Ac impedance of A4C60 fullerides under pressure

New Journal of Physics Volume 17 February 2015.

Bertil Sundqvist1,2, Ove Andersson1, Chen Gong2, Bingbing Liu2, Bounphanh Tonpheng1,3, Junchun Yu1,4 and Mingguang Yao2

1 Department of Physics, Umeå University, SE-90187 Umeå, Sweden
2 State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China
3 Faculty of Natural Sciences, National University of Laos (NUOL), Vientiane, Laos
4 Present address: University of Borås, SE-50190 Borås, Sweden.

 

Abstract

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.

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