Synthesis and structural characterization of Li₃K₃Eu₇(BO₃)₉

2.1 Synthesis

As mentioned above, our synthesis was based on the system Li₂O–K₂O–Eu₂O₃–B₂O₃ leading to the new compound Li₃K₃Eu₇(BO₃)₉. The starting materials Li₂CO₃ (99.5%, Alfa Aesar, Karlsruhe, Germany), K₂CO₃ (99.99%, ChemPur, Karlsruhe, Germany), Eu₂O₃ (99.99%, Smart Elements, Wien, Austria) and H₃BO₃ (99.5%, Merck, Darmstadt, Germany) were weighed, finely ground in an agate mortar according to the molar ratio of 3:6:1:7, and filled into a platinum FKS 95/5 crucible (feinkornstabilisiert, 95% Pt, 5% Au, Ögussa, Wien, Austria).

The sample was positioned in an electric resistance furnace (Nabertherm muffle furnace) and heated up to 900°C with a rate of 90°C h⁻¹ and maintained at that temperature for 24 h. After that period, the temperature was lowered to 500°C with a rate of 2°C h⁻¹ followed by switching off the furnace. The product cooled down to room temperature by natural rate.

The product Li₃K₃Eu₇(BO₃)₉ was obtained in form of colorless, air and water resistant crystals. Any effort to synthesize Li₃K₃Eu₇(BO₃)₉ from a stoichiometric mixture of the starting materials failed. The synthesis was only possible with the exact composition and heating process mentioned above.

2.2 Crystal structure analysis

The powder X-ray diffraction (PXRD) pattern of the reaction product was measured in transmission geometry using a Stoe Stadi P powder diffractometer with Ge(111)-monochromatized MoK_α1 (λ=70.93 pm) radiation. The comparison of the experimental and theoretical PXRD pattern simulated from single-crystal data of Li₃K₃Eu₇(BO₃)₉ showed that they match well (Fig. 1). However, the experimental PXRD pattern has some additional reflections (marked with red asterisks), which do not belong to Li₃K₃Eu₇(BO₃)₉ and arise from side products, which could not be assigned until now.

Fig. 1:

Top: experimental powder pattern of Li₃K₃Eu₇(BO₃)₉. The reflections marked with red asterisks could not be assigned until now. Bottom: theoretical powder pattern of Li₃K₃Eu₇(BO₃)₉ based on single-crystal diffraction data.

For the single-crystal structure analysis, small crystals of Li₃K₃Eu₇(BO₃)₉ were isolated by mechanical fragmentation and picked using a polarization microscope. A Bruker D8 Quest Kappa diffractometer with MoK_α radiation (λ=71.073 pm) was used to collect the single-crystal intensity data at room temperature of 298 K. A multi-scan absorption correction (Sadabs-2014 [16]) was applied to the intensity data sets. All relevant details of the data collection and the refinement are listed in Table 1. The orthorhombic space group Pca2₁ was derived from the systematic extinctions of Li₃K₃Eu₇(BO₃)₉. The structure solution and parameter refinement were successfully performed with Shelxl-13 [17], [18]. All atoms were refined anisotropically and the final difference Fourier synthesis did not reveal any significant residual peaks. The structural refinement of Li₃K₃Eu₇(BO₃)₉ was based on the positional parameters of the isotypic compound Li₃K₃Y₇(BO₃)₉ [12]. Table 2 lists the interatomic distances. Atomic coordinates and anisotropic displacement parameters are listed in the Tables S1 and S2 of the Supplementary Information. Graphical representations of the structure were produced with the program Diamond [19].

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-433210.

2.3 Vibrational spectra

The transmission FT-IR spectrum of a single-crystal of Li₃K₃Eu₇(BO₃)₉ 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). One hundred and twenty 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.

2.4 Luminescence

The title compound’s emission signal was measured by exciting a single-crystal (exhibiting the above mentioned unit-cell metrics) with a 460 nm laser (model Sapphire 460/10, 10 mW; COHERENT). The converted light was collected using a multi-mode optical fiber (QP 600-2-VIS/BX; Ocean Optics) and finally detected in a spectrometer (QE 65000; Ocean Optics).

3.1 Crystal structure of Li₃K₃Eu₇(BO₃)₉

As already mentioned, Li₃K₃Eu₇(BO₃)₉ crystallizes isotypically to Li₃K₃Y₇(BO₃)₉ [12] in the orthorhombic space group Pca2₁ (no. 29) with lattice parameters of a=21.126(2), b=6.502(2), c=17.619(2) Å, V=2420.1(2) ų, and Z=4. The structure features distorted LiO₆ octahedra, which are connected via planar BO₃ groups, forming [Li₃B₄O₂₁] units in the ac plane (Fig. 2). These units are furthermore connected by additional BO₃ groups, forming tunnels of 12-membered rings along the b axis (Fig. 2). The K⁺ and Eu³⁺ cations are arranged in the cavities of the structure (Fig. 3). K(1) is eight-fold, K(2) is 10-fold, and K(3) is seven-fold coordinated by oxygen atoms. Eu(1), Eu(3), Eu(4) and Eu(5) are all coordinated by eight oxygen atoms, whereas Eu(2), Eu(6) and Eu(7) are nine-fold coordinated by oxygen atoms. For a detailed description of the crystal structure, the reader is referred to the description of the isotypic compound Li₃K₃Y₇(BO₃)₉ [12]. Below, a comparison of the two isotypic compounds Li₃K₃RE₇(BO₃)₉ (RE=Y, Eu) is given.

Fig. 2:

Top: [Li₃B₄O₂₁] units connected to each other via BO₃ groups along the crystallographic b axis. Bottom: connection of LiO₆ octahedra via BO₃ groups along the a axis.

Fig. 3:

Top: View of the structure of Li₃K₃Eu₇(BO₃)₉ along the crystallographic b axis. Bottom: structure along the a axis.

Table 3 shows a comparison of the lattice parameters of Li₃K₃Eu₇(BO₃)₉ and Li₃K₃Y₇(BO₃)₉. The increase of the lattice parameters corresponds to the increase of the ionic radii from Y³⁺ [1.159 Å for coordination number (CN) eight and 1.215 Å for CN nine] to Eu³⁺ (1.206 Å for CN eight and 1.260 Å for CN nine) [20]. The differences of the lattice parameters also cause an increase of the interatomic distances in Li₃K₃Eu₇(BO₃)₉ compared to Li₃K₃Y₇(BO₃)₉, see Table 4. Due to the increase of the lattice parameters and interatomic distances, the coordination environment of several cations changes. Table 5 compares the interatomic distances of K(3)–O and of Eu(7)–O in comparison with Y(7)–O on the two compounds, which lead to a change of the CN of K(3) from six to seven, and from eight for Y(7) to nine for Eu(7). All other CNs remain the same as reported in Li₃K₃Y₇(BO₃)₉.

In Li₃K₃Eu₇(BO₃)₉, the K–O distances range from 2.640(4) to 3.175(6) Å, with average values of 2.854 (K1), 2.902 (K2) and 2.973 Å (K3). The distances within the EuO_x polyhedra are between 2.250(3) and 2.802(4) Å, with mean values of 2.387 (Eu1), 2.489 (Eu2), 2.418 (Eu3), 2.401 (Eu4), 2.451 (Eu5), 2.498 (Eu6) and 2.512 Å (Eu7). The average values of the B–O bonds range from 1.372 to 1.381 Å, which fit very well to the literature value of 1.37 Å [21] for common B–O distances in BO₃ groups. The Li–O distances range from 1.945(13) to 2.941(18) Å with mean values of 2.232, 2.241, and 2.231 Å for Li(1), Li(2) and Li(3), respectively.

We additionally calculated the bond valence sums for Li₃K₃Eu₇(BO₃)₉ with the bond-length/bond-strength concept (ΣV) [22], [23] and the CHARDI concept (charge distribution in solids, ΣQ) [24]. The results of these calculations are listed in Table 6 and correspond well with the expected values of the formal ionic charge of each specific atom.

MAPLE values (Madelung Part of Lattice Energy) [25], [26], [27] of Li₃K₃Eu₇(BO₃)₉ were compared with those received from the summation of the data for the starting compounds Li₂O [28], K₂O [29], Eu₂O₃ [30] and B₂O₃ [31]. A value of 158777 kJ mol⁻¹ was obtained in comparison to 155254 kJ mol⁻¹ (deviation=0.03%) calculated from the binary oxides [1½ Li₂O (5254 kJ mol⁻¹)+1½ K₂O (3767 kJ mol⁻¹)+3½ Eu₂O₃ (47574 kJ mol⁻¹)+4½ B₂O₃ (98659 kJ mol⁻¹)]. The good accordance between the value of the product and the sum of the values of the binary compounds prove the plausibility of the crystal structure solution.

3.2 IR spectroscopy

Figure 4 shows the experimental FT-IR spectra of Li₃K₃Eu₇(BO₃)₉ and Li₃K₃Y₇(BO₃)₉ [12] in the range between 600 and 4000 cm⁻¹. The absorption bands of the two compounds correspond well with negligible shifts in wavelengths and relative intensities. As already reported for the isotypic compound Li₃K₃Y₇(BO₃)₉ [12], an assignments of the vibrational modes can be done based on previous investigations of borates containing trigonal [BO₃]³⁻ groups [32], [33], [34], [35], [36]. The absorption bands between 600 and 800 cm⁻¹ could be assigned as deformation and out-of-plane bending of BO₃ groups. The bands in the region from 900 to 965 cm⁻¹ are typical for B–O symmetric stretching vibrations. The strong bands observed in the spectral range between 1150 and 1500 cm⁻¹ can be related to the antisymmetric B–O stretching vibrations. According to Mak et al. [36] the region between 2100 and 2650 cm⁻¹ can be related to combination- and overtone-bands of the BO₃ groups. The absorption peaks in the range of 2800–3000 cm⁻¹ most likely belong to the grease, which was used to fix the crystal on a glass fiber.

Fig. 4:

FT-IR absorbance spectra of Li₃K₃Eu₇(BO₃)₉ (black) and the isotypic compound Li₃K₃Y₇(BO₃)₉ (red).

3.3 Luminescence

Figure 5 shows the title compound’s emission signal on excitation at 460 nm. The profile can be assigned to typical Eu³⁺ luminescence regarding the transition ⁵D₀→⁷F_J (J=0–4). From the relative intensities, one can conclude that there is no inversion symmetry around the Eu³⁺ activator ion which is in good agreement with the results of the crystal structure determination. To our astonishment, concentration quenching of Li₃K₃Eu₇(BO₃)₉ was not observed. Ortner et al. also reported this phenomenon in the europium borate Eu[B₆O₈(OH)₅]·H₃BO₃ [37], which could be expected due to the fact that the shortest Eu–Eu distance is 4.459(1) Å. Obviously an Eu–Eu distance of 3.556(2) Å in Li₃K₃Eu₇(BO₃)₉ does not lead to concentration quenching of the Eu³⁺ activator ion.

Fig. 5:

Luminescence spectrum of a single-crystal of Li₃K₃Eu₇(BO₃)₉, obtained on excitation at 460 nm.