Synthesis and characterization of the alkali borate-nitrates Na_3–x K_x[B₆O₁₀]NO₃ (x=0.5, 0.6, 0.7)

2.1 Hydrothermal syntheses

All three alkali borate-nitrates were synthesized hydrothermally by charging Teflon-lined stainless-steel autoclaves (8 mL) with Na₂B₄O₇·10 H₂O (p.a., Merck KGaA, Darmstadt, Germany) and KNO₃ (≥99%, Carl Roth GmbH, Karlsruhe, Germany). The mixtures were ground together, heated to and kept at 513 K for 3 days, cooled slowly down to 330 K (2 K h⁻¹), and finally quenched to room temperature. Colorless micro-crystals (approx. 5 μm in diameter and <1 μm thickness) were isolated after removing excess educts through washing the products with hot deionized water. The reported borate-nitrates were obtained in the Na₂B₄O₇·10 H₂O:KNO₃ ratio ranging from 5:1 to 1:1 for Na_2.5K_0.5[B₆O₁₀]NO₃, from 1:2 to 1:6 for Na_2.4K_0.6[B₆O₁₀]NO₃, and from 1:10 to 1:50 for the phase richest in potassium Na_2.3K_0.7 [B₆O₁₀]NO₃. A brief graphical overview of the molar educt ratios is given in Fig. 1.

Fig. 1:

Na₂B₄O₇·10 H₂O to KNO₃ educt ratios yielding Na_2.5K_0.5[B₆O₁₀]NO₃ (green squares), Na_2.4K_0.6[B₆O₁₀]NO₃ (red squares), and Na_2.3K_0.7[B₆O₁₀]NO₃ (blue squares).

2.2 Crystal structure analyses

Powder X-ray diffraction patterns were collected with a Stoe Stadi P powder diffractometer with transmission geometry and MoK_α1 (λ=70.93 pm) radiation with a focusing Ge(111) primary beam monochromator and a Mythen 1K detector (Dectris, Switzerland) in the 2θ range 2.0–50.0° with a step size of 0.005°. The crystal structure determinations were performed by Rietveld refinements using the program suite Topas 4.2 [19], [20]. Hardware parameters and reflection shape refinements were carried out with a LaB₆ standard. The single-crystal structure data of the compound Na₃(NO₃)[B₆O₁₀] of Yakubovich et al. [18] served as starting parameters for the refinements.

All relevant details of the data collections and the refinements using the Rietveld method are listed in Table 1. Table 2 shows the positional parameters, occupation factors, and displacement parameters. Selected interatomic distances and bond angles are given in Tables 3 and 4 , respectively.

Further details of the crystal structure investigations 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 numbers CSD-432296, CSD-432295, and CSD-432294, for Na_2.5K_0.5[B₆O₁₀]NO₃, Na_2.4K_0.6 [B₆O₁₀]NO₃, and Na_2.3K_0.7[B₆O₁₀]NO₃, respectively.

2.3 Vibrational spectroscopy

The transmission FT-IR spectra were collected in the spectral range of 600–4000 cm⁻¹with a Bruker Vertex 70 FT-IR spectrometer (spectral resolution 4 cm⁻¹), equipped with an MCT (Mercury Cadmium Telluride) detector, and attached to a Hyperion 3000 microscope. As mid-infrared source, a Globar (silicon carbide) rod was used, whereby 32 scans of the micro-crystals of each phase placed on a BaF₂ sample holder were acquired. Primary data was recorded with support of the spectrometer software Opus [21].

Confocal Raman spectra recorded in the range of 400–4000 cm⁻¹ with a HORIBA Jobin Yvon LabRam-HR 800 Raman micro-spectrometer. The samples were excited using a frequency doubled 100 mW Nd-YAG-laser (λ=532.22 nm) under an Olympus 100×objective lens with a numerical aperture of 0.9. The scattered light was dispersed by an optical grating with 1800 lines mm⁻¹, and collected by a 1024×256 open electrode CCD detector. Measurement time of 100 s for each sample was chosen, whereby the intense NO₃ bands around 1050 cm⁻¹ were recorded additionally with 10 s measurement time in the range of 900–2000 cm⁻¹. All spectra were measured with a resolution higher than 2 cm⁻¹ determined by measuring the Rayleigh line. A manual background correction was applied [22].

2.4. Elemental analyses

The atomic mass ratio of the alkali cations was determined by energy-dispersive X-ray spectrometry (EDX). The samples were prepared by placing various crystals of each of the three phases on a carbon pad, coating them with a thin carbon film in order to obtain conductivity. The equipment used was a high-resolution electron microscopy system (Jeol JSM-6010 LV) with a Bruker QUANTAX system and a Peltier-cooled Bruker XFlash 410-M silicon drift detector. Spectra acquisition was performed at 10–15 kV accelerating voltage with an output count rate of 2800 cps. The measured spot sizes were 40–50 μm². The lifetime during spectrum collection was 60 s and the dead time was less than 2%. The Bruker Esprit 1.9 software was used for standard-less spectra evaluation through the peak-to-background model and subsequent ZAF correction. Due to the standard-less method, the totals were normalized to 100%. Backscattered imaging was obtained by scanning electron microscopy (SEM).

3.1 Synthetic conditions

The title compounds were obtained hydrothermally, as described in the experimental section. A schematic representation of the educt ratios yielding different Na₂B₄O₇·10 H₂O:KNO₃ ratios of 5:1–1:1 for Na_2.5K_0.5[B₆O₁₀]NO₃, 1:2–1:6 for Na_2.4K_0.6[B₆O₁₀]NO₃, and 1:≥10 for Na_2.3K_0.7[B₆O₁₀]NO₃ is given in Fig. 1. Syntheses with an even higher excess of Na₂B₄O₇·10 H₂O (>5:1) did not lead to any product formation. To promote single-crystal growth, a low cooling rate of 2 K h⁻¹ was applied. Even slower cooling and/or temperature variations did, however, not lead to crystallite sizes larger than 5 μm.

Various syntheses were also performed using potassium borate K₂B₄O₇·4 H₂O and KNO₃ which lead to the formation of the known K₄B₁₀O₁₅(OH)₄ [23]. Analogously, Na₂B₄O₇·10 H₂O and NaNO₃ educt mixtures were tested, which gave the known Na₃(NO₃)[B₆O₁₀] as the sole reaction product. Whenever the reaction temperature was lowered to 473 K, and/or the reaction time was shortened (1–2 days instead of 3 days at maximum temperature), the known compound Na₃(NO₃)[B₆O₁₀] was obtained from educt mixtures of Na₂B₄O₇·10 H₂O and KNO₃ ranging from 10:1 to 1:10. All further synthetic attempts including also the use of lithium borate Li₂B₄O₇ did not yield any additional solid solutions.

3.2 Crystal structure discussion

The three alkali borate-nitrates Na_3−x K_x[B₆O₁₀]NO₃ (x=0.5, 0.6, 0.7) are substitution variants of the known sodium borate-nitrate Na₃(NO₃)[B₆O₁₀] reported by Yakubovich et al. in 2002 [18]. All phases crystallize with four formula units (Z=4) in the orthorhombic space group Pnma (no. 62). The lattice parameters of the new compound with the smallest potassium content, Na_2.5K_0.5[B₆O₁₀]NO₃, are a=1261.72(5), b=1004.19(5), c=770.55(3) pm, and V=0.97630(6) nm³. The second phase, Na_2.4K_0.6[B₆O₁₀]NO₃, shows unit cell parameters of a=1265.74(3), b=1006.94(2), c=772.06(2) pm, and V=0.98400(3) nm³. The borate-nitrate Na_2.3K_0.7[B₆O₁₀]NO₃ accordingly displays the largest lattice parameters of a=1267.12(5), b=1007.96(4), c=774.38(3) pm, and V=0.98905(6) nm³. The growing cell volume of the structures in accordance with the increasing potassium content is also reflected in their X-ray powder diffraction patterns. Fig. 2 shows all three experimentally obtained patterns in direct comparison to the isotypic compound Na₃(NO₃)[B₆O₁₀]. A clear shift towards smaller diffraction angles compared to the pure sodium phase can be detected, agreeing with the increase in cell volume.

Fig. 2:

Top to bottom: in black, the experimental X-ray powder diffraction patterns (MoK_α1; λ=70.93) of Na_2.5K_0.5[B₆O₁₀]NO₃, Na_2.4K_0.6[B₆O₁₀]NO₃, and Na_2.3K_0.7[B₆O₁₀]NO₃ are given. The theoretical powder pattern simulated from single-crystal data of Na₃(NO₃)[B₆O₁₀] by Yakubovich et al. [18] is indicated in red in each diagram for comparison.

Figure 3 shows the Rietveld plots including the difference curves obtained for the fits. All refinements showed good reliability factors as presented in Table 1. Table 2 gives the positional parameters and the site occupation factors of all positions in one of the three solid solutions based on Na₃(NO₃)[B₆O₁₀] obtained through the Rietveld refinement. The position Na1 is fully occupied by sodium in all synthesized phases. The original Na2 and Na3 sites of Na₃(NO₃)[B₆O₁₀] are mixed occupied with sodium and potassium in the title compounds with values as listed in Table 2 and Table S1 of the Supporting Information. From these site occupation factors, a sodium content of 2.51(4) Na⁺ per unit cell and a corresponding K⁺ content of 0.50(4) were refined leading to the formula Na_2.5K_0.5[B₆O₁₀]NO₃. The refinement of the second synthesized phase yielded a Na⁺ content per unit cell of 2.44(1) and a respective K⁺ content of 0.56(1) resulting in the formula Na_2.4K_0.6[B₆O₁₀]NO₃. For the phase with the highest amount of K⁺, the site occupation factors of the solid solution yielded a Na⁺ content per unit cell of 2.27(4) and a respective K⁺ content of 0.74(4) giving an idealized formula Na_2.3K_0.7[B₆O₁₀]NO₃. These calculated values were supported by EDX analyses (vide infra).

Fig. 3:

Top to bottom: In black, the experimental powder patterns of Na_2.5K_0.5[B₆O₁₀]NO₃, Na_2.4K_0.6[B₆O₁₀]NO₃, and Na_2.3K_0.7[B₆O₁₀]NO₃ are given. The patterns calculated via Rietveld analyses are given in red, the difference curves in blue.

Figure 4 shows the unit cell of Na_2.3K_0.7[B₆O₁₀]NO₃ viewed along [010], representative for all three solid solutions. The mixed occupied positions Na2/K2 and Na3/K3 are indicated with gray spheres, the unsubstituted, pure sodium positions Na1 are given as black spheres. The three different cation positions show eight-fold (Na1; distorted cubic), six-fold (Na2/K2; octahedral), and nine-fold oxygen coordination (Na3/K3; a distorted hexagonal bipyramid with one split top can be drawn as the coordination polyhedron). In direct comparison to Na₃(NO₃)[B₆O₁₀], Na_2.5K_0.5[B₆O₁₀]NO₃ shows coordination distances that vary only within the estimated standard deviations. In Na_2.4K_0.6[B₆O₁₀]NO₃, a shift towards larger bond lengths can be seen: the averaged value for Na1–O increases from 256.3 to 257.0(3) pm, the Na2–O distances rise from 250.4 to 251.3(3) pm for the equivalent Na2/K2–O coordination, and the values for Na3–O rise from 269.2 to 272.4(3) pm in the case of Na3/K3–O on average. With increasing potassium content, the cation-to-oxygen distances tend to augment: in Na_2.3K_0.7[B₆O₁₀]NO₃, the Na1–O coordination partners are found at a mean value of 260(1) pm from each other. However, the average Na2/K2–O distance does not show a significant growth compared to the pure sodium site occupation. The average Na3/K3–O distance reaches a significantly larger value of 273(1) pm. Fig. 5 shows the three different coordination polyhedra with clear trends in bond lengths indicated in colors.

Fig. 4:

Unit cell of Na_2.3K_0.7[B₆O₁₀]NO₃ as viewed along [010]. The mixed alkali positons are given as gray spheres.

Fig. 5:

Coordination polyhedra in Na_2.3K_0.7[B₆O₁₀]NO₃. Bonds elongated compared to Na₃(NO₃)[B₆O₁₀] are highlighted in red, shortened bonds are given in green.

In analogy to the sodium borate-nitrate published by Yakubovich et al., all three solid solutions are built up from trigonal-planar as well as tetrahedral borate groups. The basic structural motif is formed by three BO₄ groups sharing a common apex with the O1 position forming an OB₃ group. Three additional BO₃ units extend this borate to a three-dimensional network. This structural feature was first reported in the mineral tunellite [24]. The nitrate triangles are isolated and participate in the cation coordination. Fig. 6 shows the fundamental building units in Na_2.3K_0.7[B₆O₁₀]NO₃ representatively for all three structures. Bond lengths and angles within the trigonal-planar BO₃ and NO₃ units, as well as of the tetrahedral borate groups are listed in Tables 3 and 4 as well as Tables S2 and S3 given in the Supporting Information. All distances and angles concur well with reported values [5], [25], [26]. Interestingly, in all substitution variants the trigonal-planar borate group centered by B4 shows equally larger overall bond lengths of 141(1), 139.9(4), and 140(1) pm, compared to 136.4 pm in Na₃(NO₃)[B₆O₁₀].

Fig. 6:

Fundamental building unit in Na_2.3K_0.7[B₆O₁₀]NO₃ representative for all three substitution variants.

3.3 Vibrational spectra

The IR and Raman spectra of Na_2.5K_0.5[B₆O₁₀]NO₃, Na_2.4K_0.6[B₆O₁₀]NO₃, and Na_2.3K_0.7[B₆O₁₀]NO₃ are given in Figs. 7 and 8. All IR and Raman spectra display similar vibrational bands, assignments were done by comparison to literature. In general, the spectra confirm the structural findings, so bending and coupled vibrations of BO₃ and BO₄ units are visible between 900 and 1400 cm⁻¹ [27]. From 850 to 1600 cm⁻¹, stretching vibrations of B–O units are detected, whereby absorptions of BO₄ tetrahedra dominate in the region of 850–1100 cm⁻¹ [28], [29], [30] and absorptions of BO₃ units between 1200 and 1450 cm⁻¹[29], [30], [31], [32]. Bands at 1770 cm⁻¹ and ~2430 cm⁻¹ visible in the IR spectra are likely to stem from the vibrational absorptions of the nitrate groups [33]. No water bands were visible in the IR experiments. The sharp intense bands clearly visible in all Raman spectra at 1059 and 1057 cm⁻¹ are characteristic symmetric ν₁ stretching modes of the NO₃ groups. Since the local D_3h symmetry of the nitrate triangle is lowered due to the coordination to the cations, split peak shapes are observed [34], [35]. The pronounced bands at ~1380 cm⁻¹ are likely to contain contributions of N–O stretching modes of the nitrate groups [36].

Fig. 7:

Top to bottom: IR spectra of Na_2.5K_0.5[B₆O₁₀]NO₃, Na_2.4K_0.6[B₆O₁₀]NO₃, and Na_2.3K_0.7[B₆O₁₀]NO₃ in the range of 600 to 3500 cm⁻¹.

Fig. 8:

Top to bottom: Raman spectra of Na_2.5K_0.5[B₆O₁₀]NO₃, Na_2.4K_0.6[B₆O₁₀]NO₃, and Na_2.3K_0.7[B₆O₁₀]NO₃ in the range of 400 to 2000 cm⁻¹. Upper right corners: 10 s measurement time in the range of 1000–1100 cm⁻¹ displaying the full, intense NO₃ group vibrations.

3.4. Elemental analyses

The sodium-to-potassium ratio obtained through the refinement of the powder diffraction data via the Rietveld method as discussed above was confirmed in the course of energy dispersive X-ray spectroscopy (EDX) within the accuracy permitted by the method. Various micro crystals of all title compounds were measured as described in detail in the experimental section. An average atomic ratio of Na⁺/K⁺=5.3(3) was obtained for Na_2.5K_0.5[B₆O₁₀]NO₃. For the compound Na_2.4K_0.6[B₆O₁₀]NO₃, the average atomic ratio of Na⁺/K⁺=4.6(3) was acquired and for the last phase Na_2.3K_0.7[B₆O₁₀]NO₃ a Na⁺/K⁺ ratio of 2.8(3) was detected. In summary, the measured ratios Na⁺/K⁺ of 5.3(3), 4.6(3), and 2.8(3) correspond to the ratios of 5.0(4), 4.4(1), and 3.1(4) obtained in the course of the Rietveld refinements, respectively.

Figure 9 shows an image obtained through backscattered electron spectroscopy of the Na_2.5K_0.5[B₆O₁₀]NO₃ plate-like crystallites produced hydrothermally, representative for all three phases, obtained with the SEM equipment.

Fig. 9:

BSE SEM image of the Na_2.5K_0.5[B₆O₁₀]NO₃ plate-like crystallites obtained hydrothermally at 450-fold magnification, representative for the habitus of all three phases.