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Article

The Cobalt(II) Oxidotellurate(IV) Hydroxides Co2(TeO3)(OH)2 and Co15(TeO3)14(OH)2

by
Felix Eder
1,
Matthias Weil
1,*,
Prativa Pramanik
2 and
Roland Mathieu
2
1
Institute for Chemical Technologies and Analytics, Division of Structural Chemistry, TU Wien, Getreidemarkt 9/164-5-01, 1060 Vienna, Austria
2
Department of Materials Science and Engineering, Uppsala University, Box 35, 751 03 Uppsala, Sweden
*
Author to whom correspondence should be addressed.
Crystals 2023, 13(2), 176; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst13020176
Submission received: 22 December 2022 / Revised: 11 January 2023 / Accepted: 12 January 2023 / Published: 19 January 2023
(This article belongs to the Special Issue Different Kinds of Hydrogen Bonds in Crystal Structures)
Browse Figures
Versions Notes

Abstract

:
Previously unknown Co2(TeO3)(OH)2 and Co15(TeO3)14(OH)2 were obtained under mild hydrothermal reaction conditions (210 °C, autogenous pressure) from alkaline solutions. Their crystal structures were determined from single-crystal X-ray diffraction data. Co2(TeO3)(OH)2 (Z = 2, P 1 ¯ , a = 5.8898(5), b = 5.9508(5), c = 6.8168(5) Å, α = 101.539(2), β = 100.036(2), γ = 104.347(2)°, 2120 independent reflections, 79 parameters, R[F2 > 2σ(F2)] = 0.017) crystallizes in a unique structure comprised of undulating 2∝[Co2(OH)6/3O3/3O2/2O1/1]4− layers. Adjacent layers are linked by TeIV atoms along the [001] stacking direction. Co2(TeO3)(OH)2 is stable up to 450 °C and decomposes under the release of water into Co6Te5O16 and CoO. Magnetic measurements of Co2(TeO3)(OH)2 showed antiferromagnetic ordering at ≈ 70 K. The crystal structure of Co15(TeO3)14(OH)2 (Z = 3, R 3 ¯ , a = 11.6453(2), c = 27.3540(5) Å, 3476 independent reflections, 112 parameters, R[F2 > 2σ(F2)] = 0.026) is isotypic with Co15(TeO3)14F2. A quantitative structural comparison revealed that the main structural difference between the two phases is connected with the replacement of F by OH, whereas the remaining part of the three-periodic network defined by [CoO6], [CoO5(OH)], [CoO5] and [TeO3] polyhedra is nearly unaffected. Consequently, the magnetic properties of the two phases are similar, namely being antiferromagnetic at low temperatures.

1. Introduction

Cobalt compounds in the ternary Co/Te/O system are known to exist solely with an oxidation state of +II for Co, whereas the oxidation state of Te can be +IV or +VI. Next to the structural variety of corresponding cobalt(II) oxidotellurates resulting from the two possible oxidation states of Te and the condensation grade of the oxidotellurate anions, some of the phases in this system are of interest due to their interesting magnetic and electronic behaviors. This includes CoTeIVO3 [1,2], CoTeVIO4 [3], Co3TeVIO6 [4,5,6,7,8] and Co5TeVIO8 [9]. Most of these phases have been prepared by conventional solid-state reactions at varying pressure conditions [1,2,3,4,7,8,9], or by the application of chemical vapor transport reactions [5,6,10]. Other phases in the Co/Te/O system, for which crystal structure determinations have been carried out so far, include Co6TeIV5O16 [11], CoTeIV6O13 [12] and Co2TeIV3O8 [13]. The latter two phases were prepared by hydrothermal synthesis. Under the conditions typically applied for this method, an incorporation of water or OH groups into the resulting solids is not uncommon, which, in the case of cobalt oxidotellurates, yielded non-centrosymmetric Co3(TeIVO3)2(OH)2 [14]. Subsequent re-investigations of this phase likewise revealed interesting magnetic and electric properties [15], as well as a possible incorporation of foreign components into the channels of the crystal structure, where parts of the OH groups can be replaced by other anions and/or water molecules [16]. During the latter study and during related hydrothermal formation studies for phases with zemannite-type structures [17], we obtained two new cobalt(II) oxidotellurates(IV) with additional OH groups in the form of side products, viz. Co2(TeO3)(OH)2 and Co15(TeO3)14(OH)2.
We report here on our efforts to increase the yield of Co2(TeO3)(OH)2 and Co15(TeO3)14(OH)2, together with the results of their crystal structure analyses and physical property measurements.

2. Materials and Methods

2.1. Synthesis

All employed chemicals were of pro analysi quality and were purchased from Merck (Darmstadt, Germany). Co2(TeO3)(OH)2 and Co15(TeO3)14(OH)2 were originally obtained as minor by-products during hydrothermal phase formation studies for the intended synthesis of Co3(TeO3)2(OH)2 [16] or Na2[Co2(TeO3)3]·3H2O [17]. In representative experiments targeted for 0.5 g of the intended phase, CoCO3, TeO2 and KOH (molar ratio 3:2:4) and CoO (prepared by thermal decomposition of CoCO3), TeO2 and Na2CO3 (molar ratio 2:3:10), respectively, were mixed and placed in Teflon containers with an inner volume of ≈ 4 mL. The containers were subsequently filled to two-thirds of their volume with water, sealed, placed in steel autoclaves and were heated under autogenous pressure for one week at 210 °C. The obtained solid products were filtered off with a glass frit, washed with water and ethanol and then dried in air. The obtained crystals could be distinguished due to their different colors and forms. Co2(TeO3)(OH)2 crystallizes in the form of pink needles up to 0.5 mm in lengths, Co15(TeO3)14(OH)2 in form of dark blue-to-violet isometric crystals up to 0.2 mm in length (Figure 1), and both Na2[Co2(TeO3)3]·3H2O and Co3(TeO3)2(OH)2 in form of violet thin hexagonal prisms.
Powder X-ray diffraction (PXRD) measurements of the bulk products revealed Co15(TeO3)14(OH)2, Co2(TeO3)(OH)2 and Co3(TeO3)2(OH)2 in approximate weight percentages of 58%, 27% and 15%, respectively, for the batch with CoCO3, TeO2 and KOH. For the batch starting with CoO, TeO2 and Na2CO3, the products were Na2[Co2(TeO3)3]·3H2O, Co15(TeO3)14(OH)2 and Co3(TeO3)2(OH)2 in approximate weight percentages of 60%, 25% and 15%, respectively.
Increasing the amount of KOH to a CoCO3:TeO2:KOH ratio of 3:2:9 led to a brownish polycrystalline product that was leached with diluted sulfuric acid (0.1 M) for ten minutes at room temperature. After the remaining solid was filtered off and washed with water and ethanol, a change to a dark pink color was observed. This product corresponds to single-phase Co2(TeO3)(OH)2.
For subsequent physical measurements, single-phase material of polycrystalline Co2(TeO3)(OH)2 was used, whereas for Co15(TeO3)14(OH)2 single crystals were hand-picked under an optical microscope.

2.2. X-ray Diffraction Measurements and Crystal-Structure Analysis

PXRD measurements were performed on a PANalytical X’Pert II Pro-type PW 3040/60 diffractometer using Cu-Kα1,2-radiation and an X’Celerator detector (Malvern Panalytical, Malvern, United Kingdom). For phase analysis of the reaction products, the Highscore+ software suite [18] (version 5.1) was employed.
Single-crystal X-ray diffraction measurements were performed on a Bruker Kappa APEX II single-crystal diffractometer using graphite-monochromatized Mo-Kα radiation equipped with a CCD detector (Bruker AXS, Madison, WI, USA). Instrument software (Apex-4, Saint [19]) was used for optimized measurement strategies (>99% completeness at θmax) and for data reduction; correction for absorption effects was performed with SADABS [20]. The crystal structures were solved with SHELXT [21], refined with SHELXL [22] and graphically represented with ATOMS [23]. In the case of Co2(TeO3)(OH)2, hydrogen atoms, which are part of an OH group, could clearly be located from a difference-Fourier map. Their positions were freely refined with Uiso(H) = 1.5Ueq of the parent O atom. In the case of Co15(TeO3)14(OH)2, the hydrogen atom of the OH group could not be located and thus is not part of the structure model. For the latter structure, atom labels and coordinates were assigned in accordance with the previously reported isotypic crystal structure of Mn15(TeO3)14(OH)2 [24].
Crystal structures and refinement data are listed in Table 1. Further details of the crystal structure investigations may be obtained from the joint CCDC/FIZ Karlsruhe online deposition service: https://www.ccdc.cam.ac.uk/structures/ by quoting the deposition numbers specified at the end of Table 1.
Bond valence sums (BVS) [25] were calculated using the bond valence parameters provided by Brese & O’Keeffe [26]. For the TeIV–O pair, the revised bond valence parameters by Mills & Christy [27] were additionally used, then they were put under consideration of all oxygen atoms within a distance of 3.5 Å.
Isotypic structures were quantitatively compared using the compstru program [28] available at the Bilbao crystallographic server [29].

2.3. Magnetic Measurements

The magnetic properties of the two materials were investigated as a function of temperature and magnetic field using a superconducting quantum interference device (SQUID) magnetometer from Quantum Design Inc (San Diego, CA, USA).

2.4. IR Spectroscopy

IR measurements were carried out in an ATR set-up on a Perkin Elmer Spectrum Two FT-IR spectrometer (with a diamond UATR unit; Perkin Elmer, Waltham, MA, USA). After the determination of the background (air), transmission was recorded in a range of 4000–400 cm⁻1. Samples were ground to fine powder prior to the investigation. The spectra were obtained as an average of four consecutive individual measurements.

2.5. Thermal Analysis

Thermogravimetry (TG) and differential scanning calorimetry (DSC) measurements were carried out in the temperature range 30–580 °C under flowing argon atmosphere (20 mL min−1) conditions on a Netzsch TG 209 F3 Tarsus (heating rate 10 °C‧min−1) and a Netzsch DSC 200 F3 Maia instrument (heating/cooling rate 10 °C‧min−1), respectively (Netzsch, Selb, Germany). For the TG measurements, an alumina crucible with an inner volume of 85 μL and with a pierced alumina lid was used as sample container. A correction measurement of the empty crucible was conducted and afterwards subtracted from the measurement data. For the DSC measurements, the samples were placed into aluminum crucibles (inner volume of 25 μL) that were cold-welded with a pierced aluminum lid.

3. Results and Discussion

3.1. Synthesis

PXRD of the product, formed from the more highly concentrated KOH solution, revealed single-phase Co2(TeO3)(OH)2 (see Supplementary Figure S1). In comparison with the other formed phases containing an additional OH group, i.e., Co15(TeO3)14(OH)2 and Co3(TeO3)2(OH)2, the amount of OH in Co2(TeO3)(OH)2 is the highest. Consequently, an increase in the OH− concentration of the solution favors the formation of this product. On the other hand, Na2[Co2(TeO3)3]·3H2O without OH groups in the structure solely forms in Na2CO3-containing solutions. Needless to say that this compound requires Na+ cations to be formed, but the lower alkalinity of the soda solution compared to the caustic potash solution appears to govern that Co2(TeO3)(OH)2 is not formed from Na2CO3 solutions.

3.2. Crystal Structures

All atoms in the asymmetric unit of Co2(TeO3)(OH)2 (two Co, one Te, five O, two H) are situated on a general 2 i position of space group P 1 ¯ . Each of the two CoII atoms is surrounded by six O atoms in form of a distorted octahedron, whereby Co1 shows a coordination with four OH groups (associated with O1 and O2) and two O ligands, and Co2 does so with two OH groups and four O ligands. The [Co1(OH)4O2] octahedron shows a more disparate bond lengths distribution (with one considerably longer bond of 2.3422(17) Å to an O ligand) than the [Co2(OH)2O4] octahedron. Nevertheless, the average Co—O distances of 2.124 Å for Co1 and of 2.110 Å for Co2 are similar and match with the overall mean value of 2.108(62) Å, calculated for 243 [CoO6] polyhedra [30]. The [Co1(OH)4O2] octahedra are fused together by edge-sharing their OH groups, leading to the formation of 1∝[Co1(OH)4/2O2/1] chains propagating parallel to [010]. Two [Co2(OH)2O4] octahedra are linked into centrosymmetric dimers through a common edge (O4---O4). These [Co2(OH)2/1O2/2O2/1]2 dimers in turn link neighboring 1∝[Co1(OH)4/2O2/1] chains through a common O1H---O2H edge and through corner-sharing the O4 and O5 atoms, thereby forming undulating layers 2∝[Co2(OH)6/3O3/3O2/2O1/1]4− extending parallel to (001). In these layers, both OH groups are bonded to three CoII atoms (Figure 2).
The 2∝[Co2(OH)6/3O3/3O2/2O1/1]4− layers stack along [001] and are linked through the Te1IV atoms that flank the layers on both sides. The Te1 atom is bonded to three O atoms in the shape of a trigonal pyramid, the most common coordination polyhedron for a [TeO3] unit [31]. In the crystal structure (Figure 3), the [TeO3] units are isolated from each other, having a connectivity of Q3000 in the notations of Christy et al. [31].
Additional stabilization of the structural arrangement is provided by a medium-strong hydrogen bond between one of the OH groups in one layer and an O atom in an adjacent layer (HO2···O3 = 2.7296(19) Å). Interestingly, the second OH group (O1) has no potential acceptor O atom in a distance < 3.5 Å and apparently does not participate in hydrogen bonding interactions. The two kinds of hydrogen bonding interactions are reflected in the BVS values. Atom O3 shows considerable underbonding (Table 2) that is compensated for by its role as an acceptor atom of a medium-strong hydrogen bond. The BVS values of the other potential acceptor atoms O4 and O5 are close to the expected valence of −2, and thus involvement in a noticeable hydrogen bonding interaction is not observed. The BVS values of the Te and the two Co atoms deviate only slightly from the expected values of +4 and +2, respectively.
The crystal structure of Co2(TeO3)(OH)2 shows a strong topological relationship to the selenium(IV) analog Co2(SeO3)(OH)2 [32]. Both crystal structures comprise the same set-up of chains and dimers condensed into layers, which are interlinked by the chalcogen(IV) atoms and consolidated by a hydrogen bond of the type O−H···O. However, the crystal systems of the two structures are different, viz. triclinic (P 1 ¯ , Z = 2) for the tellurium and monoclinic (P21/n, Z = 4) for the selenium compound.
The crystal structure of Co15(TeO3)14(OH)2 (Figure 4) is isotypic with those of Mn15(TeO3)14(OH)2 [23] and Co15(TeO3)14F2 [33]. Since the latter two crystal structures have been discussed in detail, we describe here only the main features. The asymmetric unit of Co15(TeO3)14(OH)2 comprises three Co, three Te, and eight O atoms (the H atom was not determined). Atoms Te3 and O8 are located at sites with symmetry 3. (Wyckoff position 6 c), Mn2 at a site with symmetry 1 ¯ (9 e), and all other atoms at a general site (18 f) of space group R 3 ¯ . Each of the three Te atoms is coordinated by three oxygen atoms, with distances between 1.86 and 1.89 Å, in the form of a trigonal pyramid. Like in Co2(TeO3)(OH)2, the correspondent [TeO3] units are isolated from each other in the crystal structure of Co15(TeO3)14(OH)2, thus having a connectivity of Q3000 [31].
Co1 and Co3 exhibit a coordination number of 6, with a distorted octahedral arrangement of the oxygen ligands. The average Co–O distances of 2.106 Å and 2.102 Å, respectively, perfectly agree with the overall mean of 2.108(62) Å [30]. Co2 exhibits a [4 + 1] coordination with five O atoms, with one considerably longer Co–O distance (2.332(2) Å) than the other four (1.989(2)–2.058(2) Å). Again, the average Co–O distance of 2.085 Å matches with the reference value of 2.066(177) Å, calculated for 16 [CoO5] polyhedra [30]. In Co15(TeO3)14(OH)2, the shape of the resulting [Co2O5] coordination polyhedron is closer to a (distorted) square pyramid than to a trigonal bipyramid, as expressed by the τ5 descriptor [34] of 0.391 (τ5 = 0 for an ideal square pyramid and τ5 = 1 for an ideal trigonal bipyramid). The [CoO6], [CoO5OH] and [CoO5] polyhedra share corners and edges to assemble into a framework structure, with the TeIV atoms and their associated non-bonding electron lone pairs occupying some of the remaining space (Figure 4).
The hydroxide group present in Co15(TeO3)14(OH)2 is associated with the O8 atom (BVS 1.17 v.u.) and shows the shortest Co–O distance of the [Co3O5OH] octahedron. All in all, O8 binds to three symmetry-related Co3 atoms. This situation is comparable to the O1 and O2 atoms in Co2(TeO3)(OH)2, but the mean (H)O–Co distances are considerably shorter in Co15(TeO3)14(OH)2 (2.042 Å) than in Co2(TeO3)(OH)2 (2.105 and 2.078 Å).
The BVS values of the Co, Te, and the other O atoms are inconspicuous, with individual values slightly deviating from the expected valence of 2, 4 and −2, respectively (Table 2).
The quantitative structural comparison between Co15(TeO3)14(OH)2, as the reference structure, with isotypic Mn15(TeO3)14(OH)2 and Co15(TeO3)14F2, is provided in Table 3. Atomic displacements for atom pairs in the structures, numerical values for the degree of lattice distortion (S), the arithmetic mean (dav) of all distances and the measure of similarity (Δ) are compiled. On the whole, the absolute displacements for atom pairs are greater with respect to Mn15(TeO3)14(OH)2 than to Co15(TeO3)14F2. Except for the OH group, which is substituted with an F atom, all atoms in Co15(TeO3)14F2 remain the same, whereas all transition metal atoms are replaced with respect to Mn15(TeO3)14(OH)2. In the latter case, the larger ionic radii of MnII (0.75 Å for a coordination number of 5, 0.83 Å for a coordination number of 6 compared to CoII with 0.67 and 0.745 Å, respectively [35]) are responsible for the higher atomic displacements, and consequently cause higher numbers for S and dav, and therefore a structure with a lower similarity. However, in both cases, the highest displacement is observed for atom O8, which is associated with the OH group. The correspondent functionality as a donor for hydrogen bonding interactions, despite being weak in the present case, strongly influences its displacement in the isotypic structures. On the one hand, the replacement of OH through F leads to the disappearance of hydrogen bonding interactions. On the other hand, the larger MnII ions evoke an expansion of the entire structure, which also has an impact on the hydrogen bonding scheme with modified donor···acceptor distances.

3.3. Magnetic Properties

Figure 5a and Figure 6a show the temperature dependence of the magnetization M collected under zero-field-cooled (ZFC) and field-cooled (FC) conditions in a small dc magnetic field (H = 50 Oe) for the two samples. The ZFC and FC curves of Co2(TeO3)(OH)2 reveal a sharp peak around 70 K (Figure 5a), suggesting an antiferromagnetic (AFM) ordering from a paramagnetic phase. In the case of Co15(TeO3)14(OH)2, an antiferromagnetic-like peak is observed at T ≈ 18 K, with an apparent excess moment [36] observed in low fields (see also the inset of Figure 6a). A weak irreversibility can be detected in the magnetization curves recorded in low fields below ≈ 70 K.
The magnetic field dependence of the magnetization was recorded for both samples at T = 2 K, as shown in Figure 5b and Figure 6b. Both M(H) curves show the typical linear AFM behavior, with nearly zero coercive field. In the case of Co15(TeO3)14(OH)2, a slight upturn of the magnetization seems to be observed above 40 kOe, possibly related to the reorientation discussed in the isotypic Co15(TeO3)14F2 [33].
The inverse of the magnetic susceptibility χ = M/H of the samples, recorded for H = 5000 Oe, was plotted vs. the temperature in Figure 5c and Figure 6c in order to check the Curie–Weiss behavior of the susceptibility; χ = C/(T − θCW), where C and θCW represent the Curie constant and Curie−Weiss temperature, respectively. Good fits could be obtained, yielding the θCW and effective moment μeff (C = NA μeff2/3kB, where NA and kB are the Avogadro number and Boltzmann constant, respectively); values are listed in Table 4. For both samples, the obtained μeff value is higher than the spin-only value (μspin = 3.87 μB) of CoII (3d7, S = 3/2), which implies a significant orbital moment contribution. In general, μeff values exceeding the spin-only value are commonly observed for CoII in an oxidic environment, including tellurium-containing corundum-related Co3TeO6 [7], A2CoTeO6 perovskites [37] or Co15(TeO3)14F2 [33]. The negative sign of θCW and its magnitude confirm the significant AFM interaction between the nearest CoII spins present in both samples. While the Néel temperature TN is found to appear near −θCW in the case of Co2(TeO3)(OH)2, TN is significantly lower than −θCW for Co15(TeO3)14(OH)2, which suggests some magnetic frustration [38] in the latter case. Co2(TeO3)(OH)2 undergoes an AFM transition below 70 K, akin to Co2(SeO3)(OH)2 [32], which shows a similar structural set-up, with chains and dimers condensed into a layered arrangement.
Expectedly, the overall magnetic behavior of Co15(TeO3)14(OH)2 is qualitatively similar to that of isotypic Co15(TeO3)14F2 [33], where only the F and OH group is interchanged. In the case of Co15(TeO3)14(OH)2, the frustration parameter |θCW/TN| is slightly lower than that for the oxidofluoride (≈ 4.3 vs. 6.6). The observed magnetic frustration in Co15(TeO3)14(OH)2, the excess moment detected in low magnetic fields, and the high-field non-linearity in M(H) curves, suggest a complex spin structure associated with the specific coordination of the magnetic CoII cations.

3.4. IR Spectroscopy

The IR spectra of the title compounds (Figure 7 and Figure 8) can be divided into two vibrational parts, viz. the one characteristic for OH vibrations and the one from the [TeO3] groups and lattice vibrations of the different kinds of polyhedra around CoII.
Co2(TeO3)(OH)2 shows two OH stretching vibration bands at 3570 cm−1 and at 3202 cm−1, respectively. The first band is associated with the OH group (O1) without an acceptor group for hydrogen bonding, which explains the rather sharp band profile and the high wavenumber. In comparison, the broad band and the red-shift of about 370 wave numbers for the second OH vibration (O2) indicates a clear participation in a hydrogen bonding interaction of a medium–strong nature. The application of Libowitzky’s empirical correlation between OH stretching and O−H···O hydrogen bond lengths [39] results in an expected O···O distance of 2.705 Å, deviating only slightly from the experimental value of 2.730 Å as determined from the X-ray diffraction study. The bending modes for the two OH vibrations are observed at 1016 and 917 cm−1, in agreement with other solids containing OH groups, e.g., for various zinc hydroxy compounds, where these bands were observed between 1015 and 755 cm−1 [40], or for the phyllomanganate birnessite that contains MnII−OH and MnIII−OH groups, the bending vibrations of which were assigned in the range 1170–900 cm−1 [41].
In agreement with the crystal structure of Co15(TeO3)14(OH)2, which comprises only one OH function (O8), the IR spectrum shows one OH stretching vibration band at 3425 cm−1, albeit with a very weak intensity. The latter might be correlated with the low amount of OH in the compound (two OH groups related to an overall of 75 atoms in the formula). The position of this band suggests a significantly weaker hydrogen bonding interaction than that for the second OH group in Co2(TeO3)(OH)2. The correlation function reveals an expected value of 2.817 Å. In fact, a possible O acceptor atom (O3) is located at 2.877 Å from O2. However, the corresponding (H)O8···O3 donor···acceptor group is not associated with an interpolyhedral distance (as usual) but is part of the [Co3O5(OH)] polyhedron, which makes a direct participation of O3 in hydrogen bonding unlikely. Without a clear localization of the corresponding H atom, the true nature of the hydrogen bonding situation thus remains unclear. The bending mode of the OH vibration at 871 cm−1 is in the same range as the ones given above.
The lower wavenumber part of the spectra is dominated by the Te−O vibrations, with a typical range between 800 and 600 cm−1 for the Te−O stretching vibrations [42], with the most prominent bands positioned at 768, 740, 654 and 613 cm−1 for Co2(TeO3)(OH)2 and 741, 681 and 635 cm−1 for Co15(TeO3)14(OH)2. Bands with lower wavenumbers between 600 and 500 cm−1 are assigned to Te−O bending vibrations [43,44], or may already occur from Co−O lattice vibrations.

3.5. Thermal Behavior

Under the conditions chosen for the TG/DSC study, Co2(TeO3)(OH)2 is stable up to ≈450 °C, as indicated by the very similar onsets of the endothermic DSC signal and of the mass loss in the TG curve (Figure 9). The DSC signal is split (maxima at 494 and 506 °C), indicating two separate incidents that, however, are not resolved in the TG curve. The continuous mass loss of 5.6% lasts to 525 °C and corresponds to the loss of one water molecule per formula unit (theory 5.5%). The products after heat treatment, as revealed by PXRD, are Co6Te5O16 and CoO in an approximate ratio of 3:1.
Co15(TeO3)14(OH)2 shows remarkable thermal stability. TG and DSC measurements are featureless, indicating neither a structural change nor a decomposition in the chosen temperature range (30–580 °C). In fact, the sample that had been subjected to the DSC measurement exhibited the same IR spectrum after heat treatment (Figure 8). Moreover, a single crystal from the DSC sample after heat treatment, selected for the X-ray diffraction study, showed an unchanged crystal structure. The same applies for polycrystalline material (see supplementary Figure S2).

4. Conclusions

The cobalt(II) oxidotellurates(IV) Co2(TeO3)(OH)2 and Co15(TeO3)14(OH)2 were simultaneously obtained with other reaction products under hydrothermal formation conditions. Through the variation of the OH− concentration to higher pH values and subsequent processing of the solid reaction product, single-phase material of Co2(TeO3)(OH)2 could eventually be obtained, whereas Co15(TeO3)14(OH)2 was always present as part of product mixtures. The unique crystal structure of Co2(TeO3)(OH)2 is built up from 2∝[Co2(OH)6/3O3/3O2/2O1/1]4− layers held together by stereoactive TeIV atoms. Two hydrogen bonds of the type O−H···O are observed. One is an interaction of medium strength and consolidates the layered set-up of the crystal structure, whereas the other is of a very weak nature, as revealed by possible O···O donor···acceptor interactions and OH vibration bands. The crystal structure of Co2(TeO3)(OH)2 resembles that of the selenium(IV) analog Co2(SeO3)(OH)2, but exhibits a different crystal system (triclinic versus monoclinic). Magnetic measurements of Co2(TeO3)(OH)2 revealed an antiferromagnetic ordering temperature below 70 K. Above 450 °C, this phase decomposes into Co6Te5O16 and CoO, in contrast to Co15(TeO3)14(OH)2, which shows a remarkable thermal stability up to 580 °C without a structural change. The crystal structure of Co15(TeO3)14(OH)2 is isotypic with previously reported Co15(TeO3)14F2. Since only the F and OH groups are interchanged, the magnetic properties of the two compounds are similar, with antiferromagnetic ordering temperatures below 10 K for Co15(TeO3)14F2 and below 18 K for Co15(TeO3)14(OH)2, respectively.

Supplementary Materials

The following supporting information can be downloaded at: https://0-www-mdpi-com.brum.beds.ac.uk/article/10.3390/cryst13020176/s1, Figure S1: Co2(TeO3)(OH)2—measured PXRD data; Figure S2: Co15(TeO3)14(OH)2—measured PXRD data.

Author Contributions

F.E.: Conceptualization; Investigation; Visualization; Data curation; Formal analysis; Writing—review and editing. M.W.: Conceptualization; Investigation; Visualization; Data curation; Resources; Supervision; Writing—original draft; Writing—review and editing. P.P.: Investigation; Data curation; Writing— original draft. R.M.: Investigation; Data curation; Writing—original draft. All authors have read and agreed to the published version of the manuscript.

Funding

We thank Stiftelsen Olle Engkvist Byggmästare (Grant No. 207-0427) for financial support.

Data Availability Statement

Part of the data presented in this study are available in The Cambridge Crystallographic Data Centre (CCDC) and can be obtained free of charge via www.ccdc.cam.ac.uk/structures.

Acknowledgments

The X-ray centre of the TU Wien is acknowledged for providing free access to single crystal and powder X-ray diffraction instruments.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Crystals 13 00176 g001 550
Figure 1. Typical crystal forms of hydrothermally grown crystals of Co2(TeO3)(OH)2 (designated as Co2 in the inset) and Co15(TeO3)14(OH)2 (Co15).
Figure 1. Typical crystal forms of hydrothermally grown crystals of Co2(TeO3)(OH)2 (designated as Co2 in the inset) and Co15(TeO3)14(OH)2 (Co15).
Crystals 13 00176 g001
Crystals 13 00176 g002 550
Figure 2. A perpendicular view of the 2∝[Co2(OH)6/3O3/3O2/2O1/1]4− layer in the crystal structure of Co2(TeO3)(OH)2. The [Co1O6] octahedron is given in turquoise, the [Co2O6] octahedron in dark blue. Atoms are displayed with anisotropic displacement ellipsoids at a 90% probability level.
Figure 2. A perpendicular view of the 2∝[Co2(OH)6/3O3/3O2/2O1/1]4− layer in the crystal structure of Co2(TeO3)(OH)2. The [Co1O6] octahedron is given in turquoise, the [Co2O6] octahedron in dark blue. Atoms are displayed with anisotropic displacement ellipsoids at a 90% probability level.
Crystals 13 00176 g002
Crystals 13 00176 g003 550
Figure 3. The crystal structure of Co2(TeO3)(OH)2 in a projection along [010]. [TeO3] polyhedra are red; O−H···O hydrogen bonding interactions are indicated by green lines. Other color codes and displacement ellipsoids are as in Figure 2.
Figure 3. The crystal structure of Co2(TeO3)(OH)2 in a projection along [010]. [TeO3] polyhedra are red; O−H···O hydrogen bonding interactions are indicated by green lines. Other color codes and displacement ellipsoids are as in Figure 2.
Crystals 13 00176 g003
Crystals 13 00176 g004 550
Figure 4. The crystal structures of Co15(TeO3)14(OH)2 in a projection along [100]. [CoO6] and [CoO5OH] octahedra (Co1, Co3) are dark blue, [CoO5] polyhedra (Co2) are turquoise and [TeO3] polyhedra are red; the O8 atoms associated with the OH function are emphasized in green. Displacement ellipsoids are as in Figure 2.
Figure 4. The crystal structures of Co15(TeO3)14(OH)2 in a projection along [100]. [CoO6] and [CoO5OH] octahedra (Co1, Co3) are dark blue, [CoO5] polyhedra (Co2) are turquoise and [TeO3] polyhedra are red; the O8 atoms associated with the OH function are emphasized in green. Displacement ellipsoids are as in Figure 2.
Crystals 13 00176 g004
Crystals 13 00176 g005 550
Figure 5. (a) Temperature dependence of the magnetization (M) in the field of 50 Oe of Co2(TeO3)2(OH)2. (b) M versus H at 2 K. (c) Inverse susceptibility (χ−1) versus T.
Figure 5. (a) Temperature dependence of the magnetization (M) in the field of 50 Oe of Co2(TeO3)2(OH)2. (b) M versus H at 2 K. (c) Inverse susceptibility (χ−1) versus T.
Crystals 13 00176 g005
Crystals 13 00176 g006 550
Figure 6. (a) M versus T of Co15(TeO3)14(OH)2 at 50 Oe. Inset shows magnetic susceptibility χ = M/H vs. T recorded for different magnetic fields. (b) M versus H at 2 K. (c) (χ − χ0) −1 vs. T after correcting for a background contribution χ0 (−0.0168 emu/mol‧Oe).
Figure 6. (a) M versus T of Co15(TeO3)14(OH)2 at 50 Oe. Inset shows magnetic susceptibility χ = M/H vs. T recorded for different magnetic fields. (b) M versus H at 2 K. (c) (χ − χ0) −1 vs. T after correcting for a background contribution χ0 (−0.0168 emu/mol‧Oe).
Crystals 13 00176 g006
Crystals 13 00176 g007 550
Figure 7. IR spectrum of Co2(TeO3)(OH)2.
Figure 7. IR spectrum of Co2(TeO3)(OH)2.
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Crystals 13 00176 g008 550
Figure 8. IR spectrum of Co15(TeO3)14(OH)2 prior and after heat treatment at 580 °C.
Figure 8. IR spectrum of Co15(TeO3)14(OH)2 prior and after heat treatment at 580 °C.
Crystals 13 00176 g008
Crystals 13 00176 g009 550
Figure 9. Co2(TeO3)(OH)2: TG curve (black) and DSC curves on heating (red) and cooling (blue).
Figure 9. Co2(TeO3)(OH)2: TG curve (black) and DSC curves on heating (red) and cooling (blue).
Crystals 13 00176 g009
Table
Table 1. Data collection and refinement details.
Table 1. Data collection and refinement details.
Co2(TeO3)(OH)2Co15(TeO3)14(OH)2
Mr654.953376.37
Space group, NoP 1 ¯ , 2R 3 ¯ , 148
Z13
Temperature/°C−17323
Crystal form, colorlath, pinkblock, dark blue
Crystal size/mm30.14 × 0.04 × 0.030.11 × 0.09 × 0.06
a/Å5.8898(5)11.6453(2)
b/Å5.9508(5)11.6453(2)
c/Å6.8168(5)27.3540(5)
α/°101.539(2)90
β/°100.036(2)90
γ/°104.347(2) 120
V/Å3220.40(3)3212.57(12)
X-ray density/g‧cm−34.9355.236
Radiation typeMo KαMo Kα
µ/mm−113.9215.11
Tmin, Tmax0.350, 0.5680.294, 0.438
DiffractometerBruker AXS APEX-II Bruker AXS APEX-II
Absorption correctionSADABS [20]SADABS [20]
No. of measured, independent and observed [I > 2σ(I)] reflections6273 2120 2068 17537 3476 2875
Rint0.0200.051
(sin θ/λ)max/Å−10.8330.834
No. of reflections21203476
No. of parameters79112
H-atom treatmentH-atom coordinates refinedH-atom parameters not defined
R[F2 > 2σ(F2)], wR(F2), S0.017, 0.044, 1.190.026, 0.051, 1.02
Δρmax, Δρmin (e Å−3)1.13, −1.701.40, −1.38
CSD-code22265602226561
Table
Table 2. Selected bond lengths/Å, angles/°, details of hydrogen bonding interactions as well as bond valence sums (BVS)/v.u. (values using the revised parameters for TeIV–O bonds [27] in parentheses).
Table 2. Selected bond lengths/Å, angles/°, details of hydrogen bonding interactions as well as bond valence sums (BVS)/v.u. (values using the revised parameters for TeIV–O bonds [27] in parentheses).
Co2(TeO3)(OH)2
Co1—O22.0320(13) Co2—O52.0690(13)
Co1—O12.0569(13) Co2—O3v2.0737(13)
Co1—O1i2.0912(13) Co2—O22.0759(13)
Co1—O5ii2.1048(13) Co2—O4vi2.1226(13)
Co1—O2iii2.1239(13) Co2—O4ii2.1449(13)
Co1—O4iv2.3433(13) Co2—O1vi2.1722(13)
Te1—O31.8715(13) O3—Te1—O598.32(6)
Te1—O51.8852(12) O3—Te1—O494.73(6)
Te1—O41.8973(12) O5—Te1—O498.82(5)
D—H···AD—HH···AD···AD—H···A
O2—H2···O3iv0.90(3)1.87(3)2.7296(19)160(3)
BVS: Co1 1.93, Co2 1.95, Te1 3.86 (3.93), O1 1.00 (without H), O2 1.06 (without H), O3 1.61, O4 1.95, O5 1.90
Symmetry codes: (i) −x + 2, −y + 2, −z + 1; (ii) −x + 1, −y + 1, −z + 1; (iii) −x + 2, −y + 1, −z + 1; (iv) x + 1, y, z; (v) −x + 1, −y + 1, −z + 2; (vi) x, y − 1, z.
Co15(TeO3)14(OH)2
Co1—O5i2.018(2) Te1—O1iv1.886(2)
Co1—O5ii2.018(2) Te1—O1v1.886(2)
Co1—O4iii2.141(2) Te1—O11.886(2)
Co1—O4iv2.141(2) Te2—O31.868(2)
Co1—O1ii2.159(2) Te2—O2v1.877(2)
Co1—O1i2.159(2) Te2—O51.896(2)
Co2—O31.989(2) Te3—O61.855(2)
Co2—O7i2.019(2) Te3—O41.866(2)
Co2—O12.024(2) Te3—O7v1.889(2)
Co2—O5iv2.058(2) O1iv—Te1—O1v98.58(9)
Co2—O22.331(2) O1—Te1—O1iv98.58(9)
Co3—O82.0419(6) O1—Te1—O1v98.58(9)
Co3—O2i2.046(2) O3—Te2—O2v91.85(10)
Co3—O22.083(2) O3—Te2—O594.86(10)
Co3—O32.102(2) O2v—Te2—O587.18(10)
Co3—O62.141(2) O6—Te3—O499.11(11)
Co3—O72.202(2) O6—Te3—O7v98.75(10)
O4—Te3—O7v98.95(10)
BVS: Co1 1.99, Co2 1.82, Co3 2.00, Te1 3.85 (3.89), Te2 3.89 (4.01), Te3 4.01 (3.81), O1 1.89, O2 2.13, O3 2.03, O4 1.85, O5 2.05, O6 1.85, O7 1.86, O8 1.17.
Symmetry codes: (i) −x + 2/3, −y + 1/3, −z + 1/3; (ii) x + 1/3, y−1/3, z−1/3; (iii) x − y + 1, x, −z; (iv) −x + y, −x, z; (v) −y, x − y, z.
Table
Table 3. Comparison of Co15(TeO3)14(OH)2 as the reference structure with isotypic Mn15(TeO3)14(OH)2 and Co15(TeO3)14F2. Atom pairs are given with their absolute distances |u|/Å, as well as the degree of lattice distortion (S), the arithmetic mean of the distances (dav/Å) and the measure of similarity (Δ).
Table 3. Comparison of Co15(TeO3)14(OH)2 as the reference structure with isotypic Mn15(TeO3)14(OH)2 and Co15(TeO3)14F2. Atom pairs are given with their absolute distances |u|/Å, as well as the degree of lattice distortion (S), the arithmetic mean of the distances (dav/Å) and the measure of similarity (Δ).
Atom/Wyckoff PositionMn15(TeO3)14(OH)2Co15(TeO3)14F2
M19 e00
M218 f0.04210.0188
M318 f0.03960.0054
Te16 c0.00930.0047
Te218 f0.05560.0158
Te318 f0.07060.0225
O118 f0.06360.0073
O218 f0.02630.0166
O318 f0.06600.0229
O418 f0.03070.0213
O518 f0.04570.0146
O618 f0.02820.0208
O718 f0.09920.0320
O8/F16 c0.37970.1441
S (a) 0.01650.0009
dav./Å (b) 0.05730.0204
Δ (c) 0.0160.005
(a) The degree of lattice distortion (S) is the spontaneous strain (sum of the squared eigenvalues of the strain tensor divided by 3). (b) The arithmetic mean (dav) of all distances between atom pairs. (c) The measure of similarity (Δ) is a function of the differences in atomic positions (weighted by the multiplicities of the sites) and the ratios of the corresponding unit cell parameters of the structures.
Table
Table 4. Parameters obtained from the M(T) curves and Curie–Weiss fitting of χ−1 (T). The ZFC/FC peak temperature is given as an estimation of TN.
Table 4. Parameters obtained from the M(T) curves and Curie–Weiss fitting of χ−1 (T). The ZFC/FC peak temperature is given as an estimation of TN.
CompoundTN (K)θCW (K)C (emu⋅K/mol⋅Oe)μeff/Co2+ (μB)
Co2(TeO3)(OH)270−71.510.126.36
Co15(TeO3)14(OH)218−76.956.145.47
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Eder, F.; Weil, M.; Pramanik, P.; Mathieu, R. The Cobalt(II) Oxidotellurate(IV) Hydroxides Co2(TeO3)(OH)2 and Co15(TeO3)14(OH)2. Crystals 2023, 13, 176. https://0-doi-org.brum.beds.ac.uk/10.3390/cryst13020176

AMA Style

Eder F, Weil M, Pramanik P, Mathieu R. The Cobalt(II) Oxidotellurate(IV) Hydroxides Co2(TeO3)(OH)2 and Co15(TeO3)14(OH)2. Crystals. 2023; 13(2):176. https://0-doi-org.brum.beds.ac.uk/10.3390/cryst13020176

Chicago/Turabian Style

Eder, Felix, Matthias Weil, Prativa Pramanik, and Roland Mathieu. 2023. "The Cobalt(II) Oxidotellurate(IV) Hydroxides Co2(TeO3)(OH)2 and Co15(TeO3)14(OH)2" Crystals 13, no. 2: 176. https://0-doi-org.brum.beds.ac.uk/10.3390/cryst13020176

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