Electrical transport in Na, K, Rb, and Cs fullerides: Phase formation, microstructure, and metallicity

The electrical properties of the alkali-metal fullerides have been investigated via parallel resistivity measurements and photoelectron spectroscopy. Phase formation associated with the incorporation of Na, K, Rb, and Cs into ${\mathrm{C}}_{60}$ films is reflected by the dependence of resistivity on temperature and alkali-metal concentration. Photoemission spectra show the details of the filling of states derived from the lowest unoccupied molecular level and the consequences of electron correlation. For K-${\mathrm{C}}_{60}$ and Rb-${\mathrm{C}}_{60}$ films, the resistivity and photoemission results indicate metallic character only for the ${\mathit{A}}_3$ ${\mathrm{C}}_{60}$ phase. Changes in the temperature coefficient of resistivity for Rb-${\mathrm{C}}_{60}$ as a function of doping reveal the granular nature of the film and show the transition from insulating to metallic character. In contrast, the Na and Cs fullerides are insulators for all concentrations. The temperature-dependent resistivity results for ${\mathrm{Na}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ show a phase transformation at 226 K for x between two and three, but neither phase is metallic.