Intramolecular mobility and phase transitions in ammonium oxofluoroniobates (NH4)2NbOF5 and (NH4)3NbOF6, a NMR and DFT study |
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| Authors: | V.Ya. Kavun S.P. Gabuda I.A. Tkachenko |
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| Affiliation: | a Institute of Chemistry, Far Eastern Branch of RAS, Pr. Stoletiya 159, 690022 Vladivostok, Russia
b A.V. Nikolaev Institute of Inorganic Chemistry, Siberian Branch of RAS, Lavrentieva Str. 3, 630090 Novosibirsk, Russia |
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| Abstract: | Molecular structure, ionic mobility and phase transitions in six- and seven-coordinated ammonium oxofluoroniobates (NH4)2NbOF5 and (NH4)3NbOF6 were studied by 19F, 1H NMR and DFT calculations. Equatorial fluorine atoms (Feq) in [NbOF5]2− and [NbOF6]3− are characterized by high 19F NMR chemical shifts while axial fluorine atoms (Fax) have those essentially lower. The high-temperature ionic mobility in (NH4)2NbOF5 does not stimulate the ligand exchange Feq ↔ Fax, whereas it is observed in (NH4)3NbOF6 as pseudorotation typical for seven-coordinated polyhedra. The transformation of pentagonal bipyramidal structure (BP) of [NbOF6]3− into capped trigonal prismatic (CTP) one takes place during the phase transition (PT) at 260 K. The PT of order-disorder type in (NH4)2NbOF5 is accompanied by transition of anionic sublattice to a rigid state. The 19F and 1H NMR data corroborate the independent motions of NH4 groups and anionic polyhedra in (NH4)2NbOF5 while they are coordinated in (NH4)3NbOF6. |
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| Keywords: | Ammonium oxofluoroniobates Ionic mobility Dynamic orientational disorder Phase transitions 19F and 1H NMR spectra Non-rigidity DFT calculations |
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