New family of superconducting fullerides, Na3(NH 3)(x)C60 (x = 2--8)

摘要

A series of A(sub 3)C(sub 60) fulleride superconductors (A is an alkali metal) with superconducting transition temperatures ((Tc)s) between 2.5 and 33 K has been reported. The (Tc)s of these materials are closely correlated to their f.c.c. lattice parameters, with the higher (Tc) materials having larger unit cell lengths. Recently, the (Tc) of the superconducting Na(sub 2)CsC(sub 60) has been raised from 10.5 K to 29.6 K by ammonia intercalation. The resulting intercalated compound, NaCs(Na(NH(sub 3))(sub 4))C(sub 60) has an expanded unit cell compared to the starting material, with a larger cation Na(NH(sub 3))(sub 4+), occupying the octahedral site. The sodium-doped C(sub 60) with nominal composition Na(sub 3)C(sub 60) however, does not show superconductivity and was reported to undergo a disproportionation reaction into two f.c.c. phases, Na(sub 2)C(sub 60) and Na(sub 6)C(sub 60) below 250 K. Here we report on the ammonia intercalation of Na(sub 3)C(sub 60). Up to four superconducting phases with (Tc)s occurring near 6.7, 11.5, 13.5 and 16.5 K were observed in the compounds Na(sub 3)(NH(sub 3))(sub x)C(sub 60) (x (approx)2 = 8). All these phases gave significant superconducting shielding fractions (between 15 and 80% at 5 K). These compounds were characterized by use of AC susceptibility and SQUID measurements, TGA and x-ray powder diffraction.