Synthesis and structural characterization of the new rare-earth borosilicates Pr₃BSi₂O₁₀ and Tb₃BSi₂O₁₀

2.1 Syntheses

The new rare-earth borosilicates RE₃BSi₂O₁₀ (RE = Pr, Tb) were synthesized from a non-stoichiometric mixture of Pr₆O₁₁ (Strem Chemicals, Newburyport, MA, USA; 99.9+ %), Tb₄O₇ (Smart Elements GmbH, Wien, Austria; 99.99 %), B₂O₃ (Strem Chemicals, Newburyport, MA, USA; 99.9+ %), H₃BO₃ (Merck, Darmstadt, Germany; 99.5 %), SiO₂ (Sigma-Aldrich, Inc., St. Louis, MO, USA; purum p.a.), and Si₃N₄ (Sigma-Aldrich, Inc., St. Louis, MO, USA; predominantly the α-modification).

The starting materials were mixed in the molar ratio 3:1:5:4:2, finely ground in an agate mortar and filled into boron nitride crucibles (Henze BNP GmbH, HeBoSint^® S100, Kempten, Germany). The boron nitride crucibles were placed in tungsten crucibles (Plansee Metall GmbH, Reutte, Austria) and heated in a radio frequency furnace [16] (Hüttinger Elektronik GmbH + Co. KG, Freiburg, Germany) under N₂ atmosphere (dried over silica gel/KOH/molecular sieves/P₂O₅, and BTS catalyst) to 1400 °C in 1 h, then the temperature was further raised to 1600 °C within 2 h. In the case of Pr₃BSi₂O₁₀, 1600 °C were maintained for 4 h and then the sample was quenched to room temperature by switching off the furnace. For Tb₃BSi₂O₁₀, the temperature of 1600 °C was lowered to 1400 °C within 1 h. Finally, the sample was quenched to room temperature. The new compound Pr₃BSi₂O₁₀ was obtained as a light green pellet, and Tb₃BSi₂O₁₀ in the form of a white pellet; both are air- and water-resistant. We presume that the syntheses of the new compounds are kinetically controlled. Only with the exact composition and heating process mentioned above it was possible to synthesize the new rare-earth borosilicates RE₃BSi₂O₁₀ (RE = Pr, Tb). A stoichiometric mixture or the omittance of one of the starting materials did not lead to the desired phases. Instead, well known compounds like PrBO₃ [17], Pr₂Si₂O₇ [18], PrBSiO₅ [2] or a mixture of two of them, and TbBO₃ [17] as well as Tb₂Si₂O₇ [19] in the case of RE = Tb were formed. In comparison to the syntheses of other rare-earth borosilicates, which were performed in evacuated silica tubes or hydrothermally [2–14], the duration of the syntheses could be markedly reduced from approximately 2 weeks to 6 h using a radio frequency furnace. One of the reasons for the reduction of the time of syntheses could be the increase of the temperature from 1000 °C to 1600 °C leading to a better diffusion.

2.2 X-ray powder diffraction analyses and Rietveld refinement

Pr₃BSi₂O₁₀ and Tb₃BSi₂O₁₀ were identified by powder X-ray diffraction measurements on flat samples of the reaction products, using a Stoe Stadi P powder diffractometer with Ge(111)-monochromatized MoK_α1 (λ = 70.93 pm) radiation. Both powder diffraction patterns (Figs. 1 and 2) showed reflections of the corresponding rare-earth borosilicates RE₃BSi₂O₁₀ (RE = Pr, Tb) and of side products. Due to the fact that both compounds RE₃BSi₂O₁₀ (RE = Pr, Tb) are isotypic to Gd₃BSi₂O₁₀ [10], a full-pattern Rietveld refinement was carried out with the program Topas [20]. The structural refinement was performed by taking the positional parameters of the isotypic phase Gd₃BSi₂O₁₀ as starting values. For Pr₃BSi₂O₁₀, constraints were set so that the B–O distances remained close to 1.37 Å and the Si–O distance close to 1.62 Å. During the refinement, the residual impurity could be identified as SiO₂ [21]. The free refinement of individual displacement parameters for boron and oxygen led to unreasonable values. Therefore, one isotropic displacement parameter was refined, and set for all atoms. The final difference Fourier syntheses did not reveal any significant peaks in the refinement. Figure 3 shows the result of the Rietveld refinement of the X-ray powder diffractogram of Pr₃BSi₂O₁₀. The residual values and all relevant details of the data collection and refinement are listed in Table 1. The Tables 2 and 3 list the positional parameters and interatomic distances for Pr₃BSi₂O₁₀. The refinement of the powder data of Tb₃BSi₂O₁₀ did not succeed until now. The program Diamond [22] was used for the graphical representation of the structure.

Fig. 1:

Top: experimental powder pattern of Pr₃BSi₂O₁₀. The reflections marked with a red asterisk arise from SiO₂ as a residual starting material. Bottom: theoretical powder pattern of Pr₃BSi₂O₁₀ based on the modified single-crystal diffraction data of Gd₃BSi₂O₁₀ (Pr³⁺ instead of Gd³⁺ and lattice parameters of Pr₃BSi₂O₁₀).

Fig. 2:

Top: experimental powder pattern of Tb₃BSi₂O₁₀. The reflections marked with a red asterisk arise from SiO₂ of residual starting material. Bottom: theoretical powder pattern of Tb₃BSi₂O₁₀ based on the modified single-crystal diffraction data of Gd₃BSi₂O₁₀ (Tb³⁺ instead of Gd³⁺ and lattice parameters of Tb₃BSi₂O₁₀).

Fig. 3:

Calculated (black) and observed (red crosses) X-ray powder diffractogram of Pr₃BSi₂O₁₀. The difference curve (observed-calculated) is plotted below (blue). The short vertical bars indicate the positions of the reflections. First row: Pr₃BSi₂O₁₀ (89.2(1) wt%), second row: SiO₂ (10.8(1) wt%).

2.3 Vibrational spectra

The FTIR-attenuated total reflection (ATR) spectra of powders were measured with a Bruker Alpha-P spectrometer with a diamond ATR-crystal (2×2 mm), equipped with a DTGS detector in the spectral range of 400– 4000 cm^–1 (spectral resolution 4 cm^–1). Twenty-four scans of the sample were acquired. A correction for atmospheric influences using the Opus 7.2 software was performed.

3.1 Crystal structure of Pr₃BSi₂O₁₀

The new rare-earth borosilicate Pr₃BSi₂O₁₀ crystallizes isotypically to the structure of Gd₃BSi₂O₁₀ in the orthorhombic space group Pbca (no. 61) with eight formula units per cell. The structure is composed of two different types of anions: ortho-silicate anions [Si2O₄]^4– and borosilicate anions [BSi1O₆]^5–, which are arranged alternatingly along the c axis (Fig. 4). The borosilicate anions consist of a [SiO₄]^4– tetrahedron and a [BO₃]^3– group sharing a common oxygen atom (Fig. 5). The three different rare-earth ions Pr1, Pr2, and Pr3 occupy the interlayer sites and are coordinated by eight (Pr1) and nine (Pr2, Pr3) oxygen atoms (Fig. 6). The Pr–O polyhedra are linked to each other through edge-sharing.

Fig. 4:

Crystal structure of Pr₃BSi₂O₁₀ (space group: Pbca).

Fig. 5:

The borosilicate anion in Pr₃BSi₂O₁₀.

Fig. 6:

Coordination polyhedra of the rare-earth ions Pr1 (left), Pr2 (center), and Pr3 (right).

The boron-oxygen distances inside of the [BO₃]^3– units range between 137.5(2) and 139.8(1) pm with a mean value of 138.5(2) pm for Pr₃BSi₂O₁₀. This mean value of the boron oxygen distances in the [BO₃]^3– units corresponds to the values of the literature, which usually range around 137 pm [23–25]. The B–O–B angles are between 112.0(1) and 121.5(2)°. The Si1–O distances vary from 159.3(1) to 163.7(2) pm with a mean value of 161.4(2) pm. The interatomic distances Si2–O are between 160.1(9) and 169.3(2) pm with a mean value of 163.2(9) pm. The bond angles range from 102.7(7) to 114.3(7)° for O–Si1–O, and from 101.0(5) to 125.5(6)° for O–Si2–O in Pr₃BSi₂O₁₀. The distances of the eightfold coordinated Pr1 to the oxygen atoms are 237.9(1)–274.0(1) pm with a mean value of 251.7(8) pm. Pr2 and Pr3 are coordinated by nine oxygen atoms; the bond lenghts vary from 234.4(9) to 267.4(9) pm with a mean value of 256.5(9) pm for Pr2 and from 237.6(9) to 291.5(1) pm with a mean value of 256.2(9) pm for Pr3. The Table 3 shows the interatomic distances of Pr₃BSi₂O₁₀. For a detailed description of the structure, the reader is referred to the paper of the isotypic compound Gd₃BSi₂O₁₀ [10]. In this work, we briefly compare the isotypic phases RE₃BSi₂O₁₀ (RE = Ce, Pr, Gd).

Due to their isotypy, there is no large difference in the structures of Pr₃BSi₂O₁₀, Ce₃BSi₂O₁₀, and Gd₃BSi₂O₁₀. Table 4 compares the unit cells, the coordination numbers of the rare-earth metal ions, and the bond lengths. The differences of the lattice parameters a, b and c correspond to the decrease in the ionic radii from Ce³⁺ to Gd³⁺, which is based on the lanthanide contraction. Due to the fact that the size difference is marginal, no greater deviations of the bond lengths and angles are observed.

3.2 IR spectroscopy

The spectra of the FTIR-ATR measurements of Pr₃BSi₂O₁₀ and Tb₃BSi₂O₁₀ are displayed in Fig. 7. The assignments of the vibrational modes are based on a comparison with the experimental data of borates containing trigonal [BO₃]^3– groups [26–30] and of borosilicates containing tetrahedral [SiO₄]^4– groups [31, 32]. For borates containing [BO₃]^3– groups, absorption bands at 1200–1450 cm^–1 (stretching vibrations) and at 600–800 cm^–1 (bending vibrations) are expected. The absorption bands for tetrahedral [SiO₄]^4– groups in borosilicates are expected at 400–600 cm^–1 (bending vibrations) and at 650–1100 cm^–1 (stretching vibrations). In the FTIR spectrum of the new praseodymium and terbium borosilicates, the expected [BO₃]^3– modes are detected between 1200 and 1450 cm^–1 and between 600 and 800 cm^–1. The [SiO₄]^4– modes are between 400 and 600 cm^–1 and between 650 and 1100 cm^–1. Furthermore, no bands for OH groups or water could be detected in the range of 3000 to 3600 cm^–1.

Fig. 7:

FT-IR reflectance spectrum of Pr₃BSi₂O₁₀ (red) and Tb₃BSi₂O₁₀ (black).