Absence of an insulator-metal transition inRb4C60up to2GPa

We present the results of direct resistance measurements on ${\mathrm{Rb}}_{4}{\mathrm{C}}_{60}$ under pressures up to $2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. At all pressures covered by this study and over the temperature range of $90--450\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ ${\mathrm{Rb}}_{4}{\mathrm{C}}_{60}$ is a semiconductor with a weakly pressure dependent band gap near $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We do not observe the insulator-to-metal transition previously reported to occur below $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, although we cannot rule out the possibility that such a transition might occur at some significantly higher pressure. The measured resistivity is surprisingly low and is dominated by carriers excited over a $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ gap. Because the corresponding conductivity increases with deformation of the sample, we assign these states to structural or orientational defects. The known structural transformation below $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ leads to a decrease in resistivity under high pressure, but the material remains semiconducting. A ${\mathrm{Rb}}_{6}{\mathrm{C}}_{60}$ control sample showed a similar behavior, also being a semiconductor under all conditions studied. At temperatures above $460\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, Rb was partially lost from our samples, resulting in metallization by a transformation into ${\mathrm{Rb}}_{3}{\mathrm{C}}_{60}$.