Synthesis, crystal structure and Raman spectrum of Ba₇[BO₃]₃Br(O_1.33F_1.33)

2.1 Synthesis

All manipulations were carried out under normal atmosphere. The stoichiometry of the starting mixture was according to that of the initial target compound “Ba₇[BO₃]₃BrF₄”. The overall mass of the educts was 1 g. Ba₇[BO₃]₃Br(O_1.33F_1.33) was obtained as minority product by reacting mixtures of H₃BO₃ (Fisher Scientific, Analytical Grade), Ba(OH)₂ (Fisher Scientific, Analytical Grade), BaF₂ (Alfa Aesar, powder, ultra dry, 99.995%) and BaBr₂·2 H₂O (Fisher Scientific, Analytical Grade). The ground educts were placed in an alumina crucible and heated over 13 h from room temperature to 1300 K. This temperature was held for an additional 13 h. After that, the furnace was shut off and allowed to cool to room temperature. Most of the sintered reaction cake was found to be amorphous, but next to some brick-shaped blocks of Ba₅[BO₃]₃Br [2], a few crystals of Ba₂[BO₃]Br [2] appeared as hexagonal prisms as the barium fluoride borates do [3, 4] while the few specimens of Ba₇[BO₃]₃Br(O_1.33F_1.33) had needle like morphologies.

The remaining crystals that were not used for X-ray or Raman measurements were heated with a few drops of concentrated sulfuric acid in a small test tube showing the etching reaction typical for a fluoride containing compound reacting with glass surfaces.

Ba₇[BO₃]₃Br(O_1.33F_1.33) reacts with air and/or moisture after a few days and yields either X-ray amorphous products or BaCO₃.

2.2 Raman spectroscopy

Single crystals of the title compounds were sealed under a protective argon atmosphere inside Mark capillaries and used for the Raman investigations (microscope laser Raman spectrometer: Jobin Yvon, 1 mW, excitation line at λ=632.817 nm (HeNe laser), 20× magnification, 360 s accumulation time, Fig. 1).

Fig. 1:

Raman spectrum of Ba₇[BO₃]₃Br(O_1.33F_1.33). Raman intensity is displayed on the vertical axis in arbitrary units.

2.3 Crystallographic studies

Needle-shaped crystals were chipped off the sintered transparent colorless mass adhering to the crucible and immersed in polybutene oil (Aldrich, M_n~320, isobutylene>90%). Suitable single crystals were selected under a polarization microscope, mounted in a drop of polybutene sustained in a plastic loop, and placed onto the goniometer. A cold stream of nitrogen (T=173(2)K) froze the polybutene oil, thus keeping the crystal stationary and protected from oxygen and moisture. Preliminary examination and subsequent data collection were performed on a Bruker X8 Apex II diffractometer equipped with a 4 K CCD detector and graphite-monochromatized MoK_α radiation (λ=71.073 pm). The intensity data were handled with the program package that came with the diffractometer [5]. An empirical absorption correction was applied using Sadabs [6]. The program Shelxs-97 [7, 8] found the positions of barium and bromine with the help of Direct Methods. The positions of the light atoms were apparent from the positions of the highest electron density on the difference Fourier maps resulting from the first refinement cycles by full-matrix least-squares calculations on F² in Shelxl-97 [9, 10]. The stoichiometry would have been “Ba₇[BO₃]₃BrF₄” at this stage, but the atoms F1 and F2 showed unusually large displacement parameters. The free refinement of the site occupation factor yielded “Ba₇[BO₃]₃BrF_2.6”, but this composition did not concur with the electroneutrality of the compound demanded by its transparency and lack of color. Since oxides and fluorides often show similar crystal chemical behavior when stabilized by a suitable host structure, a 1:1 split occupancy of these positions with F and O was introduced leading to balanced ionic charges following (Ba²⁺)₇([BO₃]³⁻)₃(Br⁻)(O²⁻)_1.33(F⁻)_1.33. No surplus electron densities were found in the hollow F2/O4 tetrahedron which indicates the absence of additional [BO₃]³⁻ moieties. For the final refinement cycles the positions with the split occupancies were fixed to the previously obtained occupation factors. The refinement converged into a stable model for the crystal structure. Additional crystallographic details are given in Table 1. Atomic coordinates and equivalent isotropic displacement coefficients are shown in Table 2, Table 3 displays the ranges of the bond lengths.

Further details of the crystal structure investigation may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany [fax: (+49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de] on quoting the deposition number CSD-431987 for Ba₇[BO₃]₃Br(O_1.33F_1.33).

3.1 Raman spectra of Ba₇[BO₃]₃Br(O_1.33F_1.33)

The Raman spectroscopic data (Fig. 1 and Table 4) show clearly the presence of the orthoborate [BO₃]³⁻ anion when compared to other compounds containing this anion. The symmetric stretching mode is clearly detectable in the Raman spectrum of the title compound at a frequency typical of many orthoborates. The symmetric stretching mode of carbonate moieties was found at 1035 cm⁻¹ – barely visible with <4% of the intensity of analog mode of the orthoborate anion. In Ba₂[BO₃]_0.9[CO₃]_0.1Cl_1.1 [14] the relation of the aforementioned intensities is 1:1/3. Therefore it can be assumed, that virtually no carbonate is present in the title compound.

3.2 The crystal structure of Ba₇[BO₃]₃Br(O_1.33F_1.33)

The crystal structure of Ba₇[BO₃]₃Br(O_1.33F_1.33) can be seen as complex 3D structure forming channels parallel to the crystallographic c axis which are filled by bromide anions rattling inside these large pores (Fig. 2). The borate anions are close to D_3h symmetry with familiar bond lengths (around 136 pm) and angles (approximately 120°) and it is surrounded in a familiar fashion by a tricapped trigonal prism of barium atoms (Fig. 3). The plane of these anions is parallel to the crystallographic c axis. Fig. 4a shows the F1/O3 position which is approximately in the centre of a distorted tetrahedron of Ba²⁺, while Fig. 4b displays the F2/O4 position which is in an environment suitable for a borate unit [3, 4], but there are no indications of any surplus electron density inside this tetrahedron that would indicate disordered [BO₃]³⁻ moieties. All bond lengths are in a familiar range (Table 4) when compared to the sum of the ionic radii suggested by Shannon [13].

Fig. 2:

View of the unit cell of Ba₇[BO₃]₃Br(O_1.33F_1.33) along the crystallographic c axis. The borate anion is displayed as cyan colored triangle, Ba–F/O bonds are shown as black lines.

Fig. 3:

Coordination polyhedron around the [BO₃]³⁻ anion. The same color code as in Fig. 2 is used. Ba–O bonds are shown as broken lines and B–O bonds as black lines.

Fig. 4:

Coordination polyhedra around the F1/O3 and F2/O4 positions. The same color code as in Fig. 3 is used.