The structural and electronic evolution of Li4C60 through the polymer–monomer transformation

Abstract

In this paper, we combine synchrotron powder x-ray diffraction, 7Li nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) experiments to study the structural evolution of Li4C60 and how its electronic ground state depends on the crystal symmetry. The compound in the two-dimensional polymer phase with mixed interfullerene bonding motifs is a band gap insulator. EPR, however, reveals the presence of intrinsic centers originating from broken C60–C60 bonds and local Li off-stoichiometry that create states in the band gap and account for the complex temperature dependence of the spin susceptibility as well as the residual temperature dependence of the 7Li NMR shift. At low temperatures, the Li+ ions are statically disordered on the 7Li NMR timescale. The observed 7Li NMR line narrowing at T>200 K is ascribed to the Li+ diffusion dynamics and above room temperature the polymer phase is already a good ionic conductor. Heating the sample to temperatures above ∼470 K results in gradual depolymerization to the metallic monomer fcc high temperature structure. The transformation is first order and polymer as well as monomer phases coexist over a broad temperature interval (130 K).