Abstract
A WIDE variety of alkali-metal fullerides (AxC60) have been prepared, with stoichiometries varying between 1⩽=x⩽=11 (refs 1,2). Most of the compounds with x = 3 are superconductors with unusually high transition temperatures (Tc) which increase with the size of the face-centred cubic unit cell3. The increase in Tc on lattice expansion is attributed to a decrease in the conduction band width, and therefore one ultimately expects a saturation or downturn of Tc followed by a metal–insulator transition. We have sought to explore this regime by expanding the A3C60 structure through intercalation of neutral molecules capable of solvating the A+ ions. Here we report that the reaction of ammonia with Na2CsC60 (Tc = 10.5K; lattice constant α = 14.132 Å) produces the compound (NH3)4Na2CsC60, which has an expanded unit cell (a = 14.473 Å) and an increased Tc of 29.6 K. The structure contains the Na(NH3)+4 cation on the octahedral site.
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Zhou, O., Fleming, R., Murphy, D. et al. Increased transition temperature in superconducting Na2CsC60 by intercalation of ammonia. Nature 362, 433–435 (1993). https://doi.org/10.1038/362433a0
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DOI: https://doi.org/10.1038/362433a0
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