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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.


The disordered structure of (NH4)2NbOF5 at room temperature (I).
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fig2: The disordered structure of (NH4)2NbOF5 at room temperature (I).

Mentions: The crystal structure of (I) (Fig. 2 ▶) consists of two crystallographically independent disordered ammonium groups and disordered [NbOF5] octahedra in which two F atoms and one O atom occupy statistically the general (X2) and special (X1) positions (Fig. 3 ▶ a). The Nb atom is randomly distributed on the 4(a) and 8(b) positions, with the probabilities 0.6554 (4) and 0.1723 (2), respectively. In the [Nb1OF5] and [Nb2OF5] octahedra (Figs. 3 ▶ b and 3 ▶ c), the O atom was identified from the Nb—X distances (Table 2 ▶). In the Nb1 environ­ment, atom O1 occupies a special site X1, while it is located at the general X2 site in the Nb2 environment. The Nb—O distances in both polyhedra are equal to 1.733 Å, and equatorial F atoms are displaced from Nb at 1.90–1.95 Å. Niobium is shifted from the equatorial plane toward the O atom by 0.23 and 0.20 Å for Nb1 and Nb2, respectively. It should be noted that a very similar [NbOF5] geometry was observed in the fully ordered structures of [4-apy]2[Cu(4-apy)4(NbOF5)2] (Izumi et al., 2005 ▶) and [pyH+]2[CuNb2(py)4O2F10]2- (Halasyamani et al., 1996 ▶). Fig. 3 ▶(a) shows that the [NbOF5] octahedra have three orientations related by local pseudo-threefold axis.


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

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

The disordered structure of (NH4)2NbOF5 at room temperature (I).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig2: The disordered structure of (NH4)2NbOF5 at room temperature (I).
Mentions: The crystal structure of (I) (Fig. 2 ▶) consists of two crystallographically independent disordered ammonium groups and disordered [NbOF5] octahedra in which two F atoms and one O atom occupy statistically the general (X2) and special (X1) positions (Fig. 3 ▶ a). The Nb atom is randomly distributed on the 4(a) and 8(b) positions, with the probabilities 0.6554 (4) and 0.1723 (2), respectively. In the [Nb1OF5] and [Nb2OF5] octahedra (Figs. 3 ▶ b and 3 ▶ c), the O atom was identified from the Nb—X distances (Table 2 ▶). In the Nb1 environ­ment, atom O1 occupies a special site X1, while it is located at the general X2 site in the Nb2 environment. The Nb—O distances in both polyhedra are equal to 1.733 Å, and equatorial F atoms are displaced from Nb at 1.90–1.95 Å. Niobium is shifted from the equatorial plane toward the O atom by 0.23 and 0.20 Å for Nb1 and Nb2, respectively. It should be noted that a very similar [NbOF5] geometry was observed in the fully ordered structures of [4-apy]2[Cu(4-apy)4(NbOF5)2] (Izumi et al., 2005 ▶) and [pyH+]2[CuNb2(py)4O2F10]2- (Halasyamani et al., 1996 ▶). Fig. 3 ▶(a) shows that the [NbOF5] octahedra have three orientations related by local pseudo-threefold axis.

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.