Electron Spin Resonance and optical reflectivity measurements demonstrate ametal-insulator transition in Na_2CsC_60 as the system passes from the lowtemperature simple cubic to the high temperature {\it fcc} structure above 300K. The non-conducting electronic state is especially unexpected in view of themetallic character of other, apparently isostructural fullerides, like K_3C_60.The occurence of this phase in Na_2CsC_60 suggests that alkali specific effectscan not be neglected in the description of the electronic properties of alkalidoped fullerides. We discuss the origin of the insulating state and therelevance of our results for the anomaly observed in the magnitude of thesuperconducting transition temperature of Na_2AC_60 fullerides.
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