Metallic phase in potassium fulleride K_xC₇₀: Electrical, magnetic, and structural properties of K₄C₇₀

The metallic phase in potassium fullerides K_xC₇₀ was researched using electrical resistivity measurements in the case of film samples and magnetic susceptibility measurements in the case of powder samples. The K concentration dependence of the electrical resistivity of the K_xC₇₀ film at 423 K showed two minima: namely, 1.0×10⁻¹ and 6.1×¹⁰⁻³ Ω cm at x=1.1 and 4.1, respectively. The temperature dependence of the resistivity of the K₄C₇₀ film showed a disordered-metallic behavior. Superconducting quantum interference device (SQUID) measurements for K₄C₇₀ powder revealed a temperature-independent magnetic susceptibility contribution of 3.47×10⁻⁴ emu/mol. This contribution was attributed to Pauli paramagnetism, and it was also observed in electron spin resonance (ESR) measurements. Futhermore, ESR spectra for K₄C₇₀ powder showed that the linewidth broadened with increasing temperature. It was clearly established that K₄C₇₀ is a metallic phase in K_xC₇₀. The crystal structure of K₄C₇₀ was found to be simple tetragonal with a=1.266 nm and c=1.098 nm. No superconductivity was observed in K₄C₇₀ above 2 K.