Structural and electronic properties of the overexpanded quaternary superconducting fulleride K0.25Rb0.25CS2.5C60

Abstract

The structural and magnetic properties of the overexpanded quatemary K0.25Rb0.25Cs2.5C60 fulleride are investigated. Rietveld analysis of the high-resolution synchrotron X-ray powder diffraction data shows that K0.25Rb0.25Cs2.5C60 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 TT 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 K0.25Rb0.25Cs2.5C60 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.