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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.


IR and Raman (R) spectra of (NH4)2NbOF5 at room temperature.
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fig1: IR and Raman (R) spectra of (NH4)2NbOF5 at room temperature.

Mentions: Structure (I) was solved, to a first approximation, by direct methods and refined against F 2 by the full-matrix least-squares method, with an anisotropic approximation to R 1 = 0.0367 by location of the Nb atom in the special position (0, Y, Z) of the space group Cmc21 (No. 36). Because of the relatively large R 1 and on the basis of our preliminary 19F NMR data concerning the reorientation motion of [NbOF5] octahedra, it was suggested that the structure of (I) is disordered. Therefore, additional refinement of the structure was carried out by the displacement of the Nb atom from the special 4(a) position to the general 8(b) position; this lowered R1 to 0.0316. In accordance with the vibrational spectra of (NH4)2NbOF5 (Fig. 1 ▶), which show two Nb states in the structure (the Nb—O stretching range contains two bands, at 933 and 912 cm−1 and at 920 and 910 cm−1 in the IR and Raman spectra, respectively), a subsequent refinement with two independent Nb atoms in special and general positions was performed; this lowered R1 to 0.0197. In these steps, F atoms were assigned, and then the final refinement to R1 = 0.0185 was made by ligand separation on O and F atoms. Atoms O1 and F1 are located on one site with different occupation parameters and equal displacement parameters, as are atoms O2 and F2. A similar procedure was used by Stomberg (1984 ▶) to discern the O and F atoms in the disordered structure of Na2NbOF5.


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

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

IR and Raman (R) spectra of (NH4)2NbOF5 at room temperature.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig1: IR and Raman (R) spectra of (NH4)2NbOF5 at room temperature.
Mentions: Structure (I) was solved, to a first approximation, by direct methods and refined against F 2 by the full-matrix least-squares method, with an anisotropic approximation to R 1 = 0.0367 by location of the Nb atom in the special position (0, Y, Z) of the space group Cmc21 (No. 36). Because of the relatively large R 1 and on the basis of our preliminary 19F NMR data concerning the reorientation motion of [NbOF5] octahedra, it was suggested that the structure of (I) is disordered. Therefore, additional refinement of the structure was carried out by the displacement of the Nb atom from the special 4(a) position to the general 8(b) position; this lowered R1 to 0.0316. In accordance with the vibrational spectra of (NH4)2NbOF5 (Fig. 1 ▶), which show two Nb states in the structure (the Nb—O stretching range contains two bands, at 933 and 912 cm−1 and at 920 and 910 cm−1 in the IR and Raman spectra, respectively), a subsequent refinement with two independent Nb atoms in special and general positions was performed; this lowered R1 to 0.0197. In these steps, F atoms were assigned, and then the final refinement to R1 = 0.0185 was made by ligand separation on O and F atoms. Atoms O1 and F1 are located on one site with different occupation parameters and equal displacement parameters, as are atoms O2 and F2. A similar procedure was used by Stomberg (1984 ▶) to discern the O and F atoms in the disordered structure of Na2NbOF5.

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.