High-pressure synthesis of REB₅O₈(OH)₂ (RE = Ho, Er, Tm)

2.1 Synthesis

All three compounds REB₅O₈(OH)₂ (RE = Ho, Er, Tm) were synthesized under high-pressure conditions utilizing a Walker-type multianvil module in a 1000 t downstroke press (both Max Voggenreiter GmbH, Mainleus, Germany) from a mixture of the rare earth metal oxides (RE₂O₃; RE = Ho, Er, Tm), boric acid H₃BO₃, and diboron trioxide B₂O₃. The total mass and the molar ratios are given in the following: Ho₂O₃:H₃BO₃:B₂O₃ = 45.30 mg (0.12 mmol):29.65 mg (0.48 mmol):25.04 mg (0.36 mmol), Er₂O₃:H₃BO₃:B₂O₃ = 45.61 mg (0.12 mmol):29.49 mg (0.48 mmol):24.90 mg (0.36 mmol), and Tm₂O₃:H₃BO₃:B₂O₃ = 45.83 mg (0.12 mmol):29.37 mg (0.48 mmol):24.80 mg (0.36 mmol).

After grinding, the mixtures were placed in 18/11 assemblies and compressed by eight tungsten carbide cubes (Hawedia, Marklkofen, Germany). Further information concerning the high-pressure assembly and its preparation can be found in the literature [3], [4], [5]. The samples were then compressed in the multianvil apparatus to 2.5 GPa within 65 min, heated to 673 K in 5 min, kept at T = 673 K for 30 min, and then cooled to room temperature in 60 min. However, in the case of TmB₅O₈(OH)₂ this procedure was not sufficient to yield single crystals of satisfactory size. The temperature was therefore increased to 723 K, kept at 723 K for 45 min, and then cooled to room temperature in 90 min, which yielded the desired single-crystal quality. After cooling, the decompression of the module was conducted over 180 min. Subsequently, the samples were separated from the surroundings and the final products were recovered as air stable crystals. Ambient pressure synthesis was attempted for all three title compounds, however these attempts were unsuccessful and resulted in yet unidentified phases.

2.2 X-ray structure determination

A STOE Stadi P diffractometer with monochromatized MoKα₁ radiation (λ = 70.93 pm; curved Ge(111)) was used to characterize polycrystalline samples by powder X-ray diffraction. Diffraction intensities were measured with a Mythen2 1K microstrip detector with 1280 strips (Dectris, Baden-Daettwil, Switzerland).

The products were ground in an agate mortar, fixed between two polyacetate films and investigated by powder X-ray diffraction (Figure 1). For clarity, the powder diffraction data of all three compounds is compared in one single Figure for HoB₅O₈(OH)₂, ErB₅O₈(OH)₂, and TmB₅O₈(OH)₂ (top to bottom). Despite several attempts, we were thus far unable to obtain the title compounds as phase pure products. The diffraction patterns indicate significant amounts of impurities (marked by asterisks), likely a combination of an unidentified side phase, boric acid, and the corresponding rare earth metal oxide (starting materials).

Figure 1:

Powder X-ray diffraction data of REB₅O₈(OH)₂ (RE = Ho, Er, Tm) compared to the theoretical pattern of DyB₅O₈(OH)₂.

Under a polarisation microscope, suitable single crystals were fixed on the tip of a MicroMount™ (MiTeGen, LLC, Ithaca, NY, USA) and placed inside the diffractometer. The intensity data was collected with a Bruker D8 Quest diffractometer (Bruker, Karlsruhe, Germany) equipped with a Photon 100 detector system and an Incoatec microfocus source generator (multi-layered optic, monochromatized MoKα radiation, λ = 71.073 pm). Concerning the ω and φ scans, the collection strategy was optimized using the Apex III [21] program package. Thus, complete data sets up to high angles with high redundancies were received. For data processing and data reduction, the program Saint [22] was employed. Thereafter, multi-scan absorption corrections were applied with the program Sadabs [23].

The structure solution and parameter refinement with anisotropic displacement parameters was done utilising the Shelxs/l-2013 [24], [25] software implemented in the program WinGX-2013.3 [26]. In the course of the structure determinations, the non-centrosymmetric monoclinic space group C2 was found to be correct, but only partial anisotropic refinement was possible for the oxygen and boron positions. Therefore, we have decided to utilize an anisotropic refinement only for the rare earth cations (holmium, erbium, and thulium) and to stick with an isotropic refinement for the oxygen and boron positions.

Relevant details of the data collection and evaluation are listed in Table 1, the atomic coordinates and the isotropic displacement parameters are given in Table 2. Interatomic distances are shown in Table 3 and bond angles in Table 4.

CCDC 2033530 (HoB₅O₈(OH)₂), 2033529 (ErB₅O₈(OH)₂), and 2033532 (TmB₅O₈(OH)₂) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

2.3 Infrared spectroscopy

A Bruker Alpha Platinum spectrometer using the ATR method (attenuated total reflection) was used to record the IR data. The spectra were measured on the bulk phase in the range from 4000 to 400 cm⁻¹. The Opus 7.2 [27] program was used to correct for atmospheric influences and to process the IR data.

3.1 Crystal structure

The title compounds HoB₅O₈(OH)₂, ErB₅O₈(OH)₂, and TmB₅O₈(OH)₂ crystallize in the monoclinic non-centrosymmetric space group C2 (no. 5), show exactly the same site symmetry, are clearly isotypic to each other, and complement the previously characterized compound DyB₅O₈(OH)₂ [20]. The change in the cell parameters with the exchange of the rare earth metal cation is shown in Figure 2. As expected, the progression to higher atomic numbers (and therefore smaller atomic radii) results in a contraction of the overall cell volume V. This however only corresponds to a decrease of the cell parameters a and b. Interestingly, the cell parameter c decreases from dysprosium to holmium, but shows a sharp increase towards the thulium compound. The monoclinic angle β steadily increases, however the same sharp increase is observed from holmium to erbium.

Figure 2:

Progression of the cell parameters in REB₅O₈(OH)₂ (RE = Dy, Ho, Er, Tm).

The fundamental building block is formed by three crystallographically different borate tetrahedra with B–O distances ranging from 142(2) to 149(2) pm and O–B–O angles ranging from 104.3(5) to 114(2)°, which both agree well with values presented in the literature [28]. The combination of fundamental building blocks yields a layer-like structure along the ab plane and a stacking along the c axis with alternating rare earth metal cation and borate domains (Figure 3). Each borate layer is built up from two layers of sechser rings, expanding along the ab plane (Figure 3; right), which are interconnected by B3 tetrahedra, thereby forming dreier rings. The term sechser ring was coined by F. Liebau, is derived from the German word “sechs” and describes a ring with six tetrahedral centres [29]. The same is applicable for dreier rings, where “drei” is used to describe a three membered ring. The layer transition depicting the rare earth metal cation between two adjacent sechser rings is shown in Figure 4. In the case of the transition from Dy³⁺ to Tm³⁺, the ionic radii decrease from 105.2 to 102 pm [30], which results in an overall smaller space requirement. While the borate layer thickness (Figure 3, orange arrow) remains virtually unchanged, the cation-cation distance (dark yellow arrow) decreases from 453.41(2) (Dy) to 451.3(1) pm (Tm). The overall layer thickness (measured as the distance from cation to cation through the layer, Figure 3, dark green arrow) decreases as well, from 883.01(7) (Dy) to 879.00(9) pm (Tm). Due to the contraction of the layer thickness, the monoclinic angle β and the c axis increase, while the a and b axes decrease, as well as the overall cell volume (see Figure 2). This modification of the cell parameters is presumably caused by the smaller space requirement of the cation and the decreasing RE–O distances.

Figure 3:

Layer-like structure stacking along the c axis (left) and detail of the sechser rings expanding in the ab plane (right) in REB₅O₈(OH)₂ (RE = Dy, Ho, Er, Tm).

Figure 4:

Rare earth metal cation surrounded by borate sechser rings.

At this point, it should be noted that identifying the position of hydrogen atoms via X-ray diffraction experiments can be difficult. Although the hydrogen atoms could be located in the structure of DyB₅O₈(OH)₂ (interconnecting the B1 and B2 tetrahedra and spanning along the sechser rings), this was not possible in the present cases. However, for charge equalization, one hydrogen position has to be added to the structure of all three compounds. Like for the isotypic compound DyB₅O₈(OH)₂, we suggest that the hydrogen atoms are situated between the O2 and the O5 oxygen atoms.

To verify the structure solution, the bond valence sums were calculated for all crystallographically different Ho/Er/Tm, B, and O atoms via the bond-length/bond-strength concept [31]. The results given in Table 5 agree well with the expected values, the only exceptions being the O2 and O5 oxygen atoms. However, this only further indicates the presence of a hydrogen atom bonded to one of these atoms. Inserting a hydrogen atom at a distance of approximately 84 pm (as found for DyB₅O₈(OH)₂) shifts the charge values towards the expected value of −2. Pronounced OH bands in the infrared spectra shown in Figure 5 further indicate the presence of OH groups.

Figure 5:

Infrared spectra of powder samples of REB₅O₈(OH)₂ (RE = Ho, Er, Tm) and of DyB₅O₈(OH)₂ reported previously [20].

3.2 Infrared spectroscopy

All three compounds show very similar infrared spectra. The distinct ν_OH band at approximately 3200 cm⁻¹ [32] indicates the presence of hydroxide groups (ν(O–H)). The absorption peaks in the region between 1100 and 800 cm⁻¹ are those typical for tetrahedrally coordinated [BO₄]⁵⁻ groups. The region below 900 cm⁻¹ is generally attributed to vibrations of [BO₃]³⁻ and [BO₄]⁵⁻ groups [33], [34]. The broad peak at 1450 cm⁻¹ is usually attributed to stretching vibrations of [BO₃]³⁻ groups. However, no [BO₃]³⁻ groups were found in the structure solution process and the bands are instead tentatively assigned to B–O–B, O–B–O, and B–O–RE (RE = rare earth metal ion) vibrations, as verified by abinitio quantum-chemical calculations for β-ZnB₄O₇ and β-CaB₄O₇ [35]. These findings are in good agreement with the observations for the isotypic compound DyB₅O₈(OH)₂ [20]. In general, the band positions vary only slightly among the four isotypic compounds.