Structural and electronic properties of fulleride superconductors

Abstract

In the present thesis, I discuss some of the current advances in research in the field of the solid state science of fullerenes. The reaction of C60 with alkali metals using both conventional solid state and low temperature solution-based synthesis techniques has led to the production of fulleride salts with interesting structural and superconducting properties. In superconducting A3C60 systems, it has been widely reported that Tc increases monotonically with interfulleride separation. Of particular interest is the family Na2Rb1-xCsxC60 (0 ≤ x ≤ 1) as they display a much steeper rate of change of Tc with interfulleride spacing. Here we discuss the related family of quaternary fullerides, Na2-xKxCsC60 in an attempt to explore the consequences of this trend and produce fulleride salts with elevated Tc's

In addition, the monotonic increase in Tc with increasing interfulleride separation has driven attempts towards the synthesis of new superconducting fullerides with very large lattice parameters. A key material among the A3C60 systems is the end member, Cs3C60, which has remained elusive in attempts to synthesise it by traditional solid state techniques due to the thermodynamic instability of this phase caused by the accommodation of the large Cs+ ion (r = 1.67 Å) in the small tetrahedral holes (r = 1.12 Å). Here we report the synthesis of “FCC rich” and "A15 rich" samples of the series, RbxCs3-xC60 (0.0 ≤ x ≤ 0.5) via low temperature synthetic techniques utilising the solvents ammonia and methylamine, respectively. This allowed us to study the effects of both chemical (by partial substitution of Cs+ by the smaller Rb+ cation) and physical pressure upon the electronic and superconducting properties of these materials.

For all samples, detailed structural studies have been performed using synchrotron X-ray powder diffraction and magnetic behaviour using SQUID magnetometry techniques.