Pressure-induced polymerization of ternary fulleride superconductors

Abstract

A reversible conversion of the cubic trianionic fulleride superconductors ${\mathrm{Na}}_2$ ${\mathrm{RbC}}_{60}$ and ${\mathrm{Na}}_2$ ${\mathrm{CsC}}_{60}$ to orthorhombic structures is observed at modest pressures (≤ 3 kbar) and ambient temperature. Structure refinement based on synchrotron powder x-ray profiles reveals polymer chains with 9.35 Å intermolecular separation, comparable to that in the metastable monoanionic dimer and polymer phases of ${\mathrm{KC}}_{60}$ and ${\mathrm{RbC}}_{60}$. In contrast, ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$, ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$, etc. are stable to much higher pressures due to their larger lattice constants at ambient pressure.