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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.


Crystal structure of (NH4)2NbOF5 (II) at 233 K after the first phase transition.
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fig5: Crystal structure of (NH4)2NbOF5 (II) at 233 K after the first phase transition.

Mentions: In the crystal structure of (II) (Fig. 5 ▶), the [NbOF5] polyhedra are identical and fully ordered, corresponding to a single orientational state of the anionic sublattice, i.e. the anions are in a static state. The octahedral geometry in (II) (Table 3 ▶) is close to that in (I). The Nb atom is displaced from the equatorial plane toward the O atom by 0.25 Å. Comparing the structures of (II) and (I), it is clear that the statistical disorder in (I) has a dynamic character. The [NbOF5] octahedra are in reorientational motion around the pseudo-threefold axis and form three spatial orientations in the crystal, which interchange with one another by a jump around the pseudo-threefold axis. The octahedra stop rotating during the (I) → (II) phase transition and their spatial orientations change into one orientation of the [Nb1OF5] octahedron. Figs. 2 ▶ and 5 ▶ show that the [NbOF5] polyhedra turned around the b axis during this process under the influence of two hydrogen bonds (Table 5 ▶).


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

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

Crystal structure of (NH4)2NbOF5 (II) at 233 K after the first phase transition.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig5: Crystal structure of (NH4)2NbOF5 (II) at 233 K after the first phase transition.
Mentions: In the crystal structure of (II) (Fig. 5 ▶), the [NbOF5] polyhedra are identical and fully ordered, corresponding to a single orientational state of the anionic sublattice, i.e. the anions are in a static state. The octahedral geometry in (II) (Table 3 ▶) is close to that in (I). The Nb atom is displaced from the equatorial plane toward the O atom by 0.25 Å. Comparing the structures of (II) and (I), it is clear that the statistical disorder in (I) has a dynamic character. The [NbOF5] octahedra are in reorientational motion around the pseudo-threefold axis and form three spatial orientations in the crystal, which interchange with one another by a jump around the pseudo-threefold axis. The octahedra stop rotating during the (I) → (II) phase transition and their spatial orientations change into one orientation of the [Nb1OF5] octahedron. Figs. 2 ▶ and 5 ▶ show that the [NbOF5] polyhedra turned around the b axis during this process under the influence of two hydrogen bonds (Table 5 ▶).

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.