β-Eu(BO₂)₃ – a new member of the β-RE(BO₂)₃ (RE=Y, Nd, Sm, Gd–Lu) structure family

2.1 Crystal structure

β-Eu(BO₂)₃ crystallizes in the orthorhombic space group Pnma (no. 62) like its isotypic compounds β-RE(BO₂)₃ (RE=Y, Nd, Sm, Gd–Lu) [12], [13], [14]. The unit cell (Z=16) exhibits the lattice parameters a=16.071(2), b=7.440(4), and c=12.362(5) Å and a volume of V=1478.1(1) ų. All relevant details of the structure refinement based on single-crystal data are listed in Table 1.

The anionic backbone of β-Eu(BO₂)₃ is built up exclusively of BO₄ tetrahedra. Three of them, namely those with the boron atoms B1, B2, and B3, are connected via the common oxygen atom O7 and form a “windmill-like” structural unit (Fig. 1, top). These “windmills” are linked by three [B₂O₇] groups developing the fundamental building block (FBB) that comprises 12 BO₄ tetrahedra (Fig. 1, bottom). Through the interconnection of the FBBs, infinite ribbons with an approximately triangular cross section are formed along [010] (Fig. 2). These ribbons are connected by the common oxygen atoms O15 to form corrugated layers parallel to the bc plane. The oxygen atom O14 constitutes the only oxygen atom that is coordinated to just one boron atom and represents the terminal corner of the triangular strand.

Fig. 1:

Top: “windmill-like” structural unit comprising three BO₄ tetrahedra that are connected via the common oxygen atom O7. Bottom: Connection of the “windmill-like” units through [B₂O₇] groups (light blue) to form infinite ribbons along [010]. Red spheres: boron; blue spheres: oxygen.

Fig. 2:

Triangular cross section of the fundamental building block (FBB) along [01̅0]. Common oxygen atoms O15 form the link between two of these units at a time. Red spheres: boron; blue spheres: oxygen.

The B–O distances and O–B–O angles lie in the ranges 1.435(3)–1.531(2) Å and 103.5(2)–115.1(2)°, respectively (Tables 2 and 3). The corresponding average values of 1.474 Å and 109.48° are similar to the those reported by Zobetz [1.476(35) Å/109.44(2.78)°] [24]. Interestingly, the two tetrahedra that exhibit both the lowest and the broadest spreading of their O–B–O angles are those which connect the triangular ribbons, B4O₄ and B6O₄, respectively.

The europium cations are located between the anionic BO layers (Fig. 3). The four crystallographically independent Eu³⁺ cations are each coordinated by eight oxygen atoms, forming irregularly shaped polyhedra (Fig. 4). The Eu–O distances range from 2.298 to 2.877 Å (see Table 4), which is in good agreement with the Sm–O and Gd–O distances in the isotypic compounds β-Sm(BO₂)₃ and β-Gd(BO₂)₃ (2.306–2.899 Å and 2.289–2.867 Å, respectively) [14]. Also in comparison to another high-pressure europium-borate, α-Eu₂B₄O₉, which was synthesized at 10 GPa/1423 K and possesses Eu–O distances between 2.26 and 3.04 Å [4], the aforementioned Eu–O distances of β-Eu(BO₂)₃ lie within the same range.

Fig. 3:

Layered structure of β-Eu(BO₂)₃ where the ribbons with approximately triangular cross section are interconnected via common vertices along [001]. The europium cations are located between the layers.

Fig. 4:

Coordination spheres of the four crystallographically independent Eu³⁺ atoms in β-Eu(BO₂)₃.

The bond valence sums were calculated for all atoms in β-Eu(BO₂)₃ according to both, the bond-length/bond-strength (ΣV) [25], [26] as well as the CHARDI (ΣQ) [27] concept (Table 5). The formal ionic charges are in good agreement with the expected values within the limits of the concepts.

Table 6 and Fig. 5 allow a comparison of the lattice parameters and cell volumes of β-Eu(BO₂)₃ with those of the already known meta-borates β-RE(BO₂)₃ (RE=Y, Nd, Sm, Gd–Lu). The new europium compound fits well into this scheme. The positional parameters as well as the isotropic and anisotropic displacement parameters of the new phase are listed in Tables 7 and 8.

Fig. 5:

Comparison of the cell parameters in the β-RE(BO₂)₃ (RE=Y, Nd, Sm-Lu) series.

Interestingly, the title compound was synthesized at 4 GPa, while its neighbors β-Sm(BO₂)₃ and β-Gd(BO₂)₃ were obtained at 7.5 GPa. We tried to receive the latter two borates at the same conditions as the Eu compound, and this turned out to be possible. In contrast, when we tried to synthesize β-Eu(BO₂)₃ at 7.5 GPa, we only received the already known compounds EuB₄O₇ [19], [20] and α-Eu₂B₄O₉ [4].

Further details of the crystal structure investigation may be obtained from The Cambridge Crystallographic Data Centre CCDC/FIZ Karlsruhe deposition service via www.ccdc.cam.ac.uk/structures on quoting the deposition number CCDC 1941204 for β-Eu(BO₂)₃.

2.2 X-ray powder diffraction

Figure 6 shows a comparison between the experimental X-ray powder diffraction pattern and the theoretical powder pattern derived from the single-crystal data of β-Eu(BO₂)₃. Obviously the product was obtained phase-pure, as no side phase is visible in the experimental pattern.

Fig. 6:

Comparison of the experimental X-ray powder diffraction data (top) and the theoretical X-ray powder pattern simulated from the single-crystal data (bottom).

2.3 Vibrational spectroscopy

The infrared (IR) spectrum of the β-Eu(BO₂)₃ powder sample in the range between 400 and 2500 cm⁻¹ in displayed in Fig. 7. Typical stretching vibrations of the BO₄ tetrahedra occur in the region between 900 and 1200 cm⁻¹ [28]. The absorptions that appear between approximately 1300 and 1400 cm⁻¹, around 1200 cm⁻¹ and below 790 cm⁻¹ would be assigned to vibrations of BO₃ groups in standard cases. Since there are no BO₃ groups present in this compound, these vibrational bands may be due to O–B–Eu, B–O–B, O–B–O, and B–O bending and stretching vibrations, as was confirmed by quantum-chemical calculations for β-ZnB₄O₇ and β-CaB₄O₇, two high-pressure compounds that also only feature exclusively BO₄ tetrahedra connected via threefold coordinated oxygen atoms [29]. BO₄ bending vibrations are observed below ~900 cm⁻¹. In the upper range of the spectrum (2000–4000 cm⁻¹), no absorption bands due to hydroxyl groups or H₂O were visible.

Fig. 7:

IR spetrum of a β-Eu(BO₂)₃ powder sample.

2.4 Luminescence properties

The photoluminescence measurements were carried out on a polycrystalline sample. An emission spectrum was collected on excitation at 448 nm (Fig. 8). For orthorhombic structures, five groups of Eu³⁺ transitions are assigned to the ⁵D₀→→⁷F_J (J=0–4) transitions as follows: a weak, single band at 579 nm is due to the ⁵D₀→⁷F₀ transition. The magnetic dipole transitions (⁵D₀→→⁷F₁) in the region between 583 and 596 nm are less intense than the electric dipole transitions (⁵D₀→→⁷F₂) between 614 and 624 nm [30]. This also confirms the presented crystal structure, in which no inversion center was found in the four crystallographic independent Eu³⁺ sites [31]. The ⁵D₀→→⁷F₃ transitions may be attributed to the weak bands around 649 nm while the ⁵D₀→⁷F₄ transitions are visible in the region from 681 to 705 nm. Interestingly, no concentration quenching is observed in this compound, even though Eu³⁺ is the sole cationic species present. For the europium borates EuBO₃, EuB₂O₄, and EuB₄O₇ also no quenching effects were observed [20], [22].

Fig. 8:

Luminescence spectrum of β-Eu(BO₂)₃, obtained upon excitation at 448 nm.

4.1 Synthesis

For the synthesis of the title compound β-Eu(BO₂)₃, the starting materials Eu₂O₃ (Smart Elements, Wien, Austria, 99.99%) and B₂O₃ (Strem Chemicals, Newburyport, MA, USA, 99.9+%) were grinded together in the stoichiometric ratio of 1:3 under ambient conditions. The homogenized mixture was filled into a Pt capsule, placed into a crucible and closed with a lid (both made of α-BN; Henze Boron Nitride Products AG, Germany). The high-pressure/high-temperature experiment was carried out in an 18/11 assembly. It was compressed by eight tungsten carbide cubes (Hawedia, Marklkofen, Germany) using a Walker-type module and a hydraulic 1000 t press (both Max Voggenreiter GmbH, Germany). A detailed description of this setup can be found in the literature [32], [33], [34].

A pressure of 4 GPa was built up within 110 min followed by heating the sample to 1473 K in the following 10 min. This temperature was kept for 10 min, before the sample was cooled to 773 K over the following 30 min. After quenching to room temperature, the 300 min decompression process started. The product was separated from its surroundings to reveal colorless crystals which were found to be β-Eu(BO₂)₃.

4.2 Single-crystal structure analysis

The intensity data of a single-crystal was collected at room temperature with a Bruker D8 Quest Kappa diffractometer equipped with a Photon 100 CMOS detector. Monochromatized MoKα radiation (λ=0.7107 Å) was generated with an Incoatec microfocus X-ray tube and a multilayer optic. Reflections were measured in the range 2.1≤θ≤38.6° and a multiscan absorption correction of the intensity data was performed with Sadabs-2014/5 [35]. For the structure solution and parameter refinement, the software Shelxs/l-2013 [36], [37] implemented in the program WinGX-2013.3 [38] was used.

4.3 X-ray powder diffraction

The powder diffraction pattern of the product was obtained in transmission geometry on a STOE Stadi P powder diffractometer (STOE & Cie GmbH, Darmstadt, Germany). The measurement was performed on a flat sample with Ge(111)-monochromatized MoKα₁ radiation (λ=70.93 pm) in the 2θ range of 2–52° with a step size of 0.015°. The reflections were detected with a Mythen 1 K detector (Dectris).

4.4 Vibrational spectroscopy

An FTIR-ATR (Attenuated Total Reflection) characterization of the β-Eu(BO₂)₃ powder sample was performed in the spectral range of 400–4000 cm⁻¹ with a Bruker ALPHA Platinum-ATR spectrometer (Bruker, Billerica, MA, USA) provided with a 2×2 mm diamond ATR-crystal and a DTGS detector. Three hundred and twenty scans of the powder sample were acquired and afterwards corrected for atmospheric influences employing the Opus 7.2 software [39].

4.5 Luminescence spectroscopy

The emission spectrum of the powder sample was collected using a setup equipped with an AvaSpec2048 spectrometer (AVANTES, Apeldoorn, Netherlands). As excitation source, a blue laser diode (THORLABS, Newton, NJ, USA) with 448 nm wavelength was used. Prior to the experiments, a tungsten-halogen calibration lamp was used for a spectral radiance calibration of the setup. The software AVA AvaSoft full version 7 was used for data handling. The excitation spectrum was measured in the range of 200–1100 nm and was background-corrected.