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

Structural phase transitions in a crystal of (NH4)2NbOF5 are the consequence of dynamic changes in its structural units as the temperature decreases. Using X-ray diffraction, it is possible to identify O and F atoms in the disordered structure of (NH4)2NbOF5 as a result of its dynamic nature.


Introduction
Noncentrosymmetric materials are a fertile topic of research owing to the important physical properties that may be observed in such materials: pyroelectricity, ferroelectricity, piezoelectricity or second harmonic generation (SHG). The first challenge one encounters in synthesizing a nonsymmetric material based on the [M V OF 5 ] 2À (M = V, Nb, Ta) or [M VI O 2 F 4 ] 2À (M = Mo, W) anions is to prevent oxide/fluoride ligand disorder around the transition metal. The second is to prevent these anions from crystallizing in a centrosymmetric arrangement (Marvel et al., 2007). In the [NbOF 5 ] 2À anion, out-of-center 'primary' electronic distortion arises from metal d-oxygen p orbital interactions. The Nb atom moves from the center of its coordination octahedron toward the O atom, forming a short Nb-O bond and a long trans Nb-F bond (Izumi et al., 2005). Secondary distortions are largely dependent on anion interactions with the extended bond network. O/F ordering in a noncentrosymmetric space group has been achieved with the [NbOF 5 ] 2À anion in inorganic-organic hybrid compounds with cluster  and chain motifs . In inorganic solid-state environments, the individual Nb-O and Nb-F bonds were recently found (Marvel et al., 2007) to be ordered in noncentrosymmetric KNaNbOF 5 , which exhibits the SHG property. Among the inorganic series A 2 NbOF 5 [A = Li (Galy et al., 1969), Na (Stomberg, 1984), K (Pinsker, 1966) or Cs (Fourquet et al., 1973)], all compounds crystallize in centrosymmetric space groups with disordered oxide and fluoride ions.
In the present work the structures of (NH 4 ) 2 NbOF 5 at room temperature and after two phase transitions are reported, with a preference for noncentrosymmetric space group in all three cases. The compound has been known for more than 140 years and it was described by Marignac (1866) for the first time, but its structure has not been determined until now.

Synthesis
(NH 4 ) 2 NbOF 5 was synthesized in a single-crystal form as colorless transparent tetrahedral prisms or polyhedra, but for the structural determination a spherical crystal was prepared. The starting materials used were of reagent grade. Niobium(V) oxide (20 g) was dissolved in 50 ml of boiling 40% hydrofluoric acid in a platinum crucible. The solution was filtered and an NH 4 F solution (NH þ 4 :H 2 NbOF 5 = 2.5) was added. Crystals were formed following slow evaporation in air. Analysis calculated for (NH 4 ) 2 NbOF 5 : NH 4 15.0, Nb 38.7, F 39.6%; found: NH 4 15.0, Nb 38.3, F 39.4%.
The ammonia content was determined by the Kjeldahl method with a precision of AE0.3 mass%. Pyrohydrolysis at 673 K was used for simultaneous determination of the fluorine and metal content. The sample (0.2-0.4 g) was placed in a Pt boat and hydrolyzed in superheated steam for 2 h. HF was water absorbed and analyzed by titration with Th(NO 3 ) 4 ; the metal was analyzed gravimetrically by weighing Nb 2 O 5 . The precision of the fluorine and metal determinations was AE0.5 mass%.

X-ray studies
A single crystal of a spherical shape was glued to the tip of a glass needle with epoxy resin. The diffracted intensities were measured at 297 (I), 233 (II) and 198 K (III) on a Bruker SMART 1000 CCD diffractometer (Mo K radiation, graphite monochromator). Scans in ! with a step size of 0.2 were performed at three ' settings with 2 = À31 and À50 at a detector distance of 45 mm. Exposures of 30 s per frame were research papers 528 Udovenko and Laptash (NH 4 ) 2 NbOF 5 Acta Cryst. (2008). B64, 527-533 Table 1 Crystal and experimental data for (NH 4 ) 2 NbOF 5 .  (10) carried out in groups of 906 frames each. All reflections were indexed in the corresponding unit cells. More details on data collection and reduction are given in Table 1. Data collection, reduction and refinement of the lattice parameters were performed using SMART (Bruker, 1998) and SAINT (Bruker, 2000). All the calculations were performed with SHELXTL (Sheldrick, 2008). Atomic coordinates and isotropic displacement parameters for all structures are available in the deposited CIF. 1 Interatomic distances and angles are listed in Tables 2, 3 and 4 and hydrogen-bond parameters are given in Table 5.

Spectroscopic measurements
Mid-IR (400-4000 cm À1 ) spectra were collected in Nujol mull using a Shimadzu FTIR Prestige-21 spectrometer operating at 2 cm À1 resolution. FT-Raman spectra of the compound were recorded with an RFS 100/S spectrometer.
The 1064 nm line of an Nd:YAG laser (130 mW maximum output) was used for excitation of the sample. The spectra were recorded at room temperature.

Results and discussion
3.1. Crystal structure of (I) 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 Cmc2 1 (No. 36). Because of the relatively large R 1 and on the basis of our preliminary 19 F NMR data concerning the reorientation motion of [NbOF 5 ] 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 R 1 to 0.0316. In accordance with the vibrational spectra of (NH 4 ) 2 NbOF 5 ( Fig. 1), which show two Nb states in the structure (the Nb-O stretching range contains two bands, at 933 and 912 cm À1 and at 920 and 910 cm À1 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 Na 2 NbOF 5 .
The occupation parameters were refined for atoms Nb1 and Nb2, and then the corresponding parameters for the F and O atoms were estimated in accordance with these refined values. The value x = 0.39 (5) of the Flack (1983) research papers Acta Cryst. (2008). B64, 527-533 Udovenko and Laptash (NH 4 ) 2 NbOF 5 529 Table 2 Selected distances (Å ) and angles ( ) for (I).
Structure determinations of (I) were carried out in another two space groups, Cmcm (No. 63) and Ama2 (No. 40), with R 1 = 0.0201 and 0.0227, respectively. It was determined that the noncentrosymmetric space group Cmc2 1 was preferable because of the lower R 1 value and the more reasonable Nb-X distances.
H atoms in (I) are not localized. Atoms N1 and N2 are surrounded by 11 O(F) atoms in the nearest environment. The electron-density difference maps around the N atoms (Figs. 4a and 4b) show the hydrogen electron density to be smeared along the c and a axes for N1H 4 and N2H 4 , respectively. Thus, the ammonium groups move in the crystal at room temperature.

Table 4
Selected distances (Å ) and angles ( ) for (III).  (Bruker, 2000). The corresponding R 1 values were 0.0193, 0.0218 and 0.0223. The Nb1-O1 and Nb2-O2 distances in the case of Cm are appreciably different (1.77 and 1.67 Å , respectively) -such a significant difference between these values is unacceptable. The octahedral parameters for C2 and C2/m are close, but the difference between two Nb-F distances and the values of four valence angles are far beyond the limits of 3. However, taking into account that structures (I) and (III) (see below) are noncentrosymmetric, we preferred the space group C2 for the structure of (II). The refinement of (II) in the space group C2 as a single crystal gave a Flack parameter of 0.57 (8), so we rerefined the structure using the twin matrix " 1 100/0 " 1 10/00 " 1 1 to R 1 = 0.0191 with a twin ratio of 0.61 (5):0.39 (5) and x equal to 0.0 (3).
In the crystal structure of (II) (Fig.  5), the [NbOF 5 ] 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 [NbOF 5 ] octahedra are in reorientational motion around the pseudothreefold 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 [Nb1OF 5 ] octahedron. Figs. 2 and 5 show that the [NbOF 5 ] polyhedra turned around the b axis during this process under the influence of two hydrogen bonds (Table 5).
The electron-density difference synthesis shows that only one of the H atoms is localized in each ammonium group. These atoms form hydrogen bonds with axial atoms in the octahedron (Table 5). Atoms N1 and N2 are surrounded by 12 and 9 O(F) atoms in the nearest environment, respectively. The electron-density difference maps evidence that the ammonium groups in phase (II) still rotate (Figs. 4c and  4d ). Some fragments of the structure of (I): the spatial orientations of the [NbOF 5 ] octahedron (a); the coordination polyhedra of Nb1 (b) and Nb2 (c).

Figure 4
Electron difference densities around N atoms in (I) (a,b) and (II) (c, d ).

Crystal structure of (III)
We failed to solve the single-crystal structure of (III) in both triclinic and monoclinic unit cells which were determined by BRAVAIS. The structure was solved in the monoclinic group Ia (No. 9) as a two-component twin with the twin law 100/010/ 00 " 1 1 and the twin ratio 0.79 (1):0.21 (1). Without taking twinning into account, R 1 was as high as 0.0529, many significant large peaks in the difference-Fourier map were observed near the Nb atoms in a difference electron density map and the H atoms were not determined. In the twin model, R 1 has decreased to 0.0279 and we managed to locate all the H atoms, whereupon R 1 dropped to 0.0254. However, the Flack parameter was 0.5 (1).
The centrosymmetric space group I2/a was recognized to be unsuitable for (III) since structure determination by direct methods and subsequent refinement resulted in a high R 1 value of 0.134. Another refinement with initial coordinates previously obtained in Ia gave an R 1 value of 0.0756 and highly weighted large peaks in the difference-Fourier map were observed in the electron density difference map. Thus, (III) was confirmed to be noncentrosymmetric. A series of single-crystal structure refinements was resumed in the Ia group, resulting in structure inversion. Final refinement with the new twin matrix " 1 100/010/001 led to R 1 = 0.0254 with a twin ratio of 0.21 (0):0.79 (0) and a Flack parameter of 0.1 (1).
The crystal structure of (III) (Fig. 6) is completely ordered. It contains two types of octahedra: [Nb1OF 5 ] and [Nb2OF 5 ]. Their Nb-O vertices are oppositely directed along the a axis. Isolated octahedra are connected via N-HÁ Á ÁO(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 [NbOF 5 ] 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.

Conclusions
It should be noted that no SHG response was observed in (NH 4 ) 2 NbOF 5 , since all three structures are twinned and appreciably pseudo-centrosymmetric. The comparison of the investigated structures shows that the orientational disorder in (I) has a dynamic nature. Both niobium octahedra and ammonium tetrahedra are reoriented dynamically, so no fixed hydrogen bonds are formed. The three spatial orientations of [NbOF 5 ] 2À around the pseudo-threefold axis arise from reorientational motion, which forces the central atom to displace research papers 532 Udovenko and Laptash (NH 4 ) 2 NbOF 5 Acta Cryst. (2008). B64, 527-533 Figure 6 Ordered structure of (NH 4 ) 2 NbOF 5 (III) at 198 K after the second phase transition.

Figure 5
Crystal structure of (NH 4 ) 2 NbOF 5 (II) at 233 K after the first phase transition. from the symmetrical position and allows us to identify O and F atoms in a separate orientation of the octahedron. Thus, it becomes possible to distinguish between O and F atoms by Xray diffraction under dynamic O/F disorder. Changes in the dynamic behavior of the complex are responsible for the phase transitions at lower temperatures.
In (II) two hydrogen bonds are formed and octahedral rotation is absent (rigid anionic sublattice), while the ammonium groups are not fully ordered. After the second phase transition [to (III)], all structural units are ordered.