Ferrimagnetic cagelike ${\text{Fe}}_4{\text{O}}_6$ cluster: Structure determination from infrared dissociation spectroscopy

Cationic iron-oxide clusters of several sizes and stoichiometries have been synthesized and studied isolated in the gas phase. Vibrational spectra of the clusters have been measured using resonant IR-induced dissociation of ${\text{Fe}}_n\text{O}_{m+2}{}^{+}\to {\text{Fe}}_n\text{O}_m{}^{+}+O_2$ in the $250–1250{\text{cm}}^{-1}$ range. Density-functional theory was used to investigate the geometry and spin configuration of the representative ${\text{Fe}}_4\text{O}_6{}^{0/+}$ cluster. Its lowest-energy state was found to be an almost tetrahedral cagelike structure with a ferrimagnetic arrangement of spins, resulting in total cluster spin of $S=5$ for the neutral cluster. These results were confirmed for ${\text{Fe}}_4\text{O}_6{}^{+}$ by the comparison of the calculated infrared spectrum to the experimentally obtained one.

Figure 1

(Color online) (a) An example mass spectrum of ${\text{Fe}}_n\text{O}_m{}^{+}$ ions. (b) Part of the mass spectrum of ${\text{Fe}}_n\text{O}_m{}^{+}$ ions illustrating its change under the influence of the IR laser radiation. The structure of the mass peaks is due to the natural isotope distribution of iron, which is illustrated by the calculated isotope pattern of ${\text{Fe}}_4\text{O}_6{}^{+}$. The inset shows the dependence of the peak intensities on the FELIX frequency.

Figure 2

(Color online) Vibrational spectra of several iron oxide cluster cations obtained via resonant dissociation of the clusters with the excess of oxygen. Cluster compositions are indicated in the figure. For comparison, bulk phonon frequencies of magnetite ${\text{Fe}}_3{\text{O}}_4$ (below the line) and hematite ${\text{Fe}}_2{\text{O}}_3$ (above the line) are indicated in the upper part of the figure.

Figure 3

(Color online) (a) Comparison of the experimental IR spectrum with the calculated IR absorption spectra for the two cationic isomers. The calculated spectra are folded with a Gaussian line-shape function of $10{\text{cm}}^{-1}$ full width at half maximum. The dashed curve shows the calculated vibrational spectrum for a neutral ${\text{Fe}}_4{\text{O}}_6$ cagelike cluster. (b) Relative energy versus spin state for the two low-energy geometries of the neutral ${\text{Fe}}_4{\text{O}}_6$ cluster.