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Orientational disorder and phase transitions in crystals of (NH4)2NbOF5.

Udovenko AA, Laptash NM - Acta Crystallogr., B (2008)

Bottom Line: As the temperature decreases, the compound undergoes two phase transitions.The structure of (III) is completely ordered.The crystals of all three phases are twinned.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Chemistry, Far Eastern Branch of RAS, Pr. Stoletiya 159, 690022 Vladivostok, Russian Federation. udovenko@ich.dvo.ru

ABSTRACT
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.

No MeSH data available.


Electron difference densities around N atoms in (I) (a,b) and (II) (c, d).
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fig4: Electron difference densities around N atoms in (I) (a,b) and (II) (c, d).

Mentions: H atoms in (I) are not localized. Atoms N1 and N2 are surrounded by 11 O(F) atoms in the nearest environment. The electron-density difference maps around the N atoms (Figs. 4 ▶ a and 4 ▶ b) show the hydrogen electron density to be smeared along the c and a axes for N1H4 and N2H4, respectively. Thus, the ammonium groups move in the crystal at room temperature.


Orientational disorder and phase transitions in crystals of (NH4)2NbOF5.

Udovenko AA, Laptash NM - Acta Crystallogr., B (2008)

Electron difference densities around N atoms in (I) (a,b) and (II) (c, d).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2553555&req=5

fig4: Electron difference densities around N atoms in (I) (a,b) and (II) (c, d).
Mentions: H atoms in (I) are not localized. Atoms N1 and N2 are surrounded by 11 O(F) atoms in the nearest environment. The electron-density difference maps around the N atoms (Figs. 4 ▶ a and 4 ▶ b) show the hydrogen electron density to be smeared along the c and a axes for N1H4 and N2H4, respectively. Thus, the ammonium groups move in the crystal at room temperature.

Bottom Line: As the temperature decreases, the compound undergoes two phase transitions.The structure of (III) is completely ordered.The crystals of all three phases are twinned.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Chemistry, Far Eastern Branch of RAS, Pr. Stoletiya 159, 690022 Vladivostok, Russian Federation. udovenko@ich.dvo.ru

ABSTRACT
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.

No MeSH data available.