RE₂B₈O₁₅ (RE = La, Pr, Nd) – syntheses of three new rare earth borates isotypic to Ce₂B₈O₁₅

2.1 Syntheses

For the syntheses of the compounds RE₂B₈O₁₅ (RE = La, Pr, Nd), mixtures of the corresponding rare earth oxides (La₂O₃, Sigma Aldrich, 99.9 %; Pr₆O₁₁, Strem Chemicals, 99.9 %; Nd₂O₃, Strem Chemicals, 99.9 %) with boron oxide (B₂O₃, Strem Chemicals, 99.9 %) were used as the starting materials according to the following equations.

RE2O3 + 4 B2O3 →1100 ∘C5.5 GPaRE2B8O15(RE = La, Nd)

Pr6O11 + 12 B2O3→1100 ∘C5.5 GPa3 Pr2B8O15 + O2

A slight stoichiometric excess of boron oxide was found to be crucial to avoid the formation of undesirable side products like δ-RE(BO₂)₃ [4, 15], γ-RE(BO₂)₃ (RE = La, Nd) [16] or λ-PrBO₃ [17] in all syntheses. The educt mixtures were finely ground under inert gas atmosphere and filled into crucibles of hexagonal boron nitride (Henze Boron Nitride Products GmbH, HeBoSint^® P100, Kempten, Germany). These were compressed inside of a Walker-type multianvil assembly using a 1000 ton hydraulic press (Voggenreiter, Mainleus, Germany) and heated via resistive heating of a graphite tube surrounding the crucible. For detailed information of the assembly, the reader is referred to references [18–22]. For the synthesis of all three new compounds, the educts were compressed to 5.5 GPa in 140 min, and this pressure was maintained during the following heating period. The temperature was raised from room temperature to 1100 °C within 10 min and kept there for 20 min, before cooling down to 500 °C within 30/300/120 min for the La, Pr, and Nd phase, respectively. Afterwards, the samples were quenched to room temperature by switching off the heating. The decompression of the assemblies required about 7 h. The recovered octahedral pressure media were broken apart and the samples carefully separated from the surrounding boron nitride crucibles. The compounds RE₂B₈O₁₅ (RE = La, Pr, Nd) were obtained as colorless (La₂B₈O₁₅), green (Pr₂B₈O₁₅), and alexandritic pink-gray (Nd₂B₈O₁₅) crystalline products that are stable under ambient conditions.

Besides the boron oxide excess in the educt mixtures, the pressure applied during the syntheses played a crucial role for the formation of the desired products RE₂B₈O₁₅ (RE = La, Pr, Nd). Lowering the pressure to 4 GPa, while keeping all other conditions of the syntheses identical, led to the formation of δ-RE(BO₂)₃ (RE = La, Nd) [4, 15] as the main product of the syntheses with RE = La, Nd, and to a high amount of λ-PrBO₃ [17] as side product of the syntheses of Pr₂B₈O₁₅. Above 6 GPa, γ-RE(BO₂)₃ (RE = Pr, Nd) [16] were the main products and the attempt to synthesize La₂B₈O₁₅ resulted in the formation of a yet unknown, very dark, and microcrystalline phase.

2.2 X-ray structure determinations

The products RE₂B₈O₁₅ (RE = La, Pr, Nd) were identified by X-ray powder diffraction on flat samples in transmission geometry using a Stoe Stadi P powder diffractometer (MoK_α1 radiation, Ge(111)-monochromatized, λ = 70.93 pm). All three new compounds RE₂B₈O₁₅ (RE = La, Pr, Nd) could be identified by comparing the reflection patterns with that of the isotypic phase Ce₂B₈O₁₅ [14]. Apart from a slight shift caused by minor differences of the lattice parameters due to the lanthanide contraction, the reflections tallied perfectly. Figure 1 shows exemplarily the powder pattern of Pr₂B₈O₁₅. As it was not possible to analyze the crystal structure of Pr₂B₈O₁₅ by means of single-crystal X-ray structure determination, the experimental powder pattern of Pr₂B₈O₁₅ (top) is compared with the theoretical powder pattern of Nd₂B₈O₁₅ (bottom) based on single-crystal diffraction data. For best comparability, the theoretical pattern was calculated with Pr³⁺ as the rare earth cation and by applying the lattice parameters obtained from indexing of the experimental powder diffraction pattern of Pr₂B₈O₁₅. By indexing the reflections of RE₂B₈O₁₅ (RE = La, Pr, Nd), we obtained the parameters a = 919.0(2), b = 421.64(5), c = 1251.2(2) pm, β = 116.61(1)°, and a volume of 0.43349(7) nm³ for La₂B₈O₁₅, a = 912.37(9), b = 420.19(4), c = 1246.3(2) pm, β = 116.82(1)°, and a volume of 0.42639(5) nm³ for Pr₂B₈O₁₅, and a = 908.87(6), b = 419.32(3), c = 1244.5(2) pm, β = 116.84°, and a volume of 0.42319(4) nm³ for Nd₂B₈O₁₅. The values for La₂B₈O₁₅ and Nd₂B₈O₁₅ are in good agreement with the lattice parameters received from the single-crystal X-ray diffraction data (Table 1) and the values obtained for Pr₂B₈O₁₅ correspond well to the progression of the lattice parameters along the series of lanthanides (vide infra).

Fig. 1:

Top: experimental powder X-ray diffraction pattern of Pr₂B₈O₁₅. The asterisk indicates a reflection caused by small amounts of λ-PrBO₃. Bottom: theoretical powder pattern based on single-crystal diffraction data of Nd₂B₈O₁₅, calculated with Pr³⁺ as the rare earth cation and with the lattice parameters obtained from indexing of the powder diffraction pattern of Pr₂B₈O₁₅.

For La₂B₈O₁₅ and Nd₂B₈O₁₅, we were able to isolate small single crystals by mechanical fragmentation that could be analyzed via single-crystal X-ray diffraction. The intensity data were collected at room temperature with a Bruker D8 Quest diffractometer (Photon 100) equipped with an Incoatec Microfocus source generator (multi layered optics monochromatized MoK_α radiation, λ = 71.073 pm). “Multi-scan” absorption corrections were applied with the program Bruker Difabs-2014/5. The positional parameters of Ce₂B₈O₁₅ [14] were used as starting values for the structure solution of La₂B₈O₁₅ and Nd₂B₈O₁₅. The parameter refinements (full-matrix least-squares against F²) were executed utilizing the Shelxs/l-97 software suite [23, 24]. For both compounds, all atoms could be refined with anisotropic atomic displacement parameters. Final difference Fourier syntheses did not reveal any significant residual peaks. All relevant details of the data collections and evaluations are given in Table 1. The atomic coordinates and isotropic equivalent displacement parameters, interatomic distances, and interatomic angles are listed in the Tables 2–4.

Further details of the crystal structure investigations may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, https://icsd.fiz-karlsruhe.de/search/basic.xhtml) on quoting the deposition numbers CSD-430713 for La₂B₈O₁₅ and CSD-430714 for Nd₂B₈O₁₅.

2.3 Vibrational spectroscopy

The ATR-FT-IR spectra of RE₂B₈O₁₅ (RE = La, Ce, Pr, Nd) were measured from small amounts of the grinded products utilizing a Bruker ALPHA-P “Platinum-ATR” FT-IR spectrometer with a spectral resolution of <2 cm⁻¹. The IR radiation with a wavelength of 850 nm was produced by a HeNe-laser and the reflected beam was detected by a DTGS detector at room temperature. For the measurements, the samples were positioned between two small diamond crystals of the IR spectrometer. 60 scans of each sample and the background were acquired using the Opus 7.0 [25] software.

3.1 Crystal structures

As already mentioned above, all three new compounds RE₂B₈O₁₅ (RE = La, Pr, Nd) crystallize isotypically to Ce₂B₈O₁₅ [14]. Figure 2 shows the crystal structures of RE₂B₈O₁₅ (RE = La, Pr, Nd), which are built up solely from corner-sharing [BO₄]⁵⁻ tetrahedra that are strongly interconnected via threefold bridging oxygen atoms to form a complex three-dimensional framework that hosts the rare earth cations (Fig. 3). The main structural motif consists of four [BO₄]⁵⁻ tetrahedra, three of them building up a three-membered ring and the fourth tetrahedron being connected to this ring via the threefold bridging oxygen atom O3. The fundamental building block can therefore be described as 4□:[Φ]<3□>|□| (after Burns et al. [26]). The rare earth cations are coordinated by 11 oxygen atoms in the form of an irregularly shaped coordination sphere as depicted in Fig. 4. For a detailed description of the crystal structure, the reader is referred to the description of the isotypic compound Ce₂B₈O₁₅ [14]. Below, a comparison of the four isotypic compounds RE₂B₈O₁₅ (RE = La, Ce, Pr, Nd) is given.

Fig. 2:

Crystal structure of RE₂B₈O₁₅ (RE = La, Ce, Pr, Nd) highlighting the fundamental building block 4□:[Φ]<3□>|□|.

Fig. 3:

Crystal structure of RE₂B₈O₁₅ (RE = La, Ce, Pr, Nd) along [010], showing the complex anionic network formed by the [BO₄]⁵⁻ tetrahedra and the channels along the b axis that host the rare earth cations.

Fig. 4:

Coordination sphere of the rare earth cations in RE₂B₈O₁₅ (RE = La, Ce, Pr, Nd).

Figure 5 shows a comparison of the lattice parameters of RE₂B₈O₁₅ (RE = La, Ce, Pr, Nd) derived from indexing of the powder diffraction patterns. The decrease of the lattice parameters along the series La³⁺–Nd³⁺ corresponds to the decreasing ionic radii of the rare earth cations. The lanthanide contraction is also the cause for slightly decreasing interatomic distances RE–O from La₂B₈O₁₅ to Nd₂B₈O₁₅. The average RE–O distance equals 267.5 pm in La₂B₈O₁₅, 266.3 pm in Ce₂B₈O₁₅, and 264.3 pm in Nd₂B₈O₁₅. The mean interatomic distances of the B–O bonds possess values of 147.7 pm in La₂B₈O₁₅, 147.6 pm in Ce₂B₈O₁₅, and 147.5 pm in Nd₂B₈O₁₅. These values are equal within the range of the respective standard deviations. The mean distances B–O of each [BO₄]⁵⁻ group within the structures of RE₂B₈O₁₅ (RE = La, Ce, Nd) are all in perfect agreement with the literature value of 147.6 pm [27, 28]. The results of n-MEFIR (Mean Fictive Ionic Radii) calculations with the MAPLE program [29] imply that La₂B₈O₁₅ has the lowest atomic packing factor (64.2 %), followed by Ce₂B₈O₁₅ (64.8 %) and Nd₂B₈O₁₅ (64.8 %). This corresponds to the calculated densities of the compounds, namely 4.62 g cm⁻³ for La₂B₈O₁₅, 4.68 g cm⁻³ for Ce₂B₈O₁₅ [14], and 4.81 g cm⁻³ for Nd₂B₈O₁₅. The interatomic angles O–B–O within the [BO₄]⁵⁻ tetrahedra show no significant variation within all three compounds RE₂B₈O₁₅ (RE = La, Ce, Nd). With average values of 109.4 to 109.5° within each [BO₄]⁵⁻ group, they are all very close to the ideal tetrahedral angle of 109.47°.

Fig. 5:

Progression of the lattice parameters (pm) of RE₂B₈O₁₅ (RE = La, Ce, Pr, Nd) with the typical decrease due to the lanthanide contraction.

The charge distribution of the atoms in the compounds RE₂B₈O₁₅ (RE = La, Nd) was calculated according to the BLBS (bond length/bond strength, ΣV) [30–34] and the CHARDI (charge distribution in solids) concept (ΣQ) [32, 33, 35]. The results are listed in Table 5 and verify the formal valence states of the cations and anions.

Additionally, we calculated the MAPLE values (Madelung Part of Lattice Energy) according to Hoppe [29, 36, 37] of RE₂B₈O₁₅ (RE = La, Nd), which were checked against the data of the binary compounds. We obtained a value of 102125 kJ mol⁻¹ for La₂B₈O₁₅ to be compared with 101986 kJ mol⁻¹ (deviation: 0.13 %) based on the binary compounds [La₂O₃ (14234 kJ mol⁻¹ [38]) + 4 × HP-B₂O₃ (21938 kJ mol⁻¹ [39])] and a value of 102401 kJ mol⁻¹ for Nd₂B₈O₁₅ to be compared with 102316 kJ mol⁻¹ (deviation: 0.08 %) based on the binary compounds [Nd₂O₃ (14564 kJ mol⁻¹ [40]) + 4 × HP-B₂O₃ (21938 kJ mol⁻¹ [39])]. The good accordance between the values of the products and the sum of the values of the binary compounds prove the plausibility of the crystal structure solutions.

3.2 FT-IR spectroscopy

Figure 6 shows the IR spectra of RE₂B₈O₁₅ (RE = La, Ce, Pr, Nd) in the range of 400–1700 cm⁻¹. The absorption bands correspond well for all four compounds with slight shifts in wavelengths and relative intensities. The spectra were recorded between 400 and 4000 cm⁻¹ and showed no vibrational bands due to OH groups or water in the upper range (2000–4000 cm⁻¹). As already reported for Ce₂B₈O₁₅ [14], a distinct assignment of the detected absorption bands to certain vibrations is difficult due to the highly cross-linked framework of the crystal structure and the resulting complexity of absorption peaks in the spectra. However, a general assignment of various regions of the spectra can be done based on previous investigations and theoretical calculations. The strong bands observed in the spectral range between 800 and 1150 cm⁻¹ are typical for stretching vibrations within [BO₄]⁵⁻ groups as in γ-RE(BO₂)₃ (RE = La–Nd), π-GdBO₃, π-YBO₃, or TaBO₄ [16, 41–43]. Absorption peaks in the range of 1200–1500 cm⁻¹ are usually assigned to stretching vibrations of trigonal [BO₃]³⁻ groups. As the crystal structures of RE₂B₈O₁₅ (RE = La, Ce, Pr, Nd) do not contain any [BO₃]³⁻ groups, the absorptions in this spectral range have to be assigned to B–O–RE, O–B–O, B–O–B, and B–O stretching and bending vibrations. This was found by ab initio quantum chemical calculations on the oxoborates β-ZnB₄O₇ and β-CaB₄O₇ [44]. Their crystal structure is similarly built up exclusively from [BO₄]⁵⁻ groups forming a complex anionic network structure and show IR absorption bands in the range between 1200 and 1500 cm⁻¹ as well. Furthermore, the existence of strongly shortened B–O bond lengths caused by the tight interconnections of [BO₄]⁵⁻ groups in the crystal structures of RE₂B₈O₁₅ (RE = La, Ce, Pr, Nd) can also be named as a reason for the finding of absorptions in the range of 1200–1500 cm⁻¹. The interatomic distances B3–O7 of only 140.9(2) pm in La₂B₈O₁₅ and Ce₂B₈O₁₅ as well as of 141.1(2) pm in Nd₂B₈O₁₅ are more in the range of classical [BO₃]³⁻ groups (137.0 pm [45]) rather than [BO₄]⁵⁻ groups (147.6 pm [27, 28]).

Fig. 6:

IR spectra of RE₂B₈O₁₅ (RE = La, Ce, Pr, Nd).

An interference caused by small amounts of amorphous side products that could not be detected by means of powder diffractometry cannot be excluded. However, it is worth mentioning that the IR spectra measured on the bulk sample of Ce₂B₈O₁₅ correspond well to those of the single-crystal measurement of Ce₂B₈O₁₅ as reported in Ref. [14].