An Electron Diffraction and XRPD Study of Superlattice Ordering in the Elpasolite-Related Oxyfluoride K3MoO3F3

A combined diffraction (XRD, electron and neutron) and electrical study of Na3MoO3F3

“…M VI = Mo, W) oxyfluoride phases is understood in a qualitative fashion, as was shown in earlier work on the related K 3 MoO 3 F 3 (12,13,14), the question of why this distortion takes such a different form in the case of Na 3 MoO 3 F 3 remains to be understood? For K 3 MoO 3 F 3 and its closely related family members (12,13,14) it was shown that large amplitude transverse anion shifts associated with MoX 6 octahedral rotation together with Mo ion shifts in response to O/F ordering, were each required on the local scale in order to satisfy the bond valence requirements of the constituent ions. However, the exact pattern of octahedral rotations (and hence the direction of the anion shifts) could not be determined.…”

“…(Note that both additional satellite reflections as well as a highly structured diffuse intensity distribution have been observed in the α polymorphs of other oxyfluoride phases (3,(12)(13)(14)). out the presence of an inversion centre so that the resultant space group symmetry is P1.…”

“…When the alkali element in the interstitial, nominally 12-coordinate, A cation site (see Fig.1) is K, Rb, Tl or Cs (A = K, Rb, Tl or Cs), the metric symmetry of the underlying parent sub-structure reported for the intermediate β polymorph (as well as for the lower temperature α polymorphs, despite the often complicating presence of additional satellite reflections) has invariably been tetragonal or pseudo-tetragonal (1- 6,[12][13][14]. (Note that the strongest reflections in all polymorphic forms are those corresponding to the F-centred parent substructure, labelled with the subscript p in what follows).…”

The Importance of Multisite Correlations in Disordered Structures

“…A recent electron diffraction study [3] has, however, reported the existence of a complex three-dimensional continuous diffuse intensity distribution and we show here that this arises from the O/F ordering and the associated structural distortions acting to improve the local crystal chemistry. Figure 2 shows examples of 2D sections of this 3D diffuse distribution.…”

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

“…Over the years a number of other 'disordered' compounds have also been reported to crystallize with a NaCl-type average structure, in particular the oxyfluoride compound Li 4 NbO 4 F [13,14] (and related compounds such as Li 3 TiO 3 F), but without much effort either to search for evidence of local short range ordering or to interpret any such evidence in terms of local ordering rules. Given growing interest in the synthesis and physical properties of mixed anion compounds [15,16] as well as our own recent experiences in the local structural characterization of a range of nominally 'disordered' metal oxyfluoride compounds [17][18][19], it was decided to re-investigate both the average, as well as the 'disordered', crystal structure of Li 4 NbO 4 F via coupled neutron and X-ray powder diffraction (XRPD) studies as well as electron diffraction (ED) studies of structured diffuse scattering. Bond valence sum as well as ab initio DFT based theoretical calculations were also carried out in order to provide additional insight into the local crystal chemistry of this compound.…”