The Cobalt(II) Oxidotellurate(IV) Hydroxides Co 2 (TeO 3 )(OH) 2 and Co 15 (TeO 3 ) 14 (OH) 2

: Previously unknown Co 2 (TeO 3 )(OH) 2 and Co 15 (TeO 3 ) 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. Co 2 (TeO 3 )(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 [ F 2 > 2 σ ( F 2 )] = 0.017) crystallizes in a unique structure comprised of undulating 2 ∝ [Co 2 (OH) 6/3 O 3/3 O 2/2 O 1/1 ] 4 − layers. Adjacent layers are linked by Te IV atoms along the [001] stacking direction. Co 2 (TeO 3 )(OH) 2 is stable up to 450 ◦ C and decomposes under the release of water into Co 6 Te 5 O 16 and CoO. Magnetic measurements of Co 2 (TeO 3 )(OH) 2 showed antiferromagnetic ordering at ≈ 70 K. The crystal structure of Co 15 (TeO 3 ) 14 (OH) 2 ( Z = 3, R 3, a = 11.6453(2), c = 27.3540(5) Å, 3476 independent reﬂections, 112 parameters, R [ F 2 > 2 σ ( F 2 )] = 0.026) is isotypic with Co 15 (TeO 3 ) 14 F 2 . 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 [CoO 6 ], [CoO 5 (OH)], [CoO 5 ] and [TeO 3 ] polyhedra is nearly unaffected. Consequently, the magnetic properties of the two phases are similar, namely being antiferromagnetic at low temperatures.


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 CoTe IV O 3 [1,2], CoTe VI O 4 [3], Co 3 Te VI O 6 [4][5][6][7][8] and Co 5 Te VI O 8 [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 Co 6 Te IV  5 O 16 [11], CoTe IV  6 O 13 [12] and Co 2 Te IV 3 O 8 [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 Co 3 (Te IV O 3 ) 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.Co 2 (TeO 3 )(OH) 2 and Co 15 (TeO 3 ) 14 (OH) 2 .
We report here on our efforts to increase the yield of Co 2 (TeO 3 )(OH) 2 and Co 15 (TeO 3 ) 14 (OH) 2 , together with the results of their crystal structure analyses and physical property measurements.

Synthesis
All employed chemicals were of pro analysi quality and were purchased from Merck (Darmstadt, Germany).Co 2 (TeO 3 )(OH) 2 and Co 15 (TeO 3 ) 14 (OH) 2 were originally obtained as minor by-products during hydrothermal phase formation studies for the intended synthesis of Co 3 (TeO 3 ) 2 (OH) 2 [16] or Na 2 [Co 2 (TeO 3 ) 3 ]•3H 2 O [17].In representative experiments targeted for 0.5 g of the intended phase, CoCO 3 , TeO 2 and KOH (molar ratio 3:2:4) and CoO (prepared by thermal decomposition of CoCO 3 ), TeO 2 and Na 2 CO 3 (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.Co 2 (TeO 3 )(OH) 2 crystallizes in the form of pink needles up to 0.5 mm in lengths, Co 15 (TeO 3 ) 14 (OH) 2 in form of dark blue-to-violet isometric crystals up to 0.2 mm in length (Figure 1), and both Na 2 [Co 2 (TeO 3 ) 3 ]•3H 2 O and Co 3 (TeO 3 ) 2 (OH) 2 in form of violet thin hexagonal prisms.
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.

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.Increasing the amount of KOH to a CoCO 3 :TeO 2 :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 Co 2 (TeO 3 )(OH) 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-K α1,2 -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-K α 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 θ max ) and for data reduction; correction for absorption effects was performed with SAD-ABS [20].The crystal structures were solved with SHELXT [21], refined with SHELXL [22] and graphically represented with ATOMS [23].In the case of Co 2 (TeO 3 )(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 U iso (H) = 1.5U eq of the parent O atom.In the case of Co 15 (TeO 3 ) 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 Mn 15 (TeO 3 ) 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 Te IV -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].

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).

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.

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.

Synthesis
PXRD of the product, formed from the more highly concentrated KOH solution, revealed single-phase Co 2 (TeO 3 )(OH) 2 (see Supplementary Figure S1).In comparison with the other formed phases containing an additional OH group, i.e., Co 15 (TeO 3 ) 14 (OH) 2 and Co 3 (TeO 3 ) 2 (OH) 2 , the amount of OH in Co 2 (TeO 3 )(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, Na 2 [Co 2 (TeO 3 ) 3 ]•3H 2 O without OH groups in the structure solely forms in Na 2 CO 3 -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 Co 2 (TeO 3 )(OH) 2 is not formed from Na 2 CO 3 solutions.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.[31].In the crystal structure (Figure 3), the [TeO 3 ] units are isolated from each other, having a connectivity of Q 3000 in the notations of Christy et al. [31].

Crystal Structures
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 The 2 ∝[Co2(OH)6/3O3/3O2/2O1/1] 4− layers stack along [001] and are linked through the Te1 IV 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 Q 3000 in the notations of Christy et al. [31].
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).
Crystals 2023, 12, x FOR PEER REVIEW Co2(TeO3)(OH)2, the correspondent [TeO3] units are isolated from each other in the structure of Co15(TeO3)14(OH)2, thus having a connectivity of Q 3000 [31].The quantitative structural comparison between Co 15 (TeO 3 ) 14 (OH) 2 , as the reference structure, with isotypic Mn 15 (TeO 3 ) 14 (OH) 2 and Co 15 (TeO 3 ) 14 F 2 , 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 (d av ) of all distances and the measure of similarity (∆) are compiled.On the whole, the absolute displacements for atom pairs are greater with respect to Mn 15 (TeO 3 ) 14 (OH) 2 than to Co 15 (TeO 3 ) 14 F 2 .Except for the OH group, which is substituted with an F atom, all atoms in Co 15 (TeO 3 ) 14 F 2 remain the same, whereas all transition metal atoms are replaced with respect to Mn 15 (TeO 3 ) 14 (OH) 2 .In the latter case, the larger ionic radii of Mn II (0.75 Å for a coordination number of 5, 0.83 Å for a coordination number of 6 compared to Co II with 0.67 and 0.745 Å, respectively [35]) are responsible for the higher atomic displacements, and consequently cause higher numbers for S and d av , 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 Mn II ions evoke an expansion of the entire structure, which also has an impact on the hydrogen bonding scheme with modified donor•••acceptor distances.(TeO 3 ) 14 (OH) 2 as the reference structure with isotypic Mn 15 (TeO 3 ) 14 (OH) 2 and Co 15 (TeO 3 ) 14 F 2 .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 (d av /Å) and the measure of similarity (∆).

Magnetic Properties
Figures 5a and 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 Co 2 (TeO 3 )(OH) 2 reveal a sharp peak around 70 K (Figure 5a), suggesting an antiferromagnetic (AFM) ordering from a paramagnetic phase.In the case of Co 15 (TeO 3 ) 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.

Figures 5a and 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 antiferromagneticlike 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 Figures 5b and 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 Co 15 (TeO 3 ) 14 F 2 [33].
The inverse of the magnetic susceptibility χ = M/H of the samples, recorded for H = 5000 Oe, was plotted vs. the temperature in Figures 5c and 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 µeff 2 /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 Co II (3d 7 , S = 3/2), which implies a significant orbital moment contribution.In general, µeff values exceeding the spin-only value are commonly observed for Co II in an oxidic environment, including tellurium-containing corundum-related Co3TeO6 [7], A2CoTeO6 perovskites [37] or Co 15 (TeO 3 ) 14 F 2 [33].The negative sign of θCW and its magnitude confirm the significant AFM interaction between the nearest Co II 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.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.The magnetic field dependence of the magnetization was recorded for both samples at T = 2 K, as shown in Figures 5b and 6b.Both M(H) curves show the typical linear AFM behavior, with nearly zero coercive field.In the case of Co 15 (TeO 3 ) 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 Co 15 (TeO 3 ) 14 F 2 [33].
The inverse of the magnetic susceptibility χ = M/H of the samples, recorded for H = 5000 Oe, was plotted vs. the temperature in Figures 5c and 6c 4. For both samples, the obtained µ eff value is higher than the spin-only value (µ spin = 3.87 µ B ) of Co II (3d 7 , S = 3/2), which implies a significant orbital moment contribution.In general, µ eff values exceeding the spin-only value are commonly observed for Co II in an oxidic environment, including tellurium-containing corundum-related Co 3 TeO 6 [7], A 2 CoTeO 6 perovskites [37] or Co 15 (TeO 3 ) 14 F 2 [33].The negative sign of θ CW and its magnitude confirm the significant AFM interaction between the nearest Co II spins present in both samples.While the Néel temperature T N is found to appear near −θ CW in the case of Co 2 (TeO 3 )(OH) 2 , T N is significantly lower than −θ CW for Co 15 (TeO 3 ) 14 (OH) 2 , which suggests some magnetic frustration [38] in the latter case.Co 2 (TeO 3 )(OH) 2 undergoes an AFM transition below 70 K, akin to Co 2 (SeO 3 )(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 Co 15 (TeO 3 ) 14 (OH) 2 is qualitatively similar to that of isotypic Co 15 (TeO 3 ) 14 F 2 [33], where only the F and OH group is interchanged.In the case of Co 15 (TeO 3 ) 14 (OH) 2 , the frustration parameter |θ CW /T N | is slightly lower than that for the oxidofluoride (≈ 4.3 vs. 6.6).The observed magnetic frustration in Co 15 (TeO 3 ) 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 Co II cations.

IR Spectroscopy
The IR spectra of the title compounds (Figures 7 and 8) can be divided into two vibrational parts, viz. the one characteristic for OH vibrations and the one from the [TeO 3 ] groups and lattice vibrations of the different kinds of polyhedra around Co II .
Co15(TeO3)14(OH)2, the excess moment detected in low magnetic fields, and the hig non-linearity in M(H) curves, suggest a complex spin structure associated with the s coordination of the magnetic Co II cations.

IR Spectroscopy
The IR spectra of the title compounds (Figures 7 and 8) can be divided into brational parts, viz. the one characteristic for OH vibrations and the one from the groups and lattice vibrations of the different kinds of polyhedra around Co II .Co 2 (TeO 3 )(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 Mn II −OH and Mn III −OH groups, the bending vibrations of which were assigned in the range 1170-900 cm −1 [41].
In agreement with the crystal structure of Co 15 (TeO 3 ) 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 Co 2 (TeO 3 )(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 [Co3O 5 (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 Co 2 (TeO 3 )(OH) 2 and 741, 681 and 635 cm −1 for Co 15 (TeO 3 ) 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.

Thermal Behavior
Under the conditions chosen for the TG/DSC study, Co 2 (TeO 3 )(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 Co 6 Te 5 O 16 and CoO in an approximate ratio of 3:1.
Co 15 (TeO 3 ) 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).

Figure 2 .
Figure 2. A perpendicular view of the 2 ∝ [Co 2 (OH) 6/3 O 3/3 O 2/2 O 1/1 ] 4− layer in the crystal structure of Co 2 (TeO 3 )(OH) 2 .The [Co1O 6 ] octahedron is given in turquoise, the [Co2O 6 ] octahedron in dark blue.Atoms are displayed with anisotropic displacement ellipsoids at a 90% probability level.The 2 ∝ [Co 2 (OH) 6/3 O 3/3 O 2/2 O 1/1 ] 4− layers stack along [001] and are linked through the Te1 IV 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 [TeO 3 ] unit[31].In the crystal structure (Figure3), the [TeO 3 ] units are isolated from each other, having a connectivity of Q 3000 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 (Table2) 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

Figure 3 .
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. Additional stabilization of the structural arrangement is provided by a mediumstrong 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 (Table2) 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 (P1 , Z = 2) for the tellurium and monoclinic (P21/n, Z = 4) for the selenium compound.

Figure 3 .
Figure 3.The crystal structure of Co 2 (TeO 3 )(OH) 2 in a projection along [010].[TeO 3 ] 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.The crystal structure of Co 2 (TeO 3 )(OH) 2 shows a strong topological relationship to the selenium(IV) analog Co 2 (SeO 3 )(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 (P1, Z = 2) for the tellurium and monoclinic (P2 1 /n, Z = 4) for the selenium compound.The crystal structure of Co 15 (TeO 3 ) 14 (OH) 2 (Figure4) is isotypic with those of Mn 15 (TeO 3 ) 14 (OH) 2[23] and Co 15 (TeO 3 ) 14 F 2[33].Since the latter two crystal structures have been discussed in detail, we describe here only the main features.The asymmetric unit of Co 15 (TeO 3 ) 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 R3.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 Co 2 (TeO 3 )(OH) 2 , the correspondent [TeO 3 ] units are isolated from each other in the crystal structure of Co 15 (TeO 3 ) 14 (OH) 2 , thus having a connectivity of Q 3000[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 [CoO 5 ] polyhedra[30].In Co 15 (TeO 3 ) 14 (OH) 2 , the shape of the resulting [Co2O 5 ] coordination polyhedron is closer to a (distorted) square pyramid than to a trigonal bipyramid, as expressed by the τ 5 descriptor[34] of 0.391 (τ 5 = 0 for an ideal square pyramid and τ 5 = 1 for an ideal trigonal bipyramid).The [CoO 6 ], [CoO 5 OH] and [CoO 5 ] polyhedra share corners and edges to assemble into a framework structure, with the Te IV atoms and their associated non-bonding electron lone pairs occupying some of the remaining space (Figure4).
(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 (d av ) of all distances between atom pairs.(c) The measure of similarity (∆) 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.
in order to check the Curie-Weiss behavior of the susceptibility; χ = C/(T − θ CW ), where C and θ CW represent the Curie constant and Curie−Weiss temperature, respectively.Good fits could be obtained, yielding the θ CW and effective moment µ eff (C = N A µ eff 2 /3k B , where N A and k B are the Avogadro number and Boltzmann constant, respectively); values are listed in Table

Table 1 .
Data collection and refinement details.

Table 3 .
Comparison of Co 15

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 T N .