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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.

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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.


Ordered structure of (NH4)2NbOF5 (III) at 198 K after the second phase transition.
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fig6: Ordered structure of (NH4)2NbOF5 (III) at 198 K after the second phase transition.

Mentions: The crystal structure of (III) (Fig. 6 ▶) is completely ordered. It contains two types of octahedra: [Nb1OF5] and [Nb2OF5]. Their Nb—O vertices are oppositely directed along the a axis. Isolated octahedra are connected via N—H⋯O(F) hydrogen bonds (Table 5 ▶). The distances in the Nb2 octahedron are appreciably longer than those in the Nb1 octahedron (Table 4 ▶), probably as a result of the influence of hydrogen bonds. As in (I) and (II), the Nb atoms in (III) are shifted toward the O atom (by 0.26 Å). The [NbOF5] polyhedra are turned around the c axis during the (II) → (III) phase transition under the influence of hydrogen bonds (Table 5 ▶), as shown in Figs. 5 ▶ and 6 ▶.


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

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

Ordered structure of (NH4)2NbOF5 (III) at 198 K after the second phase transition.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig6: Ordered structure of (NH4)2NbOF5 (III) at 198 K after the second phase transition.
Mentions: The crystal structure of (III) (Fig. 6 ▶) is completely ordered. It contains two types of octahedra: [Nb1OF5] and [Nb2OF5]. Their Nb—O vertices are oppositely directed along the a axis. Isolated octahedra are connected via N—H⋯O(F) hydrogen bonds (Table 5 ▶). The distances in the Nb2 octahedron are appreciably longer than those in the Nb1 octahedron (Table 4 ▶), probably as a result of the influence of hydrogen bonds. As in (I) and (II), the Nb atoms in (III) are shifted toward the O atom (by 0.26 Å). The [NbOF5] polyhedra are turned around the c axis during the (II) → (III) phase transition under the influence of hydrogen bonds (Table 5 ▶), as shown in Figs. 5 ▶ and 6 ▶.

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.