Synthesis and structural characterization of Li₃Y(BO₃)₂

2.1 Synthesis

As mentioned above, our first synthesis was based on the system Li₂CO₃–Cs₂CO₃–Y₂O₃–H₃BO₃, leading to the new compound Li₃Y(BO₃)₂. In contrast to Jubera et al., we used a non-stoichiometric mixture of Li₂CO₃ (99.5%, Alfa Aesar, Karlsruhe, Germany), Cs₂CO₃ (99.99%, ChemPur, Karlsruhe, Germany), Y₂O₃ (99.99%, ChemPur), and H₃BO₃ (99.5%, Merck, Darmstadt, Germany) in a molar ratio of 3:3:1:6, which was finely ground in an agate mortar 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), heated up to 900°C at 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 at a rate of 2°C h⁻¹ before switching off the furnace. The product cooled down to room temperature by natural rate.

The product Li₃Y(BO₃)₂ was obtained in the form of colorless, air-, and water-resistant crystals. The powder diffraction pattern (Fig. 1) showed the reflections of Li₃Y(BO₃)₂ as the major phase. The reflections marked with an asterisk could not be assigned until now.

Fig. 1:

Top: experimental powder pattern of Li₃Y(BO₃)₂. The reflections marked with a red asterisk could not be assigned until now. Bottom: theoretical powder pattern of Li₃Y(BO₃)₂ based on single-crystal diffraction data.

2.2 Crystal structure analysis

The powder diffraction pattern of Li₃Y(BO₃)₂ 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 (Fig. 1) shows that they match well. However, the experimental powder pattern has many additional reflections which do not belong to Li₃Y(BO₃)₂.

Small single crystals of Li₃Y(BO₃)₂ were isolated by mechanical fragmentation 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. A multi-scan absorption correction (Sadabs-2014 [14]) was applied to the intensity data sets. All relevant details of the data collection and the refinement are listed in Table 1. The monoclinic space group P2₁/c was derived from the systematic extinctions. The structural refinement was performed with the standardized positional parameters (Platon [15]) of Li₃Gd(BO₃)₂ [4] as starting values (Shelxl-13 [16], [17]). All atoms were refined anisotropically and the final difference Fourier synthesis did not reveal any significant residual peaks. Tables 2–4 list the atomic coordinates, anisotropic displacement parameters, and interatomic distances. Graphical representations of the structure were produced with the program Diamond [18].

Further details of the crystal structure investigation may be obtained from 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-432026.

2.3 Vibrational spectra

The transmission FT-IR spectrum of a single crystal of Li₃Y(BO₃)₂ 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). A total of 320 scans of the sample were acquired using a Globar (silicon carbide) rod as a 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 Li₃Y(BO₃)₂

The lithium yttrium borate Li₃Y(BO₃)₂ crystallizes isotypically to Li₃Gd(BO₃)₂ [4] in the monoclinic space group P2₁/c (no. 14). The structure is composed of YO₈ polyhedra, which form dinuclear units [Y₂O₁₄] by a common edge arranged in the bc plane (Fig. 2). Along the a axis, the [Y₂O₁₄] layers are interconnected to each other by planar B(1)O₃ groups (Fig. 3). Furthermore, LiO₄ tetrahedra, connected via common edges and corners, form chains as shown in Fig. 4. These chains additionally connect the [Y₂O₁₄] units as shown in Fig. 5.

Fig. 2:

Projection of the YO₈ polyhedra forming Y₂O₁₄ dinuclear units viewed along the a axis.

Fig. 3:

Connection between the Y₂O₁₄ units layers to each other by a planar B(1)O₃ group along the a axis.

Fig. 4:

[Li₃O₆]_n chains built up of three edge sharing LiO₄ tetrahedra. The chains are connected to each other via vertex sharing.

Fig. 5:

Crystal structure of Li₃Y(BO₃)₂ along the b axis.

The Y atoms are eightfold coordinated by oxygen atoms forming distorted cubic antiprisms. The Y–O distances range from 2.284(9) to 2.673(9) Å with a mean value of 2.396(9) Å. The values correspond to the Y–O distances of Na₃Y(BO₃)₂ with an average value of 2.323 Å [10]. A Y₂O₁₄ unit is formed by two YO₈ polyhedra sharing a common edge O(6a)–O(6b). These Y₂O₁₄ units are connected to four other ones via vertex sharing (O(5a) and O(5b)) (Fig. 2). The structure contains two crystallographically independent B atoms, which are coordinated to three oxygen atoms forming isolated BO₃ groups with B–O distances from 1.364(2) to 1.399(2) Å. The average value over the B–O distances is 1.379(4) Å, which fits very well with that of other compounds containing isolated BO₃ groups, for example, 1.364(3)–1.397(3) Å for Li₃Gd(BO₃)₂ [4], 1.360–1.399 Å for LiGd₆O₅(BO₃)₃ [7], and the literature value of 1.37 Å for compounds containing BO₃ groups in general [19]. B(1) shares a corner O(2) and an edge O(1)–O(3) with Y atoms of two consecutive [Y₂O₁₄] dimer layers, whereas B(2) shares two edges O(4)–O(5) and O(5)–O(6) with [Y₂O₁₄] units (Fig. 3). All three lithium atoms are tetrahedrally coordinated to oxygen atoms with distances between 1.867(3) and 2.104(3) Å. The mean value of all Li–O distances in Li₃Y(BO₃)₂ comes to 1.968(3) Å, fitting perfectly to values of other compounds containing fourfold coordinated Li atoms as found, for example, in Li₃Gd(BO₃)₂ (mean value 1.973 Å) [4]. The average values of the bond angles in the LiO₄ tetrahedra are 109.3(1)° for Li(1), 109.2(2)° for Li(2), and 108.5(1)° for Li(3), and fit well to the ideal tetrahedral bonding angle of 109.5°. The three LiO₄ tetrahedra are connected to each other by common edges [O(2)–O(3) between Li(1) and Li(2); O(3)–O(5) between Li(2) and Li(3)], and corners [O(1) between Li(1) and Li(2); O(3) and O(4) between Li(1) and Li(2)] building up [Li₃O₆]_n chains along the b axis (Fig. 4). The LiO₄ tetrahedra are connected to the BO₃ triangles via corner sharing. Tables 4 and 5 list all interatomic distances and important bond angles of Li₃Y(BO₃)₂. Table 6 shows a comparison of the unit cell parameters of Li₃Y(BO₃)₂ and of the isostructural compound Li₃Gd(BO₃)₂ [4].

We calculated the bond valence sums for Li₃Y(BO₃)₂ with the bond-length/bond-strength concept (ΣV) [20], [21] and the Chardi concept (charge distribution in solids, ΣQ) [22]. The results of these calculations are listed in Table 7 and correspond well with the expected values of the formal ionic charge of each specific atom.

Maple values (Madelung Part of Lattice Energy) [23], [24], [25] of Li₃Y(BO₃)₂ were compared with those received from the summation of the data for the starting compounds Li₂O [26], Y₂O₃ [27], and B₂O₃ [28]. A value of 8319.1 kcal mol⁻¹ was obtained in comparison with 8318.1 kcal mol⁻¹ (deviation=0.01%) calculated starting from the binary oxides [1½ Li₂O (1255.2 kcal mol⁻¹)+½ Y₂O₃ (1825.1 kcal mol⁻¹)+1 B₂O₃ (5237.5 kcal mol⁻¹)].

3.2 IR spectroscopy

Figure 6 displays the result of the IR spectroscopic measurement, which was performed on a single crystal of Li₃Y(BO₃)₂. The spectrum shows a series of different absorption bands with frequencies below 1500 cm⁻¹, from 2100 to 2650 cm⁻¹, and from 2800 to 3000 cm⁻¹. The assignments of the vibrational modes are based on a comparison with the experimental data of borates containing trigonal [BO₃]³⁻ groups [29], [30], [31], [32], [33]. The two absorption bands between 600 and 800 cm⁻¹ could be assigned as BO₃ deformation and out-of-plane bending. The bands at 900–965 cm⁻¹ belong to the B–O symmetric stretching vibrations. The region between 1150 and 1450 cm⁻¹ can be related to the anti-symmetric B–O stretching vibrations. Interestingly, several unusual absorption bands were observed in the range of 2100–2650 cm⁻¹, which can be interpreted as combination and overtone bands of the BO₃ groups according to Mak et al. [33]. The absorption bands between 2800 and 3000 cm⁻¹ belong to the grease, which was used to fix the crystal on the class fiber.

Fig. 6:

FT-IR reflectance spectrum of Li₃Y(BO₃)₂.