts during SWCNT growth.…”