Abstract
Single crystals of the hydrous cadmium borate Cd6B22O39·H2O were obtained through a high-pressure/high-temperature experiment at 4.7 GPa and 1000 °C using a Walker-type multianvil apparatus. CdO and partially hydrolyzed B2O3 were used as starting materials. A single crystal X-ray diffraction study has revealed that the structure of Cd6B22O39·H2O is similar to that of the type M6B22O39·H2O (M=Fe, Co). Layers of corner-sharing BO4 groups are interconnected by BO3 groups to form channels containing the metal cations, which are six- and eight-fold coordinated by oxygen atoms. The compound crystallizes in the space group Pnma (no. 62) [R1=0.0379, wR2=0.0552 (all data)] with the unit cell dimensions a=1837.79(5), b=777.92(2), c=819.08(3) pm, and V=1171.00(6) Å3. The IR and Raman spectra reflect the structural characteristics of Cd6B22O39·H2O.
1 Introduction
In the years 2003 and 2005, the two phases β-ZnB4O7 and β-HgB4O7 were synthesized for the first time at conditions of 10.5 GPa/1100 °C and 7.5 GPa/600 °C, respectively [1, 2]. The existence of a corresponding high-pressure phase “β-CdB4O7” is still an open question. However, all attempts to synthesize an analogous phase of the second group-12 element were not successful. Instead, a phase with the composition CdB2O4 was formed at 7.5 GPa and 1100 °C [3]. In 2012, density functional theory (DFT) calculations suggested that “β-CdB4O7” would form at markedly lower pressures around 1.9 GPa [4]. However, from these calculations the structure type could not be predicted unequivocally; hence, the exciting question about the structure type of “β-CdB4O7” remained. With this new information and the yet open question about the structure type of “β-CdB4O7” (β-ZnB4O7 or β-HgB4O7), we tried again to synthesize the compound “β-CdB4O7”.
In the system Cd-B-O, the crystal structures of five phases are known: CdB4O7 [5], the already mentioned high-pressure phase CdB2O4 [3], Cd2B2O5 [6], and two polymorphs of Cd3B2O6 [7, 8]. Since boric acid is commonly used as a starting reagent, it is also worth to mention the structures containing hydrogen: Cd4(BO3)2(OH)2 [9], Cd(H2O)6(B12H12) [10], Cd3(B7O13)(OH) [11], and Cd(B10O14(OH)4)(H2O) [12]. Furthermore, the phases Cd(BO2)2·7 H2O [13], CdB4O7·5 H2O [14], Cd6B14O27·3–3.8 H2O [15], and Cd6B6O15·4 H2O [16] are described in the literature, but a detailed characterization is missing.
In a series of about 25 experiments, all attempts to synthesize “β-CdB4O7” were not successful. Yet, we obtained a new compound with the composition Cd6B22O39·H2O. In this work, its crystal structure, the powder diffraction pattern, as well as the IR and Raman spectra are presented and discussed.
2 Experimental section
2.1 Synthesis
Cd6B22O39·H2O was synthesized from a 1:2 molar mixture of CdO (99 %, Fluka, Buchs, Switzerland) and partially hydrolyzed B2O3 (99.9+ %, Strem Chemicals, Kehl, Germany) in a high-pressure/high-temperature experiment. The chemicals were finely ground together in an agate mortar, filled into a platinum capsule (0.025 mm foil, 99.9 %, ChemPur, Karlsruhe, Germany), put into a crucible made of hexagonal boron nitride (HeBoSint® P100, Henze BNP GmbH, Kempten, Germany), and compressed by eight tungsten carbide cubes (HA-7 %Co, Hawedia, Marklkofen, Germany) via an 18/11 assembly.
A pressure of 4.7 GPa was generated by a hydraulic press (mavo press LPR 1000-400/50, Max Voggenreiter GmbH, Mainleus, Germany) equipped with a Walker-type module (also Max Voggenreiter GmbH) within 2 h. The sample was heated to 1000 °C within 10 min and kept at these conditions for a further 10 min. Prior to quenching by turning off the heating, the sample was cooled to 400 °C within 60 min. The decompression of the assembly lasted 6 h. Further details on the assembly and its preparation are described in the references [17–20].
The colorless crystals found in the sample after separation from the surrounding octahedral pressure medium (MgO, Ceramic Substrates and Components Ltd., Newport, Isle of Wight, U.K.) and the other parts of the assembly were found to be Cd6B22O39·H2O with the water originating from B2O3, which was partially hydrolyzed during the preparations for the experiment.
2.2 Crystal structure analysis
The powder diffraction pattern of Cd6B22O39·H2O was collected on a STOE Stadi P powder diffractometer equipped with a Dectris Mythen 1K detector with curved Ge(111)-monochromatized CuKα1 radiation (λ=145.06 pm) in transmission geometry. It is displayed in Fig. 1 in combination with the theoretical powder pattern simulated from the single crystal data [21]. Three hundred reflections of the powder pattern were indexed and refined [22] leading to the lattice parameters listed in Table 1. They fit well to the lattice parameters obtained from the single crystal data. Single crystals of Cd6B22O39·H2O were selected with a polarization contrast microscope. The single crystal diffraction data set was collected with a Nonius Kappa-CCD diffractometer with graphite-monochromatized MoKα radiation (λ=71.073 pm) at room temperature. Scalepack [23] was used to correct the intensity data for absorption based on equivalent and redundant intensities. The space groups Pnma (no. 62) and Pmn21 (no. 31) were derived from the systematic extinctions. The structure was solved and refined in both space groups (full-matrix least-squares on F2) [24, 25]. The refinement in Pnma (no. 62) yielded better results, and the Adsymm routine of Platon verified that there is no additional symmetry [26]. Except Cd4, O13, and O14, all other sites were refined with anisotropic displacement parameters. Due to the high electron density at the cadmium sites, the proton positions were not visible in the residual electron density map; hence, they were not included in the model. However, the bond distances and angles, the IR spectrum, as well as charge neutrality indicate that two protons are present next to O14 representing crystal water. In the two related compounds, M6B22O39·H2O (M=Fe, Co) [27], the presence of protons was derived in a similar way. As already mentioned, no significant peaks could be found in the final difference Fourier syntheses. All relevant details of the data collection and evaluation are listed in Table 1. Tables 2–5 show the positional parameters, anisotropic displacement parameters, selected interatomic distances, and bond angles.
Empirical formula | Cd6B22O39·H2O |
Molar mass, g mol–1 | 1554.24 |
Crystal system | Orthorhombic |
Space group | Pnma (no. 62) |
Powder diffractometer | Stoe Stadi P |
Radiation; wavelength, pm | CuKα1; λ =154.060 |
Powder data | |
a, pm | 1837.65(2) |
b, pm | 777.836(8) |
c, pm | 819.062(8) |
V, Å3 | 1170.76(3) |
Single crystal diffractometer | Enraf-Nonius Kappa-CCD |
Radiation; wavelength, pm | MoKα; λ =71.073 |
(graphite monochromator) | |
Single crystal data | |
a, pm | 1837.79(5) |
b, pm | 777.92(2) |
c, pm | 819.08(3) |
V, Å3 | 1171.00(6) |
Formula units per cell, Z | 2 |
Calculated density, g cm–3 | 4.41 |
Crystal size, mm3 | 0.03 × 0.03 × 0.05 |
Temperature, K | 296(2) |
Absorption coefficient, mm–1 | 5.6 |
F(000), e | 1440 |
θ range, deg | 2.22–32.54 |
Range in hkl | –27 ≤ h ≤ 27, –11 ≤ k ≤ 11, –12 ≤ l ≤ 12 |
Total no. of reflections | 14,185 |
Independent reflections/Rint/Rσ | 2256/0.0628/0.0378 |
Reflections with I > 2 σ(I) | 1933 |
Data/ref. parameters | 2256/177 |
Absorption correction | Multiscan [23] |
Goodness-of-fit on F2 | 1.043 |
Final R1/wR2 [I > 2 σ(I)] | 0.0284/0.0524 |
R1/wR2 (all data) | 0.0379/0.0552 |
Largest diff. peak/hole, e Å–3 | 0.99/–1.31 |
Atom | Wyckoff position | x | y | z | Uiso/Ueq | s.o.f. |
---|---|---|---|---|---|---|
Cd1 | 4c | 0.53238(8) | 3/4 | 0.58922(4) | 0.00740(8) | 1 |
Cd2 | 4c | 0.89829(2) | 3/4 | 0.32036(4) | 0.00889(8) | 1 |
Cd3 | 4c | 0.69588(3) | 3/4 | 0.03866(5) | 0.01159(9) | 0.94 |
Cd4 | 4c | 0.6700(4) | 3/4 | 0.0082(8) | 0.007(2) | 0.06 |
B1 | 8d | 0.7432(2) | 0.5843(4) | 0.4110(4) | 0.0049(6) | 1 |
B2 | 8d | 0.8160(2) | 0.4214(4) | 0.2002(4) | 0.0051(6) | 1 |
B3 | 8d | 0.6724(2) | 0.4152(4) | 0.1817(4) | 0.0056(6) | 1 |
B4 | 8d | 0.6190(2) | 0.4212(4) | 0.4705(4) | 0.0060(4) | 1 |
B5 | 8d | 0.5583(2) | 0.5817(4) | 0.2422(4) | 0.0060(6) | 1 |
B6 | 8d | 0.4867(4) | 0.4179(9) | 0.0391(9) | 0.007(2) | 1/2 |
O1 | 8d | 0.8115(2) | 0.5457(3) | 0.3326(3) | 0.0063(4) | 1 |
O2 | 8d | 0.7460(2) | 0.4520(3) | 0.0952(3) | 0.0055(4) | 1 |
O3 | 8d | 0.6890(2) | 0.4432(3) | 0.3627(3) | 0.0057(4) | 1 |
O4 | 8d | 0.5559(2) | 0.4495(3) | 0.3683(3) | 0.0064(4) | 1 |
O5 | 8d | 0.6222(2) | 0.5505(3) | 0.1341(3) | 0.0065(4) | 1 |
O6 | 4c | 0.7155(1) | 3/4 | 0.3764(4) | 0.0067(6) | 1 |
O7 | 4c | 0.8139(2) | 1/4 | 0.2706(4) | 0.0069(6) | 1 |
O8 | 8d | 0.8775(2) | 0.4510(3) | 0.0992(3) | 0.0053(4) | 1 |
O9 | 4c | 0.6455(2) | 1/4 | 0.1418(4) | 0.0066(6) | 1 |
O10 | 4c | 0.6192(2) | 1/4 | 0.5441(4) | 0.0068(6) | 1 |
O11 | 4c | 0.5554(2) | 3/4 | 0.3161(4) | 0.0070(6) | 1 |
O12 | 8d | 0.4923(2) | 0.5649(3) | 0.1308(3) | 0.0103(4) | 1 |
O13 | 4c | 0.4718(7) | 1/4 | 0.114(2) | 0.021(3) | 1/2 |
O14 | 8d | 0.4581(8) | 0.220(2) | 0.121(2) | 0.027(4) | 1/4 |
Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.
Atom | U11 | U22 | U33 | U12 | U13 | U23 |
---|---|---|---|---|---|---|
Cd1 | 0.0059(2) | 0.0073(2) | 0.0090(2) | 0 | 0.0008(1) | 0 |
Cd2 | 0.0063(2) | 0.0078(2) | 0.0126(2) | 0 | –0.0011(2) | 0 |
Cd3 | 0.0181(2) | 0.0066(2) | 0.0101(2) | 0 | 0.0052(2) | 0 |
B1 | 0.004(2) | 0.004(2) | 0.007(2) | –0.000(1) | –0.001(1) | 0.000(2) |
B2 | 0.003(2) | 0.006(2) | 0.007(2) | 0.001(1) | 0.000(1) | –0.001(2) |
B3 | 0.006(2) | 0.006(2) | 0.005(2) | –0.001(2) | 0.001(1) | –0.001(2) |
B4 | 0.006(2) | 0.005(2) | 0.007(2) | –0.002(1) | 0.001(1) | 0.001(2) |
B5 | 0.004(2) | 0.007(2) | 0.007(2) | 0.000(2) | 0.001(1) | 0.000(2) |
B6 | 0.006(3) | 0.010(3) | 0.007(3) | 0.002(2) | –0.005(2) | 0.000(2) |
O1 | 0.0065(9) | 0.006(1) | 0.006(1) | 0.0001(7) | 0.0005(7) | –0.0013(8) |
O2 | 0.0030(9) | 0.009(1) | 0.005(1) | –0.0007(7) | 0.0002(7) | –0.0012(8) |
O3 | 0.0057(9) | 0.006(1) | 0.005(1) | –0.0018(7) | 0.0003(7) | –0.0021(8) |
O4 | 0.0055(9) | 0.0071(9) | 0.007(1) | 0.0001(7) | –0.0013(7) | 0.0007(8) |
O5 | 0.0061(9) | 0.007(1) | 0.006(1) | 0.0029(7) | 0.0006(7) | 0.0019(8) |
O6 | 0.006(2) | 0.006(2) | 0.008(2) | 0 | –0.001(2) | 0 |
O7 | 0.009(2) | 0.005(2) | 0.008(2) | 0 | 0.002(2) | 0 |
O8 | 0.0052(9) | 0.006(1) | 0.005(1) | 0.0012(7) | 0.0008(7) | 0.0011(7) |
O9 | 0.006(2) | 0.005(2) | 0.009(2) | 0 | –0.001(2) | 0 |
O10 | 0.009(2) | 0.006(2) | 0.006(2) | 0 | 0.001(2) | 0 |
O11 | 0.009(2) | 0.005(2) | 0.007(2) | 0 | –0.000(2) | 0 |
O12 | 0.009(1) | 0.013(2) | 0.009(2) | 0.0019(8) | –0.0053(8) | –0.0034(9) |
Cd1–O4 | 227.2(2) | Cd2–O1 | 225.3(2) | Cd3–O7 | 220.3(3) | Cd4–O7 | 196.8(8) |
Cd1–O4 | 227.2(2) | Cd2–O1 | 225.3(2) | Cd3–O5 | 220.4(2) | Cd4–O5 | 206.1(5) |
Cd1–O11 | 227.7(3) | Cd2–O12 | 228.4(2) | Cd3–O5 | 220.4(2) | Cd4–O5 | 206.1(5) |
Cd1–O8 | 227.9(2) | Cd2–O12 | 228.4(2) | Cd3–O2 | 253.7(2) | Cd4–O14 | 259(2) |
Cd1–O8 | 227.9(2) | Cd2–O10 | 228.6(3) | Cd3–O2 | 253.7(2) | Cd4–O14 | 259(2) |
Cd1–O14 | 239(2) | Cd2–O9 | 275.3(3) | Cd3–O6 | 279.0(3) | Cd4–O1 | 273.4(4) |
Cd1–O14 | 239(2) | av. Cd2–O | 235.2 | Cd3–O1 | 285.7(2) | Cd4–O1 | 273.4(4) |
Cd1–O13 | 244(2) | Cd3–O1 | 285.7(2) | Cd4–O13 | 279(2) | ||
av. Cd1–O | 232.5 | av. Cd3–O | 252.4 | Cd4–O2 | 279.8(4) | ||
Cd4–O2 | 279.8(4) | ||||||
av. Cd4–O | 251.4 | ||||||
B1–O6 | 141.5(3) | B2–O8 | 142.0(4) | B3–O9 | 141.5(4) | B4–O4 | 144.7(4) |
B1–O1 | 144.2(4) | B2–O7 | 145.3(4) | B3–O5 | 145.2(4) | B4–O8 | 145.0(4) |
B1–O3 | 153.4(4) | B2–O1 | 145.5(4) | B3–O3 | 153.0(4) | B4–O10 | 146.2(4) |
B1–O2 | 154.7(4) | B2–O2 | 156.6(4) | B3–O2 | 155.4(4) | B4–O3 | 156.9(4) |
av. B1–O | 148.5 | av. B2–O | 147.4 | av. B3–O | 148.8 | av. B4–O | 148.2 |
B5–O11 | 144.3(4) | B6–O12 | 137.2(7) | ||||
B5–O4 | 145.8(4) | B6–O12 | 145.1(7) | ||||
B5–O5 | 149.1(4) | B6–O13 | 146.7(9) | ||||
B5–O12 | 152.4(4) | av. B6–O | 143.0 | ||||
av. B5–O | 147.9 |
O3–B1–O2 | 101.8(2) | O1–B2–O2 | 105.2(2) | O3–B3–O2 | 104.0(2) |
O1–B1–O2 | 106.5(2) | O8–B2–O2 | 108.0(2) | O5–B3–O3 | 106.5(2) |
O1–B1–O3 | 107.6(2) | O7–B2–O1 | 108.2(3) | O5–B3–O2 | 107.3(2) |
O6–B1–O3 | 111.5(2) | O7–B2–O2 | 109.6(2) | O9–B3–O2 | 111.5(3) |
O6–B1–O2 | 114.1(3) | O8–B2–O1 | 111.8(2) | O9–B3–O5 | 112.0(3) |
O6–B1–O1 | 114.4(3) | O8–B2–O7 | 113.7(3) | O9–B3–O3 | 115.0(3) |
av. O–B1–O | 109.3 | av. O–B2–O | 109.4 | av. O–B3–O | 109.4 |
O8–B4–O3 | 107.4(2) | O5–B5–O12 | 104.9(2) | O12–B6–O12 | 115.3(5) |
O4–B4–O3 | 108.4(2) | O11–B5–O12 | 107.5(3) | O12–B6–O13 | 121.9(7) |
O8–B4–O10 | 108.9(3) | O4–B5–O5 | 109.2(2) | O12–B6–O13 | 122.1(7) |
O10–B4–O3 | 109.2(3) | O4–B5–O12 | 109.9(2) | av. O–B6–O | 119.8 |
O4–B4–O8 | 110.6(2) | O11–B5–O4 | 110.0(3) | ||
O4–B4–O10 | 112.3(3) | O11–B5–O5 | 115.2(3) | ||
av. O–B4–O | 109.5 | av. O–B5–O | 109.5 |
Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting the deposition number CSD-428634.
2.3 Vibrational spectroscopy
The IR spectrum of Cd6B22O39·H2O was recorded from a single crystal in the spectral range of 600–5800 cm–1 with a Bruker Vertex 70 FT-IR spectrometer (spectral resolution 4 cm–1) equipped with a KBr beam splitter, a Mercury Cadmium Telluride (LN-MCT) detector, and a Bruker Hyperion 3000 microscope. As mid-infrared source, a Globar (silicon carbide) rod was used. A frustum-shaped attenuated total reflection (ATR) crystal was pressed on the sample crystal, crushing it to pieces. Thirty-two scans of the sample were acquired.
Two Raman spectra of Cd6B22O39·H2O were measured from a single crystal in the spectral ranges of 120–1800 and 1800–4000 cm–1 with a Raman microspectrometer LabRAM HR-800 (Horiba Jobin Yvon) and 100-fold magnification using a frequency doubled Nd:YAG laser (λ=532.22 nm) as excitation source. The Raman-scattered light was detected through an optical grid with 1800 lines/mm. Three ranges were measured for each spectrum with a spectral resolution better than 2 cm–1. The measurement time per range was 150 s. A background correction was applied [28].
3 Results and discussion
3.1 Crystal structure of Cd6B22O39·H2O
The orthorhombic unit cell of Cd6B22O39·H2O exhibits the dimensions a=1837.79(5), b=777.92(2), c=819.08(3) pm, and V=1171.00(6) Å3. The compound crystallizes with two formula units per cell (Z=2) in the space group Pnma (no. 62). The crystal structure is built up from corner-sharing BO3 and BO4 groups. It shows great similarities to the structure type of M6B22O39·H2O (M=Fe, Co). Both structures are compared in Fig. 2. However, the latter compounds crystallize in the space group Pmn21 (no. 31), which is a maximal nonisomorphic subgroup of Pnma (no. 62) [27]. The structures consist of corrugated layers of BO4 tetrahedra that are interconnected by BO3 groups. The different space groups originate from a variance around the BO3 group. While the BO3 group in M6B22O39·H2O (M=Fe, Co) is in an intermediate state between a triangular planar BO3 group and a BO4 tetrahedron, the BO3 group in Cd6B22O39·H2O hardly differs from the trigonal planar geometry owing to a disorder at the central boron site (occupation: 50 %). Within the layers, two three-fold coordinated oxygen atoms (O[3]) are present. Between the layers, the metal cations are located in rectangular channels that are separated by the BO3 groups.
Selected bond lengths and angles can be found in Tables 4 and 5. The B–O bond lengths in Cd6B22O39·H2O [137.2(7)–146.7(9) pm, av. 143.0 pm] are smaller than in the similar structures of Fe6B22O39·H2O [142.0(5)–148.9(5) pm, av. 146.1 pm] and Co6B22O39·H2O [142.4(4)–147.7(4) pm, av. 145.7 pm] due to the differences around the BO3 group. The next oxygen atom is 188.2(6) or 169.5(4) pm apart from the boron atom in the iron and cobalt compound, respectively. This leads to an intermediate state between a planar BO3 group and an edge-sharing BO4 tetrahedron. In contrast, the distance is larger in the cadmium compound [246.5(7) pm], leading to a nearly planar BO3 group (mean O–B–O bond angle: 119.8°) (see Fig. 2). Another difference is the position of the oxygen atom carrying the protons. While in M6B22O39·H2O (M=Fe, Co) the corresponding oxygen atom is situated on a mirror plane, the O14 site is positioned next to a mirror plane in Cd6B22O39·H2O. In addition, the O13 site remains on the mirror plane. This leads to an O14–O14 distance of 47(3) pm and O13–O14 distances of 35(2) pm. Accordingly, the neighboring O13 and O14 sites cannot be occupied at the same time. Instead, there is disorder with site occupation factors of 0.5 and 0.25 for O13 and O14, respectively. Thus, the BO3 group in Cd6B22O39·H2O exhibits a specific disorder as displayed in detail in Fig. 3. If O13 is occupied, the two neighboring B6 positions are occupied, while the same positions remain unoccupied if one of the O14 positions is occupied. In both cases, this leads to pyroborate groups B2O5, which are separated from the next B2O5 group by a water molecule (O14). The bond lengths and angles within the BO4 tetrahedra are comparable to the values found in literature [137.3–169.9 pm, av. 147.6(3.5) pm and 95.7–119.4°, av. 109.4(2.8)°] [29].
Within the channels of the B–O network, four cadmium sites are present. The coordination sphere of Cd1 and Cd4 is complex, as it is affected by the disorder of the atoms O13 and O14. With respect to this disorder, Cd1 and Cd2 exhibit a distorted octahedral coordination, while Cd3 and Cd4 are asymmetrically coordinated by eight oxygen atoms as illustrated in Fig. 4. The reason for this asymmetric coordination is presumably the disorder of the Cd atom resulting in the two disordered positions Cd3 and Cd4. Six- and eight-fold coordinated Cd2+ cations lie well within the range found in other cadmium containing borates and borate hydrates (CN=4 [5]–12 [3]). The Cd–O distances are distributed from 196.8(8) to 285.7(2) pm. While Cd–Omin of 196.8(8) pm is the shortest bond distance within the system Cd–B–O(–H) [214.3(2) [8]–230.6(7) [9] pm], the distance Cd–Omax [285.7(2) pm] is longer than in the other phases of the system [222(1) [5]–285.0(2) [3] pm]. The short Cd–O distance of just 196.8(8) pm is a result of the disorder of the Cd3 and Cd4 position (s.o.f.: 0.94 and 0.06, respectively), since Cd4 is more orientated toward O7 compared to Cd3 [Cd3–O7: 220.3(3) pm; Cd4–O7: 196.8(8) pm].
3.2 Vibrational spectroscopy
The ATR-IR and Raman spectra of Cd6B22O39·H2O are presented in Figs. 5 and 6, respectively. The vibrational modes are assigned to vibrations of the different groups by comparison with other borates. Below 800 cm–1, bands of complex bending and stretching vibrations of the lattice, including the metal ions, are visible. Bands in the area between 800 and 1100 cm–1 can be assigned to stretching modes of the BO4 tetrahedra [30, 31], while absorption bands between 1200 and 1450 cm–1 are expected for borates containing BO3 groups [31, 32]. In both regions, modes are visible in the IR and Raman spectra. However, there are also modes in between, which probably arise from the distorted BO3 group, showing remarkably long B–O distances nearly comparable to those in BO4 tetrahedra. Modes between 3000 and 3500 cm–1 are typical for compounds containing water. The remaining vibrations around 1600 and 2900 cm–1 presumably arise from acetone, which was used to wash the crystals [27].
4 Conclusions
Single crystals of Cd6B22O39·H2O were obtained by a high-pressure/high-temperature experiment at 4.7 GPa and 1000 °C. In analogy to the structures of M6B22O39·H2O (M=Fe, Co), Cd6B22O39·H2O is built up from layers of corner-sharing BO4 groups that are interconnected by BO3 groups. In contrast to the iron and cobalt phase, no formation of intermediate states on the way to edge-sharing tetrahedra can be recognized. Between the layers, the metal cations are six- and eight-fold coordinated by oxygen atoms. IR and Raman spectral data support the structure model found by singe crystal X-ray diffraction, showing vibrations of BO3 and BO4 groups as well as of H2O. Even though this fascinating new structure is now at hand, the existence of “β-CdB4O7” still remains an unsolved question, which needs further investigation.
Acknowledgments
Special thanks go to University Prof. Dr. R. Stalder (University of Innsbruck) for generous access to the IR spectrometer, to Lukas Perfler for the dedicated measurement of the Raman spectrum, to Dr. Simon Penner for the swift translation of Russian literature, and to Dr. Gunter Heymann (University of Innsbruck) for the precise recording of the single crystal data set.
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