부산대학교 도서관

Alkali fullerides A{sub 2}A{prime}C{sub 60} (A, A{prime} = alkali metals) are superconducting with T{sub c} at ambient pressure ranging from 3.5 K for Na{sub 2}RbC{sub 60} to 33 K for RbCs{sub 2}C{sub 60}. The only exception is provided by Li{sub 2}CsC{sub 60} and Li{sub 2}RbC{sub 60} which do not show superconductivity down to 50 mK. Otherwise, T{sub c} increases monotonically with increasing interfullerene separation, d, as the density of states at the Fermi level, N({epsilon}{sub F}) increases. The absence of superconductivity in the Li fulleride family has been associated with the presence of strong bonding Li{sup +}-C interactions. These lead to a reduced formal charge of the C{sub 60}{sup n{minus}} ions in Li{sub 2}A{prime}C{sub 60}, found experimentally as {approximately}2.5 by Raman measurements. In an attempt to overcome the Li{sup +}-C interactions and achieve half filling (n = 3) of the t{sub lu} band, the authors synthesized the Li{sub x}CsC{sub 60} (x = 1.5--6) fullerides. By adjusting x, the authors can control the electron transfer from Li to C{sub 60} and tune the filling level of the conduction band continuously. Half filling is achieved for Li{sub 3}CsC{sub 60} which is a bulk superconductor with T{sub c} = 10.5 K. This paper reports the structural properties of the Li{sub 3}CaC{sub 60} superconductor. At low temperatures, it is primitive cubic (space group Pa{bar 3}), isostructural with the metastable Na{sub 2}A{prime}C{sub 60} phases with orientationally ordered C{sub 60}{sup 3{minus}} ions in the unit cell.