High-pressure stability of ${\mathrm{Cs}}_6{\mathrm{C}}_{60}$

The ${\mathrm{Cs}}_6{\mathrm{C}}_{60}$ molecular crystal has been studied by Raman spectroscopy measurements and ab initio calculations under high pressure. The Raman scattering data have been collected from ambient pressure up to $45.5\mathrm{GPa}$ at room temperature. It is shown that the intercalation of cesium atoms in the ${\mathrm{C}}_{60}$ crystalline structure allows us to preserve the ${\mathrm{C}}_{60}$ molecules up to the maximum studied pressure, i.e., more than twice the amorphization pressure of the solid ${\mathrm{C}}_{60}$ fullerene crystal. The calculated pressure evolution of all the observed Raman mode frequencies is in good agreement with that obtained experimentally. In this work, the high-resolution measurements allow us to observe six Raman modes and several partners in doublets in addition to the Raman lines previously observed for the same system. The symmetry of all the observed modes has been identified through our calculations.

Figure 1

Low-frequency (left panel) and high-frequency (right panel) Raman spectra of ${\mathrm{Cs}}_6{\mathrm{C}}_{60}$ at room temperature as a function of pressure. The modes are labeled with the names of the irriducible representation of the Raman active modes of the free ${\mathrm{C}}_{60}$ molecule for commodity. A continuous evolution of the intramolecular Raman modes under pressure is observed up to $45.5\mathrm{GPa}$. The Raman mode intensity has been normalized to the $H_g\left(3\right)$ (first line) intensity mode in the upper panel and to the $H_g\left(7\right)$ intensity mode in the lower panel. The Raman spectrum at $0.5\mathrm{GPa}$ shows the presence of a shoulder in the $A_g\left(2\right)$ mode coming from an impurity also found on the gasket.

Figure 2

Pressure dependence of the low-frequency and high-frequency Raman modes of ${\mathrm{Cs}}_6{\mathrm{C}}_{60}$ from ambient pressure up to 45.5 and $43\mathrm{GPa}$, as obtained from experiments (left panels) and calculations (right panels), respectively. The solid lines correspond to least-squares fits of a parabolic pressure dependence for most modes. The horizontal error bar represents the pressure uncertainty estimated from the Lorentzian width of the ruby R1 line; the vertical error bar is the half-width at half maximum of each Raman mode.