On the synthesis and crystal structure of praseodymium(III) metaborate molybdate(VI) – PrBO₂MoO₄

2.1 Synthesis of PrBO₂MoO₄

A polycrystalline sample of PrBO₂MoO₄ was synthesized from a mixture with the molar ratio 1:1:5 of Pr(NO₃)₃·6H₂O (0.28 mmol, 99.9%, Strem Chemicals), MoO₃ (0.28 mmol, >99%, Plansee, Reutte, Tyrol, Austria), and H₃BO₃ (1.36 mmol, >99.8%, Carl Roth GmbH). The starting materials were weighed and intimately ground in an agate mortar. The reactions were carried out in a platinum crucible at 850 °C under an argon atmosphere. From room temperature, the sample was heated to the reaction temperature of 850 °C at a rate of 5 K per minute. The temperature was kept constant for 48 h before it was reduced to room temperature at a cooling rate of 3 K per minute. To remove any excess of H₃BO₃, the polycrystalline sample was washed five times each with 1 ml of hot deionized water. Finally, the resulting green product could be identified as PrBO₂MoO₄ by its powder pattern. However, no single-crystals that were sufficiently sized for structure determination were found in the sample. Therefore, the compound was placed in an 18/11 assembly of a Walker-type multianvil press (Max Voggenreiter GmbH, Mainleus, Germany), on which more detailed information can be found in the related literature [18, 19]. Within 30 min, the sample was compressed to 1 GPa and subsequently heated to 850 °C in 10 min. This temperature was kept constant for the following 15 min. Within 2 h, the temperature was slowly decreased to 200 °C, and finally the reaction was stopped by quenching. After the decompression phase (90 min), green crystals of PrBO₂MoO₄, suitable for a single-crystal structure determination, were isolated from the sample under a polarization microscope.

2.2 Crystal structure determination

Single-crystals suitable for structure determination were mounted on a Bruker D8 Quest single-crystal diffractometer equipped with a Photon III C14 area detector. Data collection and processing were performed employing the programs Saint [20] and Apex4 [21]. With the program Sadabs [22, 23], a multi-scan absorption correction was carried out and the structure was solved via Shelxt [24, 25] algorithms. All atoms were refined anisotropically using Shelxl [26] algorithms incorporated into the Olex2 [27] software. The final structure was checked employing the Platon program package and standardized using the program’s Structure Tidy routine [28], [29], [30], [31], [32]. In Tables 1 –4, the refinement and structural data of PrBO₂MoO₄ are presented.

In Table 4, selected interatomic distances and bond angles of the isotypic compounds CeBO₂MoO₄, PrBO₂MoO₄, and NdBO₂MoO₄ are compared. To enable an easy comparison, we have also used the standardized crystal structure data for CeBO₂MoO₄ and NdBO₂MoO₄ by adjusting the crystal coordinates and atom names to those obtained for PrBO₂MoO₄ by the Structure Tidy-routine [33].

Further details of the crystal structure investigation may be obtained from the joint CCDC/FIZ Karlsruhe online deposition service: https://www.ccdc.cam.ac.uk/structures/ by quoting the deposition number CSD-2240481.

X-ray powder diffraction patterns were collected with a STOE Stadi P Powder Diffractometer (STOE & CIE GmbH, Darmstadt, Germany) [34, 35], equipped with a Ge(111) monochromator and a MYTHEN 1K detector system (Dectris Ltd, Baden-Daettwil, Switzerland) [36]. The measurements were carried out in transmission mode using a flat sample holder [37]. Measured and calculated powder patterns are compared in Figure 1.

Figure 1:

Comparison of the measured and calculated powder X-ray diffraction patterns (λ = 70.93 pm). Additional reflections marked with an asterisk stem from small amounts of an unidentified side-phase.

2.3 Infrared spectroscopy

An infrared spectrum of the new compound was recorded on a Bruker Alpha Platinum attenuated total reflection (ATR) spectrometer on the bulk material. The spectrum was measured in the range of 4000 to 400 cm⁻¹ and the data was processed and corrected for atmospheric influences employing the Opus 7.2 [38] software.

3.1 Crystal structure description

PrBO₂MoO₄ crystallizes in the monoclinic space group P2₁/c (no. 14) with the unit cell parameters a = 10.1735(2), b = 4.1479(1), c = 11.9090(3) Å, and β = 116.52(1)°, representing a crystal structure which is isotypic to that of CeBO₂MoO₄ [3] and NdBO₂MoO₄ [15], but not of LaBO₂MoO₄ [3]. From Tables 2, 4, and references [3, 15], it is apparent that although the standardized crystal coordinates of the structure solutions of LnBO₂MoO₄ (Ln = Ce, Pr, Nd) match quite well, there are subtle deviations, mainly resulting in slightly different distortions of the lanthanoid-centered polyhedra, as it was already observed from the comparison of CeBO₂MoO₄ [3] and NdBO₂MoO₄ [15]. Consequently, interatomic distances in the three compounds do not strictly follow the expected trend of smaller Ln–O distances with decreasing lanthanoid-cation radii. The praseodymium atoms are ten-fold coordinated by oxygen atoms forming a distorted bicapped square antiprismatic shape depicted in Figure 2. Through face- and edge-sharing, the bicapped square antiprisms form double chains of [PrO₁₀] polyhedra running along the crystallographic b-axis. The interatomic distances vary from 2.391(2) to 2.954(2) Å, which is comparable to those of other compounds containing ten-fold coordinated praseodymium [17, 39], [40], [41], [42]. These bond lengths are also plausible considering the isotypic structures [3, 15].

Figure 2:

A graphical depiction of the bicapped distorted [PrO₁₀] square antiprism.

The molybdenum-centered polyhedra are shown in Figure 3 and can either be described as isolated [MoO₄] tetrahedra with interatomic Mo–O-distances from 1.745(3) to 1.824(2) Å or as [MoO₅] trigonal bipyramids, where the fifth oxygen atom is found in a distance of 2.418(2) Å from the molybdenum atom. The tetrahedral coordination sphere is built up by the atoms O3, O4, O5, and O6, while another atom O6 completes the trigonal bipyramid. In this case, the distorted [MoO₅] trigonal bipyramids form chains running along the crystallographic b-axis. It is noteworthy that in reference [3], the structures of LaBO₂MoO₄ and CeBO₂MoO₄ are described via [MoO₅] trigonal bipyramids, while later works concerning NdBO₂MoO₄ favored the approach via isolated [MoO₄] tetrahedra [15]. A detailed look at the interatomic distances and bond angles within the molybdenum-centered polyhedra provides arguments for both points of view. Assuming coordination number four, the interatomic distances are well in the range of those reported in the literature including molybdenum oxides and rare-earth molybdates [43], [44], [45], [46]. However, with tetrahedral angles varying from 96.3(1) to 118.0(1)°, and mean values of 108.8° a relatively high deviation from the expected ideal tetrahedral angle of 109.5° [43], [44], [45], [46], [47] is observed. We assume that the interaction of the molybdenum atoms with this fifth oxygen atom leads to the observed distortion of the tetrahedra.

Figure 3:

The molybdenum-centered polyhedra can either be described as isolated [MoO₄] tetrahedra or as chains of trigonal bipyramidal [MoO₅] units with one very long Mo–O bond along the crystallographic b-axis.

Boron is centered in a trigonal planar coordination sphere with B–O-distances between 1.334(3) and 1.401(3) Å, which is close to the standard value of B–O bond lengths of 1.370(19) Å in triangular [BO₃] units [48]. The [BO₃] units are arranged in the form of “zigzag”-chains along the crystallographic b-axis via corner-sharing. For a graphical depiction of this structural motif see Figure 4.

Figure 4:

“Zigzag”-chains of corner-sharing trigonal planar [BO₃] units are formed along the crystallographic b-axis.

In Figure 5, the resulting crystal structure of PrBO₂MoO₄ is shown in its unit cell in the ac-plane. It is built up by the double-chains of square antiprismatic [PrO₁₀] polyhedra being linked along the crystallographic a-axis via the molybdenum-centered polyhedra and through the chains of [BO₃] triangles along the crystallographic c-axis.

Figure 5:

The crystal structure of PrBO₂MoO₄ with a view along [ 0 1 ‾ 0 ] .

The calculation of the bond valences according to the bond-length/bond-strength (BLBS)- and charge distribution (CHARDI)-concept [49, 50] indicates good congruency of the expected and calculated values. The results of the calculations are shown in Table 5.

3.2 Infrared spectroscopy

In the infrared spectrum of PrBO₂MoO₄ (Figure 6), several sharp and intense signals are observed. It displays a high degree of similarity with the infrared spectra of LaBO₂MoO₄ and CeBO₂MoO₄ [3], and of the molybdenum-free compound λ-PrBO₃ [41]. Hence, the vibration modes observed in PrBO₂MoO₄ can be assigned in a similar way. The signals at 1452, 1369, and 1175 cm⁻¹ are assigned to the asymmetric stretching modes of the trigonal planar [BO₃] units [3], which is in good agreement with the related literature values, where absorption bands in the range of 1200–1500 cm⁻¹ in borates are generally attributed to the stretching modes of the triangular [BO₃] units [51]. At lower wavenumbers, contributions of stretching and bending modes of the molybdenum-centered polyhedra and bending modes of the [BO₃] triangles lead to the signals between 418 and 948 cm⁻¹ [3, 52], which again display good congruency with the bands of the infrared spectra of rare-earth molybdates exhibiting crystal structures with tetrahedrally coordinated molybdenum [53, 54]. At 1063 cm⁻¹, a weak signal is observed, which was not reported for LaBO₂MoO₄ or CeBO₂MoO₄, possibly due to the low intensity of the band. Although the presence of a side-phase containing [BO₄] tetrahedra cannot be excluded [55], the weak absorption can also be explained differently. In Mo₂B₄O₉ [56], minor contributions of Mo–O vibrations were found up to wavenumbers around 1250 cm⁻¹ in theoretical investigations of the compound’s infrared spectrum.

Figure 6:

The infrared spectrum of PrBO₂MoO₄ in the range from 4000 to 400 cm⁻¹.