Synthesis and structural characterization of Ca₁₂Ge₁₇B₈O₅₈

2.1 Synthesis

According to Eq. 1, a stoichiometric mixture of the starting materials CaCO₃ (99.95%, Stream Chemicals, Newburyport, MA, USA), GeO₂ (99.99%, ChemPur, Karlsruhe, Germany), and H₃BO₃ (99.5%, Merck, Darmstadt, Germany) was finely ground in an agate mortar and filled into a platinum FKS 95/5 crucible (feinkornstabilisiert, 95% Pt, 5% Au, Ögussa, Wien, Austria).

Calcination was performed in an electric resistance furnace (Nabertherm muffle furnace). The sample was heated up to 1100°C with a rate of 275°C h^–1 and maintained at that temperature for 20 h. After that, the temperature was lowered to 500°C with a rate of 3°C h^–1 before switching off the furnace. The product naturally cooled down to room temperature.

The new compound Ca₁₂Ge₁₇B₈O₅₈ could be obtained in form of colorless, air- and water-resistant crystals. The powder diffraction pattern (Fig. 1) showed reflections of Ca₁₂Ge₁₇B₈O₅₈ as the major phase. The eight reflections marked with asterisks could not be assigned up to now.

Fig. 1:

Top: experimental powder pattern of Ca₁₂Ge₁₇B₈O₅₈. The reflections marked with a red asterisk could not be assigned until now. Bottom: theoretical powder pattern of Ca₁₂Ge₁₇B₈O₅₈ based on single-crystal diffraction data.

2.2 Crystal structure analysis

The powder diffraction pattern of Ca₁₂Ge₁₇B₈O₅₈ was obtained in transmission geometry from a flat sample of the reaction product, using a Stoe Stadi P powder diffractometer with Ge (111)-monochromatized MoK_α1 (λ = 70.93 pm) radiation. The comparison of the experimental powder pattern with the theoretical pattern simulated from the single-crystal data in Fig. 1 exhibited that they match well.

Small single crystals of Ca₁₂Ge₁₇B₈O₅₈ were selected by mechanical fragmentation using a polarization microscope. A Bruker D8 Quest Kappa diffractometer with Mo-K_α radiation (λ = 71.073 pm) was used to collect the single crystal intensity data at room temperature. A multi-scan absorption correction (Sadabs-2014 [10]) was applied to the intensity data sets. All relevant details of the data collection and the refinement are listed in Table 1. According to the systematic extinctions, the tetragonal space group P4̅ (No. 81) was derived for the crystal. Due to the fact that Ca₁₂Ge₁₇B₈O₅₈ is isotypic to Cd₁₂Ge₁₇B₈O₅₈ [2], the structural refinement was performed by using the positional parameters of Cd₁₂Ge₁₇B₈O₅₈ as starting values (Shelxl-13 [11], [12]). All atoms were refined anisotropically and the final difference Fourier synthesis did not reveal any significant residual peaks, leading to values of 0.0151 and 0.0339 for R1 and wR2, respectively. The refinement exhibited that the measured crystal was a twin, with the twin law (01̅0, 1̅00, 001̅). The atomic coordinates, anisotropic displacement parameters, and interatomic distances are listed in the Tables 2–4 . Graphical representations of the structure were produced with the program Diamond [13].

Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-informationsdienste.de/en/DB/icsd/depot_anforderung.html) on quoting the deposition number CSD-431201.

2.3 Vibrational spectra

The transmission FT-IR spectrum of a single crystal of Ca₁₂Ge₁₇B₈O₅₈ was measured in the spectral range of 600–4000 cm^–1 with a Vertex 70 FT-IR spectrometer (spectral resolution 4 cm^–1), which is equipped with a KBr beam splitter, an LN-MCT (Mercury Cadmium Telluride) detector and a Hyperion 3000 microscope (Bruker, Vienna, Austria). 320 scans of the sample were acquired using a Globar (silicon carbide) rod as mid-IR source and a 15× IR objective as focus. During the measurement, the sample was positioned on a BaF₂ window. A correction of atmospheric influences was performed with the software Opus 6.5.

3.1 Crystal structure of Ca₁₂Ge₁₇B₈O₅₈

The new calcium borogermanate Ca₁₂Ge₁₇B₈O₅₈ crystallizes isotypically to Cd₁₂Ge₁₇B₈O₅₈ [2] in the tetragonal space group P4̅ (No. 81). The structure is composed of [Ge₄O₁₂]_n chains (Fig. 2a). These chains are interconnected into a [Ge₄O_10.5]_n network via corner sharing (Fig. 2b). Together with the [Ge(B₂O₇)₄]^28– clusters (Fig. 2c) a three-dimensional [Ge₁₇B₈O₅₈]^24– framework (Fig. 2d) is built, forming tunnels of five-, six-, and seven-membered rings along the c axis which are occupied by Ca²⁺ cations (Fig. 3).

Fig. 2:

Scheme showing the [Ge₄O₁₂]_n chain (a); construction of the [Ge₄O_10.5]_n network (b); [Ge(B₂O₇)₄] unit (c); the three-dimensional [Ge₁₇B₈O₅₈]^24– anionic structure (d).

Fig. 3:

Crystal structure of Ca₁₂Ge₁₇B₈O₅₈ along the c axis.

The Ge(1)O₄, Ge(4)O₄, and Ge(5)O₄ tetrahedra as well as the Ge(2)O₆ octahedra are interconnected via vertex sharing building up the above mentioned [Ge₄O₁₂]_n chains (Fig. 2a). Each of these chains is connected with three others to form a three-dimensional [Ge₄O_10.5]_n network via corner sharing (Fig. 2b). The three-dimensional [Ge₄O_10.5]_n network builds up small four-membered and large 24-membered rings forming tunnels along the c axis (Fig. 2b). A B₂O₇ dimer is formed by the B(1)O₄ and B(2)O₄ tetrahedra sharing a common oxygen atom. Four of those dimers are connected to the Ge(3)O₄ tetrahedron via vertex sharing building up the [Ge(B₂O₇)₄]^28– clusters (Fig. 2c). The clusters are positioned in the large 24-membered ring tunnels of the three-dimensional [Ge₄O_10.5]_n network via corner sharing, forming the three-dimensional [Ge₁₇B₈O₅₈]^24– anionic structure with tunnels of five-, six-, and seven-membered rings along the c axis (Fig. 2d) where the Ca²⁺ cations are located (Fig. 3). The five-membered ring is composed of three BO₄ and two GeO₄ tetrahedra. The six-membered ring consists of two BO₄ tetrahedra, three GeO₄ tetrahedra, and one GeO₆ octahedron, whereas the seven-membered ring is formed by one BO₄ tetrahedron, four GeO₄ tetrahedra, and two GeO₆ octahedra.

All three Ca atoms are eight-fold coordinated to oxygen anions with Ca–O distances ranging from 2.311(2) to 3.011(2) Å. The values correspond very well to the Ca–O distances of Ca₁₀Ge₁₆B₆O₅₁ ranging from 2.256(5) to 3.023(4) Å [2]. The two B atoms are coordinated by four oxygen atoms forming distorted tetrahedra with B–O distances ranging from 1.387(4) to 1.581(4) Å. Like in Cd₁₂Ge₁₇B₈O₅₈ [2], the B–O bond lengths exhibit a large variation, which stems from the different coordination environments of the oxygen atoms in the BO₄ tetrahedra. The atom Ge3 is located on the four-fold axis, whereas all other Ge atoms are on general positions. Ge2 is octahedrally coordinated to oxygen anions with Ge–O distances ranging from 1.859(2) to 1.910(2) Å, whereas Ge1, Ge3, Ge4, and Ge5 are tetrahedrally coordinated by oxygen atoms with Ge–O distances between 1.733(2) and 1.796(2) Å. Table 4 shows the interatomic distances of Ca₁₂Ge₁₇B₈O₅₈. Table 5 compares the unit cell parameters of Ca₁₂Ge₁₇B₈O₅₈ and Cd₁₂Ge₁₇B₈O₅₈.

3.2 IR spectroscopy

Figure 4 displays the results of the IR spectroscopic measurement, which was performed on a single crystal of Ca₁₂Ge₁₇B₈O₅₈. The spectrum shows a series of different absorption bands with frequencies below 1500 cm^–1 and from 2800–3000 cm^–1, which can be associated with various vibrations of the different units in the crystal. The absorption bands between 645 and 875 cm^–1 can be related to various modes within the different GeO₄ tetrahedra, GeO₆ octahedra and BO₄ tetrahedra [14], [15], [16], [17], [18]. The region of 925–1200 cm^–1 is due to bands of the symmetric stretching and asymmetric stretching modes of BO₄ tetrahedra [18], [19], [20], [21]. The two small peaks between 1380 and 1450 cm^–1 might represent a combinational tone of two vibration modes of Ge–O–Ge bonds [22]. Due to the overlap of the various bands, a precise assignment of the individual bands is impossible. The absorption bands at 2800–3000 cm^–1 belong to the grease, which was used to fix the crystal on the class fiber. In summary, it can be stated that the IR spectrum confirms the existence of tetrahedrally and octahedrally coordinated germanium, and tetrahedrally coordinated boron atoms.

Fig. 4:

FT-IR reflectance spectrum of Ca₁₂Ge₁₇B₈O₅₈.