Structural and electronic properties of the overexpanded quaternary superconducting fulleride K _0.25 Rb _0.25 Cs _2.5 C ₆₀

The structural and magnetic properties of the overexpanded quaternary K _0.25 Rb _0.25 Cs _2.5 C ₆₀ fulleride are investigated. Rietveld analysis of the high-resolution synchrotron X-ray powder diffraction data shows that K _0.25 Rb _0.25 Cs _2.5 C ₆₀ adopts a cubic structure with face-centered (fcc) symmetry. Magnetization measurements at ambient pressure reveal that the material is a superconductor with a transition temperature, T _c , of 29.4 K. Temperature dependence of the magnetic susceptibility does not show a simple temperature-independent Pauli susceptibility term – instead, it shows a cusp at a certain temperature implying an insulator-to-metal crossover on cooling. Complementary high-pressure magnetization measurements to a pressure of 10.2 kbar show a non-monotonic response of T _c with a dome-shaped scaling with pressure and a maximum value of 32.4 K at 3.9 kbar. The experimental results are consistent with quaternary K _0.25 Rb _0.25 Cs _2.5 C ₆₀ following the established unconventional behavior of overexpanded fullerides. However, our results also unveil well-defined detrimental cation specific effects on the electronic properties associated with the introduction of increased disorder in the tetrahedral interstices of the fcc fulleride structure.