Increased transition temperature in superconducting Na2CsC60 by intercalation of ammonia

• Published in: Nature (London) 1993-04, Vol.362 (6419), p.433-435
• Main Authors: Zhou, O., Fleming, R. M., Murphy, D. W., Rosseinsky, M. J., Ramirez, A. P., van Dover, R. B., Haddon, R. C.
• Format: Article
• Language: English
• Subjects: Ammonia; Chemistry; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Humanities and Social Sciences; letter; multidisciplinary; Physics; Science; Science (multidisciplinary); Superconducting materials (excluding high-tc compounds); Superconductivity; Temperature; Transition temperatures
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/362433a0

A WIDE variety of alkali-metal fullerides (A x C 60 ) 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 (T c ) which increase with the size of the face-centred cubic unit cell 3 . The increase in T c on lattice expansion is attributed to a decrease in the conduction band width, and therefore one ultimately expects a saturation or downturn of T c followed by a metal–insulator transition. We have sought to explore this regime by expanding the A 3 C 60 structure through intercalation of neutral molecules capable of solvating the A + ions. Here we report that the reaction of ammonia with Na 2 CsC 60 (T c = 10.5K; lattice constant α = 14.132 Å) produces the compound (NH 3 ) 4 Na 2 CsC 60 , which has an expanded unit cell (a = 14.473 Å) and an increased T c of 29.6 K. The structure contains the Na(NH 3 ) + 4 cation on the octahedral site.