Crystal structure of Rb$_{4}$C$_{60}$ under pressure

We show that Rb$_{4}$C$_{60}$ transforms from its orientationally disordered tetragonal structure at ambient pressure to an orthorhombic phase in the neighborhood of 0.4 GPa. Lattice parameters, interfullerene distances, and closest Rb-C distances evolve continuously up to 2.2 GPa. Rietveld refinements establish that the high pressure phase is isostructural to Cs$_{4}$C$_{60}$. The previously observed conducting phase at 0.8 GPa is therefore structurally distinct from the ambient pressure insulator.