Neutron-diffraction studies ofBaC6:c-axis compressibility, carbon–carbon bond length, and charge transfer

The c-axis compressibility of the stage-1 alkaline-earth-metal intercalation compound ${\mathrm{BaC}}_6$, determined from pressure-dependent (00L) neutron diffraction, is 0.77×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^2$/dyn, the lowest of any graphite intercalation compound. In-plane diffraction at 1 bar shows that the in-plane C—C bond length is expanded 1.3% relative to graphite. Both results are consistent with valence and core-level x-ray photoemission experiments and band-structure theory; approximately one of the two Ba(6s) electrons is transferred to a graphitic ${\pi }^{\mathrm{*}}$ conduction band, while s-d hybridization internal to the Ba site takes up the rest. The localized s-d charge accounts in part for the low compressibility.