Open AccessOrientational disorder and phase transitions in crystals of (NH 4 ) 2 NbOF 5
Author(s) -
Udovenko Anatoly A.,
Laptash Natalia M.
Publication year - 2008
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
eISSN - 1600-5740
pISSN - 0108-7681
DOI - 10.1107/s0108768108021289
Subject(s) - octahedron , crystallography , phase (matter) , crystal structure , atom (system on chip) , diffraction , materials science , phase transition , crystal (programming language) , x ray crystallography , chemistry , condensed matter physics , physics , optics , organic chemistry , computer science , embedded system , programming language
Ammonium oxopentafluoroniobate, (NH 4 ) 2 NbOF 5 , was synthesized in a single‐crystal form and the structures of its different phases were determined by X‐ray diffraction at three temperatures: phase (I) at 297 K, phase (II) at 233 K and phase (III) at 198 K. The distorted [NbOF 5 ] 2− octahedra are of similar geometry in all three structures, with the central atom shifted towards the O atom. The structure of (I) is disordered, with three spatial orientations of the [NbOF 5 ] 2− octahedron related by a jump rotation around the pseudo‐threefold local axis such that the disorder observed is of a dynamic nature. As the temperature decreases, the compound undergoes two phase transitions. The first is accompanied by full anionic ordering and partial ordering of the ammonium groups (phase II). The structure of (III) is completely ordered. The F and O atoms in the structures investigated were identified via the Nb— X ( X = O and F) distances. The crystals of all three phases are twinned.