Structural and electronic properties of (NH3)xK3C60.

We have investigated the synthesis, structure, and electronic properties of ${(\mathrm{N}{\mathrm{H}}_{3})}_{x}{\mathrm{K}}_{3}{\mathrm{C}}_{60}$ 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 ${({\mathrm{C}}_{60})}^{3\ensuremath{-}}$ charge state intact, while allowing the distance between the ${\mathrm{C}}_{60}$ molecules to vary. We observe two different crystallographic structures at $x=1$ and $x=8\ensuremath{-}10$. The $x=1$ structure is face-centered orthorhombic with a single N${\mathrm{H}}_{3}$ on the octahedral interstitial site. It is single phase at 100\ifmmode^\circ\else\textdegree\fi{}C and N${\mathrm{H}}_{3}$ pressures \ensuremath{\ge}400 torr and remains single phase at 0 torr if the temperature is reduced to 25\ifmmode^\circ\else\textdegree\fi{}C. Although (N${\mathrm{H}}_{3}$)${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ exhibits no superconductivity at ambient pressure, superconductivity can be induced by hydrostatic pressure with an onset at 28 K. The details of the $x=8\ensuremath{-}10$ structure are uncertain, however it appears that this phase is body-centered tetragonal with N${\mathrm{H}}_{3}$ on both the octahedral and tetrahedral sites. It occurs at room temperature and N${\mathrm{H}}_{3}$ pressures \ensuremath{\ge}500 torr, but is not stable at lower N${\mathrm{H}}_{3}$ pressures. Resistivity measurements of N${\mathrm{H}}_{3}$-doped ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ films show that the formation of the new structure results in a rapid increase of the resistivity to values exceeding 1 \ensuremath{\Omega} cm.