Crystal structure of ${\mathrm{Rb}}_4{\mathrm{C}}_{60}$ under pressure: X-ray diffraction experiments

We show that ${\mathrm{Rb}}_4{\mathrm{C}}_{60}$ transforms from its orientationally disordered tetragonal structure at ambient pressure to an orthorhombic phase in the neighborhood of $0.4\mathrm{GPa}$. Lattice parameters, interfullerene distances, and closest Rb-C distances evolve continuously up to $2.2\mathrm{GPa}$. Rietveld refinements establish that the high pressure phase is isostructural to ${\mathrm{Cs}}_4{\mathrm{C}}_{60}$. The previously observed conducting phase at $0.8\mathrm{GPa}$ is therefore structurally distinct from the ambient pressure insulator.

Figure 1

Evolution of synchrotron x-ray diffraction pattern of ${\mathrm{Rb}}_4{\mathrm{C}}_{60}$ as a function of applied pressure collected with sample inside a diamond anvil cell at the Advanced Photon Source. The data was collected using x-rays of wavelength $0.825\mathrm{\AA }$. A small quantity $(\sim 7\mathrm{wt}\%)$ of ${\mathrm{Rb}}_3{\mathrm{C}}_{60}$ is present in the starting sample.

Figure 2

The orientationally ordered $Immm$ structure of ${\mathrm{Rb}}_4{\mathrm{C}}_{60}$ under pressure.

Figure 3

Rietveld refinement of ${\mathrm{Rb}}_4{\mathrm{C}}_{60}$ at $1\mathrm{GPa}$. The observed pattern is denoted by dots, the calculated and difference plots are shown in solid lines. Both peak markers for ${\mathrm{Rb}}_3{\mathrm{C}}_{60}$ (lower row) and ${\mathrm{Rb}}_4{\mathrm{C}}_{60}$ are shown.

Figure 4

Lattice parameters, interfullerene distances, and closest Rb-C distances of ${\mathrm{Rb}}_4{\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3{\mathrm{C}}_{60}$ as a function of pressure. Solid lines are guides to the eye. In the lower two panels, squares represent ${\mathrm{Rb}}_4{\mathrm{C}}_{60}$ and triangles, ${\mathrm{Rb}}_3{\mathrm{C}}_{60}$.