Superconductivity of CaxC60 prepared by thermal decomposition of azide: ESR and LFS microwave absorption study

Superconducting(SC) behavior of Ca-doped C60 prepared by calcium azide thermal decomposition is studied by low-field signal(LFS) of microwave absorption and conventional ESR. SC phase is obtained for the sample prepared with nominal composition of Ca6C60 and its volume fraction is significantly enlarged by annealing at 350≈400 °C for 2 h. It is suggested that the SC compound may have two SC phases with slightly different transition temperatures, TC1(8.2≈8.4 K) and TC2(8.7≈8.8 K). The SC2 phase with higher TC2 is degraded by annealing at 450≈500 °C and converts to SC1. Degradation of SC2 at high temperature is interpreted to be caused either by (a) thermal reaction of some intercalated Ca with quartz glass and/or (b) dissociation of CaN clusters in the octahedral sites of Ca5(CaN)yC60 lattice.