Ac impedance of A₄C₆₀ fullerides under pressure

Three A₄C₆₀ compounds, with A = Li, Na and K, have been studied by impedance spectroscopy between 100 and 293 K at pressures up to 2 GPa. The results are in very good agreement with earlier dc resistance studies and with data from the literature. For all three materials the measured conductivity can be fitted by a sum of at least two Arrhenius terms. The band gaps derived from the resistance data, 0.3 eV for Na₄C₆₀ and 0.5 eV for K₄C₆₀, are in excellent agreement with data measured by other methods. For Li₄C₆₀, our results disagree with a recent suggestion that the conductivity is dominated by ionic conduction. Although a certain ionic component probably exists we suggest that electronic transport dominates in our samples at and below room temperature because the derived ‘activation energy’ decreases under pressure, the derived ‘activation volume’ is negative, and we observe neither a significant electrode blocking capacitance nor any significant metal transport under dc conditions. (paper)