A type-II clathrate with a Li-Ge framework

Abstract

Abstract Na16Cs8Li x Ge136−x (x≈2.8, space group Fd3̅m) is the first intermetallic type-II clathrate with a lithium-substituted framework. The phase was obtained from the elements in sealed Ta ampoules by annealing at 650°C for 5 days. Samples were investigated by synchrotron X-ray powder diffraction, solid-state NMR, microstructure and chemical analysis. The substitution of Ge by Li atoms causes a marked shrinking of the lattice parameter. Studies by 7Li NMR confirmed the presence of Li in the clathrate phase and the 23Na and 133Cs NMR spectra consistently showed distinct changes as compared to the ternary Na16Cs8Ge136. The SEDOR technique revealed a distance between Li and Cs atoms in agreement with the result of crystal structure refinement, indicating Li substitution at site 96g. The distinct Knight shift of all NMR signals implies metallic behaviour of the clathrate phase, measurements of the magnetic susceptibility indicate diamagnetic behaviour.