Coupled Li/Nb and O/F ordering on the Na and Cl sites of the average NaCl structure of Li4NbO4F

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.