Hydrothermal synthesis and characterization of the praseodymium borate-nitrate Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O

2.1 Synthesis

Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O was synthesized hydrothermally in a Teflon-lined stainless-steel autoclave (holding approx. 8 mL) from Pr(NO₃)₃·6H₂O [100.0 mg (0.23 mmol); Strem Chemicals, Kehl, Germany, 99.9%] and H₃BO₃ [71.1 mg (1.15 mmol); Carl Roth GmbH and Co. KG, Karlsruhe, Germany, ≥99.8%] resulting in a Pr:B atomic ratio of 1:5. The educt mixture was thoroughly ground together, and heated to and kept at 513 K for 3 days. After slowly cooling down to 373 K (2 K h⁻¹), the autoclave was allowed to cool at room temperature. The product mixture was washed with 100 mL of hot, deionized water to remove unreacted educts, whereupon Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O was obtained without any by-products detectable in the powder X-ray analysis.

2.2 Crystal structure analysis

An X-ray powder diffraction pattern was collected in the 2θ range 2.0–45.0° with a step width of 0.01° on a Stadi P powder diffractometer (STOE, Darmstadt, Germany) used in Debye-Scherrer geometry equipped with Ge(111)-monochromatized MoK_α1 radiation (λ=70.93 pm) and a Mythen 1K detector (Dectris, Baden-Dättwil, Switzerland). The raw powder data were processed with WinXPOW [32] and refined with the single-crystal structure data using the Rietveld method (program suite Topas 4.2 [33], [34]). Hardware parameters and reflection shape refinements of the Rietveld calculations were carried out with a LaB₆ standard.

Single crystals of Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O were selected under a polarization contrast microscope. The single-crystal intensity data was collected on a Bruker D8 Quest diffractometer using monochromatized MoK_α radiation (λ=71.073 pm). An Incoatec microfocus X-ray tube operating at 50 kV and 1 mA with a multilayer optic was used. The diffracted beam was collected on a Photon 100 detector. Primary data correction for absorption effects was performed with the system-immanent program Sadabs [35]. The centrosymmetric space group P2₁/n (no. 14) was derived from the systematic extinctions. The structure was subsequently solved with ShelXS-2013 [36] using Direct Methods, and refined with ShelXL-2016 [37] (full-matrix least-squares on F²), both implemented in the used WinGX surface [38]. The Addsym routine of Platon [39] indicated no obvious additional symmetry. For reasons of comparability, the setting transformation to P2₁/c (no. 14), as suggested by the Structure Tidy [40] routine of Platon, was renounced. All non-hydrogen atoms were refined anisotropically. The O–H distances were restrained to 83(2) pm using the DFIX command.

Details of the single-crystal and powder data collections and evaluations are listed in Table 1. Atomic coordinates and isotropic displacement parameters are listed in Table 2, respective anisotropic displacement parameters are given in Table 3. Tables 4 and 5 show selected interatomic distances and angles, data concerning the hydrogen bonds are tabulated in Table 6.

Further details of the crystal structure investigation may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de) on quoting the deposition number CSD-433274.

2.3 Vibrational spectroscopy

An FTIR spectrum of a single crystal of Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O was measured in the range of 600–4000 cm⁻¹ in transmission mode with a Bruker Vertex 70 FT-IR spectrometer providing a spectral resolution of 4 cm⁻¹. The mid-infrared radiation of a Globar silicon carbide rod with a KBr beam splitter was used to excite the crystal on a BaF₂ sample holder. An LN-MCT (liquid nitrogen cooled mercury cadmium telluride) detector attached to a Hyperion 3000 microscope was employed to acquire 320 scans of the sample. A correction for atmospheric influences was performed by using the system-immanent Opus [41] software, the final spectrum was plotted with Origin 2017 [42].

Confocal Raman spectroscopy was performed on a single-crystal of the rare earth borate-nitrate using a Horiba Jobin-Yvon LabRam HR-800 spectrometer with a focal length of 80 cm⁻¹. The scattered light was dispersed by an optical grating with 1800 lines mm⁻¹ and collected with a Peltier-cooled Andor CCD detector (1024×256 pixels). The 632.8 nm emission line of a He-Ne laser with a confocal pinhole of 300 μm and a slit of 100 μm was used. Before the measurement, the spectral position of the Raman mode of a Si standard wafer was measured against the Rayleigh line, resulting in a value of 520.4 cm⁻¹. The spectrum was obtained in multi- window acquisition mode provided by the Labspec [43] software package in the Raman shift range of 200–3700 cm⁻¹ with a spectral resolution of 0.4 cm⁻¹. The spectrum was recorded unpolarized and is presented with a sixth-order, manual background correction (also done with the Labspec package). Final visualization was achieved with Origin 2017 [42].

3.1 Hydrothermal synthesis

All borate-nitrates reported to date were obtained through mild hydrothermal syntheses. Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O was readily formed under exactly the same conditions as previously reported by our group for Nd[B₅O₈(OH)(H₂O)_0.85]NO₃·2H₂O and Sm[B₅O₈(OH)]NO₃· 2H₂O [31]: the praseodymium nitrate hydrate and boric acid in the molar ratio of 1:5 were heated to a maximum temperature of 513 K and held for 3 days. Again, the addition of water, which is common in hydrothermal syntheses of other compounds [44], [45], [46], but was never reported to lead to the desired product in borate-nitrates, consistently led to no observable product formation. A boric acid excess seems crucial in phase formation, as it likely acts as a flux enabling the reaction [47]. A stoichiometric ratio of Pr:B=1:5, however, led to a pure product – no side phases were detectable in the powder diffraction experiments after washing the crude product. Unlike with other defect variants also synthesized in our group [31], a substantial evolution of NO_x gases was observed upon opening of the autoclave. Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O forms transparent, colorless, single-crystal columns. The bulk product has a greenish hue as expected for Pr(III), similar to the rare earth nitrate hydrate educt.

3.2 Crystal structure discussion

Firstly, Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O was characterized via X-ray powder diffraction. A Rietveld plot of the pattern obtained at ambient conditions is shown in Fig. 1, with a fit to the reflection positions obtained from single-crystal data, and the respective difference curve. No obvious side phases were detectable, also indicated by the obtained quality values of R_exp=0.0197, R_wp=0.0386, R_p=0.0306, and GOF=1.96 for the refinement of a single powder phase. All reflections were refined with pseudo-Voigt Thompson-Cox-Hasting functions. For the (020) reflection at d=7.77, a preferred orientation was refined, justified with the column-like shape of the crystals formed by Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O. Details on the data collection and Rietveld refinement are given in Table 1.

Fig. 1:

Rietveld plot with the experimental powder pattern fitted to the structural model obtained from the single-crystal data.

The single-crystal structure determination for Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O revealed that the structure variant of RE[B₅O₈(OH)(H₂O)_x]NO₃·2H₂O [RE=La (x=0; 1), Ce (x=1), Nd (x=0.85), Sm (x=0)] is crystallizing monoclinically in the space group P2₁/n (no. 14) with four formula units (Z=4) per unit cell and the following cell parameters: a=641.9(3), b=1551.8(7), c=1068.4(5) pm, with β=90.54(2)° giving V=1.0643(8) nm³. The phases containing neighboring rare earth cations display comparable unit cell dimensions: the cerium borate-nitrate with a=644.38(12), b=1557.2(3), c=1074.5(2) pm, β=90.395(3)°, and V=1.0782(3) nm³ [48] is quite significantly larger. Interestingly, Nd[B₅O₈(OH)(H₂O)_0.85]NO₃·2H₂O, with a=642.85(2), b=1552.58(4), c=1070.52(2) pm, β=90.39(2)°, and V=1.06844(5) nm³ is also slightly larger. Ultimately, Sm[B₅O₈(OH)]NO₃·2H₂O with a=644.09(3), b=1541.99(8), c=1050.97(5) pm, β=90.33(1)°, and V=1.04379(9) nm³, and no coordinating water, displays a notably smaller unit cell. The related praseodymium borate Pr[B₅O₈(OH)₂] [25] crystallizes also monoclinically with Z=4, the structure being solved from X-ray powder data in P2₁/c (no. 14). The unit cell dimensions of a=649(1), b=1041(1), c=1103(1) pm, β=113.35(1)°, and V=0.684(1) nm³ are significantly smaller.

Figure 2 shows the unit cell of Pr[B₅O₈(OH)(H₂O)_0.87]NO₃· 2H₂O projected along the a axis. The continuous borate backbone in this rare earth borate-nitrate structure type, as well as in Pr[B₅O₈(OH)₂] [25], is built up from two different, connected Dreierrings [49]: one is formed by two tetrahedral and one trigonal-planar borate groups through shared corners. The other is formed by only one tetrahedral and two trigonal-planar borate blocks also sharing corners. The two rings share one tetrahedron. The resulting fundamental building block (FBB) can be symbolized as 3□3∆: <∆2□>–<□2∆> (after Hawthorne et al. [50]). The double-rings are then again connected through a single, shared oxygen position. In summary, the FBB forms continuous chains <∆2□>–<□2∆><∆2□> –<□2∆>·, visible in Fig. 2 as corrugated layers in the ac plane.

Fig. 2:

Unit cell of Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O. [BO₄]⁵⁻ tetrahedra are given in blue, [BO₃]³⁻ groups are depicted without polyhedra (except for B5 to clarify its deflection from the planar shape). The H₂O···[BO₃]³⁻ coordination interaction is indicated with dashed grey lines. [NO₃]³⁻ groups are highlighted with green triangles.

The borate groups’ geometry is in good agreement with the previously presented structure variants and the praseodymium pentaborate Pr[B₅O₈(OH)₂] [25], [30], [31], [48]. B–O distances in Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O range from 134.8(4) to 144.0(3) pm (Ø 137.5 pm) in the [BO₃]³⁻ groups, and from 144.2(4) to 149.4(4) pm (Ø 147.5 pm) in the [BO₄]⁵⁻ units. The trigonal-pyramidal H₂O···[BO₃]³⁻ group formed by B5 is, similar as in the hydrated La³⁺, the Ce³⁺, and the Nd³⁺ borate-nitrate variants as well as in Pr[B₅O₈(OH)₂], deflected out of the oxygen plane towards the coordinating water position O7 (cf. Fig. 2). B–O bond lengths in this unit are 138.7(4) (B5–O2), 139.7(4) (B5–O4), 144.0(3) (B5–O3), and 166.3(5) pm (B5···O7). The distance between B5 and the coordinating water molecule is significantly larger than in the La³⁺ [164.4(4) pm] [29], Ce³⁺ [163.7(5) pm] [48], or Nd³⁺ equivalent [162.3(5) pm] [31]. The equivalent position B3 in Pr[B₅O₈(OH)₂] [25] shows a distance of 164.3(2) pm to the equally coordinating position O10. O–B–O bond angles in the trigonal-planar [BO₃]³⁻ (Ø 120.0°) and in the [BO₄]⁵⁻ groups including the trigonal-pyramidal H₂O···[BO₃]³⁻ unit (Ø 109.4°) agree well with commonly reported values for borates [51], [52], along with those obtained, e.g. for the La³⁺ and Ce³⁺ structures. Selected interatomic distances and angles are also listed in Tables 4 and 5, respectively.

As mentioned, the praseodymium borate-nitrate is a defect structure, differing from, e.g. the cerium equivalent in its coordinating water content. The crystal structure of the cerium borate-nitrate Ce[B₅O₈(OH)(H₂O)]NO₃·2H₂O, first discovered by Lin et al. [23], was redetermined by Mi and coworkers in 2012 [48]. The latter group stated one coordinating water molecule in addition to two crystal water molecules, instead of three non-coordinating water positions. In the defect structure we present, the coordinating water position is occupied at 87%, similar to Nd[B₅O₈(OH)(H₂O)_0.85]NO₃·2H₂O. Neither for the La³⁺ nor for the Sm³⁺ compound, a coordinating water position was found. The positions B5 in these compounds are found in perfect, trigonal-planar coordination by three oxygen anions. Matching all RE[B₅O₈(OH)(H₂O)_x]NO₃·2H₂O [RE=La (x=0; 1), Ce (x=1), Nd (x=0.85), Sm (x=0)] structures, the praseodymium compound contains two crystal water molecules at comparable positions. In Fig. 2, the coordinating as well as crystal water molecules are shown, the B5···O7 interaction is indicated with dashed grey lines.

The third structural component, besides borate groups and water molecules, present in Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O, are nitrate groups, indicated with green triangles in Fig. 2. The N–O bond lengths are 123.8(4), 123.9(4), and 125.8(4) pm (Ø 124.5 pm) – comparable values are found in all other known structure variants, e.g. 125.1(5)–125.9(5) pm for the Ce³⁺ compound. Bond angles in the trigonal-planar [NO₃]³⁻ groups (Ø 120.0°) are also in line with those commonly reported for nitrates [27] and the structure analogues, e.g. Ce[B₅O₈(OH)(H₂O)]NO₃·2H₂O [48]. The interatomic distances and angles, also of the nitrate unit, are listed in Tables 4 and 5.

The hydrogen bonds formed in Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O are presented in Table 6. Six different hydrogen bonds are formed in every structure representative, regardless of the coordinating water content. Without the additional water position, the nitrate group is free to act as an acceptor for hydrogen bonds, as reported earlier for the lanthanum [29], [30] and samarium [31] borate-nitrates. The coordinating water molecule present in the Pr³⁺ compound acts like reported, e.g. for the Nd³⁺ [31] and Ce³⁺ [48] equivalents: it reorientates the nitrate group by forming the hydrogen bonds O7–H6···O11 and O7–H7···O8. In this position, all hydrogen bonds formed by the [NO₃]³⁻ group are geometrically impossible, instead, another nitrate-water coordination O15–H2···O8 is formed. All values for the hydrogen bonds are in line with those previously reported.

The nitrate and borate groups together with the crystal water molecules all coordinate the rare earth cations. Pr³⁺ centers are ten-fold coordinated at an average distance of 256.6 pm, with effective coordination number (ECoN) [53] contributions of the oxygen anions ranging from 1.155 to 0.778. The average coordination distance is in perfect alignment with those found in the other rare earth compounds: 260.6, 258.4, 255.6, and 253.7 pm for La³⁺ [30], Ce³⁺, Nd³⁺, and Sm³⁺, respectively. The additional, boron-coordinating water positions present in the hydrated lanthanum, cerium, neodymium, and in the novel praseodymium compounds do not participate in the coordination of the rare earth cations. Regardless of its orientation, the nitrate group participates in the coordination of the central cation also in Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O. In Fig. 3, the coordination environment of Pr1 is shown. Interatomic distances in the Pr–O coordination polyhedron are given in Table 4.

Fig. 3:

Coordination environment of the praseodymium(III) cation in Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O.

The bond-valence sums for Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O were calculated using the bond-length/bond-strength concept (ΣV) [54], [55]. The charge distribution was derived using the CHARDI concept (Charge distribution in solids, ΣQ) [56], [57], [58]. The calculated formal ionic charges of all atoms are within the limits of the concepts and are listed in Table 7. For the bond-valence sum of B5, the S.O.F. of 0.87 of the coordinating O7 was taken into account.

3.3 Vibrational spectroscopy

The FTIR absorption spectrum of a single crystal of Pr[B₅O₈(OH)(H₂O)_0.87]NO₃·2H₂O is given in Fig. 4. Bands at wavenumbers below 800 cm⁻¹ can be attributed to complex lattice vibrations involving Pr³⁺. Borate group vibrations occur in the regions between 800 and 1100 cm⁻¹ ([BO₄]⁵⁻ tetrahedra) [59], [60], [61], and 1200–1450 cm⁻¹ ([BO₃]³⁻ trigonal-planar groups) [59], [62]. Absorptions stemming from ν₁ [NO₃]³⁻ vibrations around ~1300 cm⁻¹ [63], [64] overlap with borate group absorptions and are better visible in the Raman spectrum. Absorptions at 1498 cm⁻¹ can be attributed to ν₄ [NO₃]³⁻ vibrations [64]. The absorption at 1556 cm⁻¹ does not pertain to the borate-nitrate and could be assigned to oil residues on the crystal prior used for the single-crystal X-ray structure determination. Bands at 1635 and 1699 cm⁻¹ likely stem from bending vibrations of the water molecules [65], [66]. Absorptions at 3566 and ~3745 cm⁻¹ are assignable to OH group vibrations [67].

Fig. 4:

FTIR spectrum of a single crystal of Pr[B₅O₈(OH)(H₂O)_0.87]NO₃· 2H₂O.

Figure 5 shows the Raman spectrum of a single crystal of the praseodymium borate-nitrate. Lattice, [BO₄]⁵⁻, and [BO₃]³⁻ vibrations are visible below 800 cm⁻¹, between 800 and 1100 cm⁻¹, and from 1200 to 1400 cm⁻¹, respectively [68], [69]. The most intense bands near 1049 cm⁻¹ stem from the readily polarizable ν₁ stretching vibrations of the [NO₃]⁻ group [70]. Since the local D₃h symmetry of the nitrate triangle is lowered due to the coordination to the Pr³⁺ cation, split peak shapes are observed [63], [71]. The characteristic double bands at 2850 and 2902 cm⁻¹ stem from oil residues on the single crystal’s surface that could unfortunately not be eradicated. Bands at 3453 and 3615 cm⁻¹ originate from OH vibrations [67], [72].

Fig. 5:

Raman spectrum of a single crystal of Pr[B₅O₈(OH)(H₂O)_0.87]NO₃· 2H₂O.