Alkali specific effects in superconducting fullerides: the observation of a high temperature insulating phase in Na₂CsC₆0

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Electron Spin Resonance and optical reflectivity measurements demonstrate a metal-insulator transition in Na₂CsC₆0 as the system passes from the low temperature simple cubic to the high temperature fcc structure above 300 K. The non-conducting electronic state is especially unexpected in view of the metallic character of other, apparently isostructural fullerides, like K₃C₆0. The occurence of this phase in Na₂CsC₆0 suggests that alkali specific effects can not be neglected in the description of the electronic properties of alkali doped fullerides. We discuss the origin of the insulating state and the relevance of our results for the anomaly observed in the magnitude of the superconducting transition temperature of Na₂AC₆0 fullerides.