Coupled Li 1+ /Nb 5+ and O 2- /F - ordering on the Na and Cl sites of the average NaCl structure of Li 4 NbO 4 F

Abstract

The average, as well as the cation and anion 'disordered', crystal structure of Li4NbO4F has been carefully investigated via coupled neutron and X-ray powder diffraction studies as well as via electron diffraction studies. The existence of a spectacular highly structured diffuse intensity distribution in the latter provides strong evidence for coupled Li1+/Nb5+ and O2-/F- ordering on the Na and Cl sites of the average NaCl structure of Li4NbO4F. Bond valence sum calculations have been used to investigate local crystal chemistry as well as to suggest plausible local crystal chemical constraints while ab initio DFT based theoretical calculations of a 2×2×2 supercell have been carried out in order to provide additional insight into the local crystal chemistry of this compound.