Effect of Carbon Adsorption on the Isomer Stability of ${\mathrm{Ir}}_4$ Clusters

The atomic structure and electronic properties of gas-phase and MgO(100)-supported iridium tetramers are studied using density functional theory. At variance with experimental data, the most stable ${\mathrm{Ir}}_4$ isomer on MgO(100) is the square one, as in the gas phase, and the metastable tetrahedral isomer is highly distorted by interactions with the substrate. In the presence of a single carbon adatom, the most stable structure of ${\mathrm{Ir}}_4$ is tetrahedral for both environments and the structural distortion of the adsorbed cluster is reduced. On MgO(100), the binding energy of a C adatom to tetrahedral ${\mathrm{Ir}}_4$ is 1.6 eV larger than that to the square isomer, due to strong interactions between $\mathrm{C}\mathrm{\text{−}}2p$ orbitals and a low-energy unoccupied molecular orbital of tetrahedral ${\mathrm{Ir}}_4$.

Figure 1

Side and top views of the atomic structure of ${\mathrm{Ir}}_4/\mathrm{MgO}(100)$. The tetrahedral (T), butterfly (B), and square (S) configurations are represented, together with those of the transition states (TS1 and TS2). The corresponding energies (in eV) relative to that of the T state are also indicated. Red (dark gray) and green (light gray) balls indicate O and Mg atoms, respectively. Gray balls indicate Ir atoms.

Figure 2

Carbon adsorption geometries to tetrahedral (upper panel) and square ${\mathrm{Ir}}_4$ isomers (lower panel). Free ${\mathrm{Ir}}_4$ clusters, top view and side view of MgO(100)-supported ${\mathrm{Ir}}_4$ clusters are given in columns from left to right.

Figure 3

Schematic plot of the total-energy change of gas-phase and MgO(100)-supported tetrahedral and square ${\mathrm{Ir}}_4$ isomers upon C adsorption. The length of vertical lines is proportional to the C binding energy.

Figure 4

Total (dark gray, blue online) and $\mathrm{C}\mathrm{\text{−}}2p$ projected (light gray, red online) density of states of gas-phase (a) tetrahedral ${\mathrm{Ir}}_4$, (b) square ${\mathrm{Ir}}_4$, (c) tetrahedral ${\mathrm{Ir}}_4$ with an adsorbed C atom, and (d) square ${\mathrm{Ir}}_4$ with an adsorbed C atom. Arrows indicate states resulting from the interaction of $\mathrm{C}\mathrm{\text{−}}2p$ orbitals with ${\mathrm{Ir}}_4$ states close to the Fermi energy (see text for details). Gaussian broadening with a width of 0.2 eV has been applied.

Figure 5

(a) Low-energy unoccupied orbital of free tetrahedral ${\mathrm{Ir}}_4$ and (b) the bonding orbital resulting from its interaction with $\mathrm{C}\mathrm{\text{−}}2p$ states.