Coupled Li1+/Nb5+ and O2−/F− ordering on the Na and Cl sites of the average NaCl structure of Li4NbO4F

Abstract

The average, as well as the cation and anion 'disordered', crystal structure of Li 4 NbO 4 F 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 Li 1+ /Nb 5+ and O 2À /F À ordering on the Na and Cl sites of the average NaCl structure of Li 4 NbO 4 F. 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 Â 2 Â 2 supercell have been carried out in order to provide additional insight into the local crystal chemistry of this compound.

Modeling: What has it Really Contributed?

“…Rietveld refinement reports the average structure, but different atoms can occupy the same site, depending on stoichiometry, and local structure variations can exist. Understanding these local atomic arrangements and related properties is of fundamental importance, which is difficult to obtain without modeling [4]. An example is the investigation of arsenic trapping in gypsum and calcite [5], again in the geoscience field.…”