Structural and electronic properties of ${\left(\mathrm{N}{\mathrm{H}}_3\right)}_x{\mathrm{K}}_3{\mathrm{C}}_{60}$

We have investigated the synthesis, structure, and electronic properties of ${\left(\mathrm{N}{\mathrm{H}}_3\right)}_x{\mathrm{K}}_3{\mathrm{C}}_{60}$ by exposing preformed ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ to N${\mathrm{H}}_3$ vapor. Measurements were made on bulk powders and films with a variety of techniques including in-situ x-ray diffraction and monitoring of N${\mathrm{H}}_3$ pressure, Raman scattering and ac susceptibility. The reaction of N${\mathrm{H}}_3$ with ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ is completely reversible and leaves the ${\left({\mathrm{C}}_{60}\right)}^{3-}$ charge state intact, while allowing the distance between the ${\mathrm{C}}_{60}$ molecules to vary. We observe two different crystallographic structures at $x=1\mathrm{}$ and $x=8-10\mathrm{}$. The $x=1\mathrm{}$ structure is face-centered orthorhombic with a single N${\mathrm{H}}_3$ on the octahedral interstitial site. It is single phase at 100°C and N${\mathrm{H}}_3$ pressures ≥400 torr and remains single phase at 0 torr if the temperature is reduced to 25°C. Although (N${\mathrm{H}}_3$)${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ exhibits no superconductivity at ambient pressure, superconductivity can be induced by hydrostatic pressure with an onset at 28 K. The details of the $x=8-10\mathrm{}$ structure are uncertain, however it appears that this phase is body-centered tetragonal with N${\mathrm{H}}_3$ on both the octahedral and tetrahedral sites. It occurs at room temperature and N${\mathrm{H}}_3$ pressures ≥500 torr, but is not stable at lower N${\mathrm{H}}_3$ pressures. Resistivity measurements of N${\mathrm{H}}_3$-doped ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ films show that the formation of the new structure results in a rapid increase of the resistivity to values exceeding 1 Ω cm.