The open–access journal for physics New Journal of Physics The structural and electronic evolution of Li 4 C 60 through the polymer–monomer transformation D Ar ˇ con 1,2 , A Zorko 1 , M Mazzani 3 , M Belli 3 , D Pontiroli 3 , M Ricc ` o 3 and S Margadonna 4 1 Institute Jozef Stefan, Jamova 39, 1000 Ljubljana, Slovenia 2 Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia 3 CNISM and Dipartimento di Fisica, Università di Parma, Viale G P Usberti 7/A, 43100 Parma, Italy 4 School of Chemistry, University of Edinburgh, West Mains Road, EH9 3JJ Edinburgh, UK E-mail: denis.arcon@ijs.si New Journal of Physics 10 (2008) 033021 (17pp) Received 24 August 2007 Published 12 March 2008 Online at http://www.njp.org/ doi:10.1088/1367-2630/10/3/033021 Abstract. In this paper, we combine synchrotron powder x-ray diffraction, 7 Li nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) experiments to study the structural evolution of Li 4 C 60 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 C 60 –C 60 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 7 Li NMR shift. At low temperatures, the Li + ions are statically disordered on the 7 Li NMR timescale. The observed 7 Li 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). New Journal of Physics 10 (2008) 033021 1367-2630/08/033021+17$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft