High-pressure/high-temperature synthesis of Ln₂CdB₅O₁₁(OH) (Ln = Tm, Lu)

2.1 High-pressure/high-temperature synthesis

High-pressure/high-temperature syntheses were carried out in a modified Walker-type multianvil press (Max Voggenreither GmbH, Mainleus, Germany), for which a detailed description is found in the literature. ^{20 – 22} Lu₂CdB₅O₁₁(OH) was synthesized from a stoichiometric mixture of Lu₂O₃ (0.16 mmol, Smart Elements, 99.99 %), CdO (0.16 mmol, Fluka, 99 %), and H₃BO₃ (0.81 mmol, Roth, 99.9 %), which was placed in a platinum capsule and centered in an 18/11 assembly of the above-mentioned high-pressure device. Analogously, Tm₂CdB₅O₁₁(OH) was synthesized using Tm₂O₃ (0.16 mmol, Chempur, 99.90 %), CdO, and H₃BO₃ as starting materials. Within 180 min, the assembly was compressed to 7 GPa and heated to 650 °C in the following 10 min. The temperature was kept constant for 75 min before the reaction mixture was quenched. After 540 min, the sample was fully decompressed, and the polycrystalline samples were isolated from the platinum capsule. Lu₂CdB₅O₁₁(OH) formed thin, long, and heavily intergrown crystal needles, while the Tm compound was obtained as crystals of similar shape and behavior but a much smaller size. Notably, the temperature of 650 °C is high enough for the water, that forms during the reaction, to dissipate from the assembly.

2.2 X-ray diffraction structure determination

Measurements were carried out on a Bruker D8 Quest equipped with a Photon III C14 area detector. The programs Saint (version 8.40B) ²³ and Apex4 (v2021.4.0) ²⁴ were used for data collection and processing. For structure determination, a non-merohedrally twinned crystal (BASF 0.204(2)) of Lu₂CdB₅O₁₁(OH) was selected under a polarization microscope. Two twin domains (second cell rotated from the first domain) were identified in the data by the program Cell_Now. ²⁵ A multi-scan absorption correction was carried out using the program Twinabs (2012/1), ²⁶ and the structure was solved using Shelxt (2018/2) ^{27 , 28} algorithms. 17878 reflections were found to only involve domain 1, 17839 reflections involved only domain 2, and 13320 reflections involved two domains. Of these, 2195, 2192, and 1879 reflections, respectively, were considered unique in the calculations of the structure refinements. During structure refinement, eleven reflections were omitted due to their high deviation. Structure refinements were calculated on the basis of the HKLF5 file using Shelxl ²⁹ algorithms incorporated into the program Olex2 (version 1.5), ³⁰ and all atoms, with the exception of Lu2, were refined anisotropically. No signs for the formation of a superstructure were observed in the data. Measurements of similarly non-merohedrally twinned crystals of Tm₂CdB₅O₁₁(OH) did not yield satisfactory results due to the small size of the crystals. Therefore, its crystal structure could only be solved by powder diffraction based on the single-crystal structure solution of the isotypic Lu₂CdB₅O₁₁(OH). X-ray powder diffraction patterns (Figures 1 and 2) were collected on a STOE Stadi P Powder Diffractometer (STOE & Cie GmbH, Darmstadt, Germany), ³¹ using Ge(111)-monochromatized MoKα₁ radiation and a MYTHEN 1K detector system (Dectris AG, Baden-Daettwil, Switzerland). ³² The measurements were carried out overnight in Debye-Scherrer mode in a glass capillary with a diameter of 0.5 mm (Hilgenberg, Malsfeld, Germany). ³³ Rietveld refinements ^{34 , 35} were carried out based on the single-crystal structure solution of Lu₂CdB₅O₁₁(OH) with the program Topas 4.2. ³⁶ Structural and refinement data for the single-crystal measurements on Lu₂CdB₅O₁₁(OH) are shown in Tables 1, 2 and 3. Parameters for the Rietveld refinements of the experimental powder diffraction patterns of Tm₂CdB₅O₁₁(OH) and Lu₂CdB₅O₁₁(OH) are provided in Table 4, and structural data for Tm₂CdB₅O₁₁(OH) as obtained from the powder diffraction measurements is presented in Table 5. Selected interatomic distances and angles are shown in Table 6.

Figure 1:

Powder X-ray diffraction pattern and Rietveld-refinement for Lu₂CdB₅O₁₁(OH) (MoKα₁; λ = 70.93 pm).

Figure 2:

Rietveld refinement for Tm₂CdB₅O₁₁(OH) (MoKα₁; λ = 70.93 pm). No crystalline side phases are detectable.

Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 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-2326196.

2.3 Infrared spectroscopy

Infrared spectra of the bulk material of the title compounds were collected on a Bruker Alpha Platinum attenuated total reflection (ATR) spectrometer. The spectra were measured in the range of 4000–400 cm⁻¹ and the data was processed and corrected for atmospheric influences employing the Opus 7.2 software. ³⁷

2.4 SEM-EDX measurements

The chemical composition of the title compounds was further investigated with a field emission gun scanning electron microscope (FEG-SEM) Clara Ultra High Resolution (UHR) from TESCAN equipped with an energy-dispersive X-ray spectroscopy (EDX) detector Ultim Max, 65 mm² from OXFORD. Representative parts of the sample were fixed on conducting carbon tabs. Imaging and EDX measurements were performed in analysis mode at an acceleration voltage of 20 keV and a beam current of 3 nA at a working distance of 9 mm. The results of the EDX-analysis are shown in Table 7. Boron, oxygen, and hydrogen were not quantifiable with reasonable precision.

3.1 Crystal structure description

The compounds Tm₂CdB₅O₁₁(OH) and Lu₂CdB₅O₁₁(OH) crystallize in the orthorhombic space group Pmna (no. 53) with four formula units per unit cell. Due to the isotypic nature of the title compounds, we will present the crystal structure of Lu₂CdB₅O₁₁(OH) representatively for both compounds. Lu₂CdB₅O₁₁(OH) displays unit cell parameters of a = 12.772(2), b = 4.6028(7), and c = 12.481(2) Å (single-crystal data), and a crystal structure which is homeotypic to the lanthanoid borates of the type Ln₃B₅O₁₂ (Ln = Er − Lu). ⁶

In the crystal structure of Ln₂CdB₅O₁₁(OH) (Ln = Tm, Lu), four crystallographically different boron atoms are found, which are exclusively tetrahedrally coordinated by oxygen. Interatomic B–O distances in Lu₂CdB₅O₁₁(OH) vary between 1.465(7) and 1.494(7) Å around the B1 site, 1.416(7) and 1.539(8) Å around the B2 site, 1.46(2) and 1.54(2) Å around the B3 site, and 1.43(2) and 1.53(2) Å around the B4 site. This results in average B–O bond lengths of 1.480, 1.488, 1.50, and 1.49 Å in the tetrahedra centered by the B1, B2, B3, and B4 atoms, respectively. Interatomic distances in the [BO₄] tetrahedra are usually around 1.48(4) Å with O–B–O angles around 109.4°, ³⁸ hence showing good congruency of our experimental results with general expectations. It is noteworthy that the interatomic distances in Tm₂CdB₅O₁₁(OH) are comparable to those in Lu₂CdB₅O₁₁(OH), despite larger observed deviations. This is likely due to the refinement from powder diffraction instead of single-crystal data. The [BO₄] tetrahedra are connected via shared corners, thus forming layers in the ac-plane, which are built up by four-, five, and, eight-membered rings. Each four-membered ring is surrounded by four five-membered and two eight-membered rings, which is apparent in Figure 3. Along the crystallographic b-axis, the layers are connected via the lanthanoid- and cadmium-centered polyhedra. This stacking of the layers within the crystal structure is shown in Figure 4.

Figure 3:

View on the layers in the ac plane in crystals of the title compounds.

Figure 4:

Stacking of the layers along the crystallographic b axis.

Two crystallographically different Ln sites were described in Ln₃B₅O₁₂ (Ln = Er–Lu) ⁶ and are similarly observed in the title compounds. The Lu1 site in Lu₂CdB₅O₁₁(OH) is eight-fold coordinated by oxygen atoms, forming a square antiprism (Figure 5). Interatomic distances around this Lu1 position vary between 2.277(5) and 2.421(5) Å. These values closely match the interatomic distances around the Lu1 site in Lu₃B₅O₁₂, which were reported to be in the range between 2.206(3) and 2.423(4) Å. ⁶ The coordination sphere around the Lu2 site can be described as distorted square antiprismatic with Lu–O distances from 2.194(5) to 2.589(6) Å, which, again, is comparable to the interatomic distances of 2.160(3)–2.582(4) Å in Lu₃B₅O₁₂. ⁶ The Ln2 site in Ln₂CdB₅O₁₁(OH) (Ln = Tm, Lu) is shared by lanthanoid and cadmium atoms. Several observations suggest a 1:1 ratio. First of all, the free refinement of site occupation factors during the calculation of the single-crystal structure refinements of Lu₂CdB₅O₁₁(OH) led to values close to 0.5 for Lu2 and Cd2 (0.56 for Lu2 and 0.47 for Cd2). In the case of Tm₂CdB₅O₁₁(OH), the free refinement of the respective site occupation factors during the Rietveld refinements of the powder diffraction patterns also consistently led to values close to 0.5 for the ratio Tm₂/Cd₂. Furthermore, as apparent from Table 7, EDX measurements match well with the expected chemical composition Ln₂CdB₅O₁₁(OH) (Ln = Tm, Lu), which results if Ln2 and Cd2 are assumed with S.O.F.’s of 0.5 each. In borates, the coordination number eight is unusual for Cd²⁺. However, a coordination number as high as 12 was reported for the cadmium cations in CdB₂O₄. ³⁹ The interatomic distances between 2.290(7) and 2.579(7) Å match well with Cd–O distances between 2.179(2) and 2.850(2) Å, which were previously observed in borates. ^{39 – 43} To the best of our knowledge, mixed occupation of lanthanoid and cadmium atoms has not been reported before, but considering typical interatomic distances of Tm–O, Lu–O, and Cd–O in the respective borates, ^{39 – 47} shared Ln/Cd sites seem reasonable. If the ionic radii of Tm³⁺ (113.4 pm), Lu³⁺ (111.7 pm), and Cd²⁺ (124 pm) are considered, it is also expected that the cadmium atoms occupy the Ln site with slightly larger interatomic distances in Ln₂CdB₅O₁₁(OH) (Ln = Tm, Lu). ⁵ In Figure 5, the coordination spheres of the Lu1 and the Lu2/Cd2 sites in Lu₂CdB₅O₁₁(OH) are shown.

Figure 5:

Depiction of the coordination environment of the Lu1 and the mixed Lu2/Cd2 site.

Due to the lower charge of Cd²⁺ compared to Lu³⁺, one hydrogen atom is needed to achieve charge neutrality. However, from the single-crystal data it was not possible to clearly assign this hydrogen atom to one position during the refinement of the data. Therefore, we have used the bond-length/bond-strength ⁴⁸ (BLBS) and the charge distribution concept ⁴⁹ (CHARDI) to calculate the bond valence sums of all crystallographically different atoms. The results are shown in Table 8.

Despite generally good congruency with the expected charge values of +2 for cadmium, +3 for lutetium and boron, and −2 for oxygen, the results remain unclear regarding the position of the hydrogen atom. The largest deviations from the expected value of −2 for oxygen is found for the O1 and O2 atoms. We therefore assume that the hydrogen atoms could be split onto these positions within the crystal structure. The comparison of individual bond lengths in Lu₃B₅O₁₂ ⁶ and Lu₂CdB₅O₁₁(OH) shows that two B–O bonds are lengthened in the cadmium-containing compound, namely the B3–O1 and the B4–O2 bonds. This observation also suggests that the hydrogen atoms reside on these sites. Considering the multiplicity of the Wyckoff site 4h, two half-occupied sites would yield the one hydrogen atom needed to achieve charge neutrality. However, owing to these uncertainties, we refrained from including the hydrogen atoms in the structure refinement.

3.2 Infrared spectroscopy

In Figure 6, the infrared spectra of Ln₂CdB₅O₁₁(OH) (Ln = Tm, Lu) are shown. A broad signal between 2600 and 3600 cm⁻¹ is clearly observed, which is typical for the stretching mode of a hydroxyl group that is involved in hydrogen bonding. ^{50 , 51} The broad range of this signal further hints at the existence of several sites on which the hydrogen is distributed. ⁵¹ Thus, the presence of hydrogen within the crystal structure is confirmed. The residual absorption bands can similarly be assigned as in the homeotypic compounds Ln₃B₅O₁₂ (Ln = Er–Lu). ⁶ Typical stretching modes of [BO₄] tetrahedra are observed in the range 800–1300 cm⁻¹. ⁵² However, in the title compound, a strong absorption is also observed at somewhat higher wavenumbers. The respective signal in the homeotypic compounds Ln₃B₅O₁₂ (Ln = Er–Lu) was assigned to a side phase featuring triangular [BO₃] groups which however, was not detectable in our samples. During the investigation of the infrared spectroscopic properties of the title compounds, we have also conducted measurements on single-crystals, yielding patterns that did not differ significantly from those collected on the bulk material shown in Figure 6. Therefore, we deduce that this absorption stems from contributions of B–O–B, O–B–O, B–O−Ln (Ln = Tm, Lu), and B–O–Cd vibrations at higher than usual wavenumbers, which have been studied experimentally and by ab initio quantum chemical calculations on the compounds β-CaB₄O₇ and β-ZnB₄O₇. ⁵³ At lower wavenumbers, the bending modes of the [BO₄] tetrahedra and vibrations involving the cations Cd, Tm, and Lu contribute increasingly to the observed signals. ^{52 – 55}

Figure 6:

Infrared spectra of Tm₂CdB₅O₁₁(OH) and Lu₂CdB₅O₁₁(OH). For a better comparability, the spectra were scaled differently on the y-axis.