2 Linear

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Short-Range Antiferromagnetic Ordering of Netlike S = 1/2 Linear Trimeric Units in the Copper Germanate K2Cu3Ge4O12 Christiane Stoll,† Oliver Janka,‡ Rainer Pöttgen,‡ Markus Seibald,§ Dominik Baumann,§ Klaus Wurst,† and Hubert Huppertz*,† †

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Institut für Allgemeine, Anorganische und Theoretische Chemie, Westfälische Wilhelms-Universität Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria ‡ Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstraße 30, 48149 Münster, Germany § OSRAM Opto Semiconductors GmbH, Mittelstetter Weg 2, 86830 Schwabmünchen, Germany S Supporting Information *

ABSTRACT: K2Cu3Ge4O12 was synthesized via a solid-state reaction in a high-temperature experiment at 1073 K. Crystal structure analysis provided the following data: space group Cmcm (no. 63), a = 1407.9(2), b = 578.0(1), c = 1389.2(1) pm, V = 1.1305 nm3, and Z = 4. The structure consists of alternating layers of netlike arranged trimeric [Cu3O8]10− units and layers of four-membered rings of GeO4 tetrahedra. The potassium cations connect the different structural moieties. Although both structural motifs are well-known, the way they are connected in K2Cu3Ge4O12 is unique. K2Cu3Ge4O12 was further characterized via vibrational spectroscopy and SEM-EDX measurements. Magnetic measurements exhibit an antiferromagnetic behavior at low temperatures along with an unusual pseudo-2D coupling. octahedral coordination in the form of edge-sharing chains.14 Magnetic measurements of K2Cu3Ge5O14 showed no signs of magnetic ordering at low temperatures. In the following, we present the second potassium copper germanate with the composition K2Cu3Ge4O12. This germanate exhibits the same stoichiometry as Na2Cu3Ge4O12 and shows a similar trimeric [Cu3O8]10− motif. However, the arrangement of the trimeric units differs from ladderlike in Na2Cu3Ge4O12 to a netlike arrangement in K2Cu3Ge4O12 and therefore offers a new possibility to study nontrivial magnetic phenomena.

1. INTRODUCTION Low dimensional quantum-spin S = 1/2 systems are known to exhibit nontrivial magnetic behavior and are interesting specimens to study spin−spin or spin−lattice correlations.1−4 Copper(II) oxides especially attract a lot of attention due to the variety of possible connections of their fundamental building block, a square coordinated CuO4 unit. This building block can be observed as isolated moiety in Bi2CuO45 or CuSb6O8(SO4)2.6 The connection of the CuO4 units can be achieved via corners, as seen in Sr2CuO37 or BaCu2Ge3O7,1 via edges, as in CuGeO38 or Li2CuO2,9 or via a combination of both connection modes, as in malachite.10 This gives rise to various structural motifs ranging from molecular-like structures, such as dimeric11 and trimeric2,3 units up to chains.7,8,12 Embedding low-dimensional spin-carrying structures, like the trimeric motif, in a nonmagnetic oxide matrix offers the chance to study molecule-like physical properties and leads to a better understanding of magnetic phenomena. 13 Triclinic Na2Cu3Ge4O12 is one of the substances showing a trimeric [Cu3O8]10− motif that is isolated by a nonmagnetic matrix of germanium oxide.13 This substance shows nontrivial magnetic behavior, which was described in 2014 by Yasui and coworkers.3 By the exchange of the sodium cation in the quaternary system, in 2000 Monge and co-workers14 discovered the compound K2Cu3Ge5O14, which is the only known representative of the quaternary system consisting of the elements copper, germanium, oxygen, and potassium. In K2Cu3Ge5O14, copper can be found in square pyramidal and © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Synthesis. K2Cu3Ge4O12 was prepared from a mixture of K2CO3 (ChemPur, Karlsruhe, Germany, 99.9%), CuO (Merck, Darmstadt, Germany, 99%), and GeO2 (ChemPur, Karlsruhe, Germany, 99.999%) with a molar ratio of 2:3:4. The reactants were homogenized in a planetary mill (Pulverisette 7, FRITSCH, IdarOberstein, Germany) for 6 × 5 min at 800 rpm in ethanol. Subsequently, the mixture was dried at 100 °C for 3 h, transferred to a platinum crucible, and placed in a muffle furnace (HTC 03/16, Nabertherm, Lilienthal, Germany). The temperature was raised to 1073 K within 4.5 h and held for 48 h. After cooling the mixture down to room temperature, phase analysis via the Rietveld technique determined the powder composition to be 95(1) wt % K2Cu3Ge4O12 and 5(1) wt % CuO (Figure 1). Received: September 14, 2018

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DOI: 10.1021/acs.inorgchem.8b02594 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Rietveld plot of K2Cu3Ge4O12. The experimental powder diffraction pattern is shown in black, the calculated pattern in red, and the difference plot is shown in blue. The Bragg positions of K2Cu3Ge4O12 and CuO are shown in yellow and green, respectively. As calculated, the sample contains 5(1) wt % CuO and 95(1) wt % K2Cu3Ge4O12. 2.4. Magnetic Measurements. The polycrystalline sample of K2Cu3Ge4O12 was packed in a polyethylene (PE) capsule, which was attached to the sample holder rod of a Vibrating Sample Magnetometer unit (VSM) for measuring the magnetization M(T,H) in a Quantum Design Physical Property Measurement System (PPMS). The sample was investigated in the temperature range of 2.5−300 K with external magnetic fields up to 80 kOe. 2.5. X-ray Spectroscopy. The chemical composition of several crystals was analyzed by energy dispersive X-ray spectroscopy (EDX) using a SUPRA35 scanning electron microscope (SEM, Carl Zeiss, field-emission) equipped with a Si/Li EDX detector (Oxford Industries, model 7426).

Single crystals of K2Cu3Ge4O12 were synthesized in a platinum crucible. K2CO3 (15.85 mg, 0.11 mmol), CuO (18.25 mg, 0.23 mmol), and GeO2 (40 mg, 0.38 mmol) were ground in an agate mortar. The mixture was transferred to a platinum crucible, placed in a muffle oven, heated to 1073 K, and held at that temperature for 48 h. After cooling down to room temperature, the product revealed a large number of turquoise crystals (see Figure S1). 2.2. X-ray Structure Determination. 2.2.1. Single-Crystal X-ray Diffraction. A turquoise crystal was selected under a polarization microscope. The single-crystal intensity data was collected at ambient temperature using a Bruker D8 Quest diffractometer (BRUKER, Billerica, USA) with Mo-Kα radiation (λ = 71.07 pm). The diffractometer was equipped with an Incoatec microfocus X-ray tube (Incoatex, Geesthacht, Germany) and a Photon 100 detector. A multiscan absorption correction of the intensity data was performed with SADABS 2014/5.15 On the basis of the extinction conditions, space groups Cmc21 (no. 36), Cmcm (no. 63), and Ama2 (no. 40) were considered for the structure solution (SIR-92)16 and refinement, whereas space group Cmcm (no. 63) was found to be correct. The parameter refinement (full-matrix least-squares against F2) was carried out with SHELXL-201317,18 as implemented in the WinGX-2013.319 suite. The anisotropic refinement led to values of 0.0253 and 0.0424 for R1 and wR2 (all data), respectively. The positional parameters, anisotropic displacement parameters, selected interatomic distances, and bond angles are listed in Tables S1−S4. Further information on the crystal structure investigation can be obtained from the joint CCDC/FIZ Karlsruhe deposition service on quoting the deposition number CCDC 1867313 for K2Cu3Ge4O12. 2.2.2. Powder X-ray Diffraction. A powder sample of K2Cu3Ge4O12 was analyzed using a Stoe Stadi P diffractometer (Stoe, Darmstadt, Germany) in transmission geometry with Mo-Kα1 radiation (λ = 70.93 pm) utilizing a focusing Ge(111) primary beam monochromator and a Mythen 2 DCS4 detector. The measurement was performed in the 2θ range of 2.0−40.4° with a step size of 0.015°. Figure 1 shows the Rietveld analysis of the powdered sample, which was carried out using an LaB6 standard; for the fitting of the reflection shape and for hardware parameter refinements, the program suite TOPAS 4.220 was used. 2.3. Vibrational Spectroscopy. FTIR-ATR (Attenuated Total Reflection) characterization of K2Cu3Ge4O12 was performed on a Bruker Alpha-P spectrometer (BRUKER, Billerica, USA). The spectrometer was equipped with a 2 × 2 mm diamond ATR-crystal and a DTGS detector. The OPUS 7.2 software was used to correct the data set of atmospheric influences.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure. K2Cu3Ge4O12 crystallizes in the orthorhombic crystal system with space group Cmcm (no. 63), with the lattice parameters a = 1407.9(2), b = 578.0(1), and c = 1389.2(1) pm and a volume of V = 1.1305 nm3. The asymmetric unit contains four oxygen, two copper, one potassium, and one germanium position, of which only three atoms (Ge1, O1, and O3) are located at general positions (16h). Cu2 is located at the special Wyckoff position 8e, O2 at 8f, K1 and O4 at 8g, and Cu1 at 4a. The unit-cell contains 84 atoms and is comprised of four formula units. Details of the structure refinement are listed in Table 1. K2Cu3Ge4O12 possesses a layerlike structure and contains two main motifs, the [Cu3O8]10− trimeric building block (Figure 2, top) and the [Ge4O12]8− unit (Figure 2, bottom). The [Cu3O8]10− trimeric building block consists of three linear arranged CuO4 units, where the central Cu1 atom is coordinated almost square planar (coordination partners: 4 × O1) and the two terminal Cu2 atoms are coordinated distorted square planar (coordination partners: 2 × O1 and 2 × O3). The Cu−O bond lengths range from 192.9(2) to 197.1(2) pm, which is in good agreement with the bond lengths of 191.8(4) and 197.9(5) pm found in the trimeric [Cu3O8]10− units of Na2Cu3Ge4O12.13 The Cu1−O1−Cu2 bridging angle as well as the intratrimer copper distance with values of 97.3(1)° and 293.3(1) pm are relatively small and shorter compared to the values found for Na2Cu3Ge4O12 B


Article

Inorganic Chemistry

(bridging angles between 100.0(2) and 102.2(1)°, dCu1−Cu2 = 302.6(1) pm).13 The [Cu3O8]10− trimers build a quasi-netlike structure (Figure 3, bottom), which is quite different from that in

Table 1. Crystal Data and Structure Refinement of K2Cu3Ge4O12 empirical formula molar mass, g mol−1 crystal system space group

K2Cu3Ge4O12 751.18 orthorhombic Cmcm (no. 63) Powder Data powder diffractometer Stoe Stadi P radiation Mo-Kα1 (λ = 70.93 pm) a, pm 1408.2(1) b, pm 577.7(1) c, pm 1389.2(1) V, nm3 1.1301(1) Single-Crystal Data single-crystal diffractometer Bruker D8 Quest Photon 100 radiation Mo-Kα (λ = 71.07 pm) a, pm 1407.9(2) b, pm 578.0(1) c, pm 1389.2(1) V, nm3 1.1305(2) formula units per cell, Z 4 calculated density, g cm−3 4.41 temperature, K 296(2) absorption coefficient, mm−1 16.8 F(000), e 1396 2θ range, deg 5.8−72.7 range in hkl ±23, ±9, −23≤ l ≤ 20 total no. of reflections 11065 independent reflections/Rint 1470/0.0384 reflections with I > 2σ(I) 1285 data/ref parameters 1470/56 goodness-of-fit on Fi2 1.067 absorption correction semiempirical (from equivalents) final R1/wR2 (I ≥ 2σ(I)) 0.0189/0.0409 final R1/wR2 (all data) 0.0253/0.0424 largest diff. peak/hole, e Å−3 0.75/−0.91

Figure 3. (top) Chain with alternating [Ge4O12]8− units and the trimeric [Cu3O8]10− groups along the crystallographic c-axis. (bottom) Schematic quasi-netlike structure of the trimeric [Cu3O8]10− unit viewed along the crystallographic c-axis.

Na2Cu3Ge4O12, whose units are arranged in a ladderlike manner (Figure S2).13 The Cu2−O3 distance (270.1(1) pm) is quite long. The inter-trimeric copper distance Cu2−Cu2 (311.9(1) pm) in K2Cu3Ge4O12 is shorter than that in Na 2 Cu 3 Ge 4 O 12 (d Cu1−Cu2 = 334.9(1) pm, d Cu2−Cu2 = 327.3(1) pm).13 Comparisons of the crystal structure of both substances and the Cu−Cu distances are listed in Tables S6 and S7, respectively. The second main motif represented by the [Ge4O12]8− unit is build up by four GeO4-tetrahedra (coordination partners: O1, O2, O3, and O4) that are connected via two common corners (O2, O4) forming a four-membered ring. This structural motif can be found in various compounds, e.g., Y2CoGe4O12.21 In K2Cu3Ge4O12, the Ge−O bond lengths vary between 171.9(2) and 177.2(1) pm, and the O−Ge−O angles are between 103.1(1) and 120.7(1)°. This is in good agreement with the values given for Y2CoGe4O12 (Ge−O: 171.9(5)−179.3(1) pm and O−Ge−O: 107.4(1)− 126.0(4)°).21 The quasi-netlike copper layers isolate this structural motif. The connection between the two structural motifs is realized via two common oxygen atoms O1 and O3. Along the crystallographic c-axis, the two fundamental building blocks create alternating chains (Figure 3, top). Hereby, the [Ge4O12]8− group is connected via two corners (O1) to the almost square planar coordinated Cu1 atom. These chains are interconnected via the distorted square planar coordinated Cu2 atoms by sharing the O3 oxygen atoms. The potassium cations can be found between the [Ge4O12]8− units in the [Ge4O12]8− layer (Figure 4). Bond-valence sums (∑V)22,23 and CHARDI (∑Q)24 calculations were performed as an additional check on the reliability of the determined structure (Table S5). The highest discrepancy can be found at O4 (∑V = −2.25), which is the

Figure 2. (bottom) [Ge4O12]8− unit build up by four GeO4 tetrahedra viewed along the b-axis. (top) Trimeric copper group [Cu3O8]10− consisting of three distorted square planar CuO4 units in the [011] plane. C


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Inorganic Chemistry

symmetric Ge−O−Ge vibrations of the four-membered rings are visible.25 3.4. Magnetic Measurements. Figure 6 shows the temperature dependence of the magnetic susceptibility of

Figure 4. Layerlike arrangements of the [Ge4O12]8− and [Cu3O8]10− entities in the crystal structure of K2Cu3Ge4O12, viewed along [01̅−0].

only oxygen position coordinated solely by germanium and potassium cations. Although both basic building blocks are well-known, the way they are connected is unique for the presented germanate. To the best of our knowledge, there is no compound with an atomic structure similar to that of K2Cu3Ge4O12, therefore it represents a new structure type: Cmcm, Pearson code oS84, and Wyckoff sequence h3g2fea. 3.2. EDX Spectroscopy. EDX measurements of the powder sample yielded an average molar ratio of 2.2(2):2.9(2):4 for potassium, copper, and germanium if normalized to the germanium content. The cation composition is in good accordance with the nominal composition of K2Cu3Ge4O12 derived from diffraction data. 3.3. Vibrational Spectroscopy. The experimental IR spectrum recorded from a powder sample is depicted in Figure 5. It shows no absorption bands above 1800 cm−1. Two broad

Figure 6. Magnetic susceptibility measurement of K2Cu3Ge4O12 at 10 kOe. (top) fit of the data using the S = 1/2 Heisenberg trimer model in the high-temperature region and the S = 1/2 Heisenberg chain in the low-temperature region. (bottom) fit of the data using the 2D QHAF approach.

K2Cu3Ge4O12 measured with an applied external field of 10 kOe. At low temperatures, a fairly broad maximum can be observed at T = 8.3(1) K, in line with short-range 1D or 2D antiferromagnetic interactions; the respective 3D ordering must occur at even lower temperatures. A fit of the paramagnetic regime using the Curie−Weiss law yields an effective magnetic moment of μeff = 1.87(1) μB, in line with S = 1/2 for Cu2+ ions that exhibit additional spin−orbit coupling (μspin‑only = 1.73 μB). Furthermore, a negative paramagnetic Curie temperature of θP = −49.1(1) K suggests antiferromagnetic interactions in the paramagnetic temperature regime. The magnetic exchange interactions between the Cu2+ cations can be analyzed by fitting the susceptibility curve with different models. Yasui and co-workers3 have shown that the magnetic properties of Na2Cu3Ge4O12 can be explained and fitted using the isolated S = 1/2 Heisenberg trimer model at higher temperatures and the 1D S = 1/2 Heisenberg chain model at low temperatures. The coupling within a trimer is illustrated in Figure 7 (top). In the high-temperature regime, the outer spins of the trimer in Na2Cu3Ge4O12 couple antiferromagnetically with a fairly large coupling constant of J2/kB = 340(20) K, forming a nonmagnetic singlet state, while the second coupling (outer and inner spin) is rather weak (J1/kB = 30(20) K). The remaining inner spin also couples antiferromagnetically, but only at low temperatures, forming a Heisenberg chain with

Figure 5. FT-IR absorbance spectrum of the powder sample of K2Cu3Ge4O12 in the range of 1800−400 cm−1. The spectrum shows no bands in the region of 4000−1800 cm−1.

bands with low intensity are found at 1400 and 1630 cm−1 and can most likely be assigned to small amounts of unreacted K2CO3. At around 1015 cm−1, vibration bands of the fourmembered [Ge4O12]8− rings can be observed.25 Asymmetric stretching vibrations of connecting GeO4 units can be observed in the region of 775−793 cm−1.25 Between 450 and 580 cm−1, D


Article

Inorganic Chemistry

However, we have to point out that the suggested magnetic model, if applied to K2Cu3Ge4O12, has certain problems: The “chain” with the coupling constant J1 has holes, since it consists only of trimers. Furthermore, the central Cu atoms in a trimer have no neighbor in another chain, and finally, the interaction between the “chains” does not exhibit a 90° angle. Since we were unable to find a different magnetic model, which describes a case like this one, we have tried to adapt the 2D QHAF model to this particular case. When fitting the experimental data in the high-temperature region (50−300 K), the model reproduces this temperature range well, with fitting parameters of J1/kB = 56(5) K and α = 0.15(1). This suggests that the coupling between the trimers is rather weak (αJ1/kB = 0.15 × 56 K = 8.4 K) in contrast to the coupling within the trimer. The resulting intratrimer coupling is furthermore in the range of the difference of the two coupling constants J2−J1 determined by the S = 1/2 Heisenberg trimer, suggesting that this coupling represents the average coupling in the trimer. For the low-temperature fit (5−50 K), parameters J1/kB = 9(1) K and α = 0.92(1) have been obtained. The low coupling constant suggests that the interactions in the trimer now are significantly reduced, the intertrimer correlations, however, are αJ1/kB = 0.92 × 9 K = 8.3 K and therefore in the same range as observed before. The J1 coupling constant furthermore matches the peak temperature obtained by susceptibility measurements well, again indicating short-range ordering. The lower peak temperature of the potassium compound (T = 8.3(1) K) can be finally explained by distortions within the trimer compared to the sodium member (T ∼ 11 K3). Figure 9 depicts the structural angles in both

Figure 7. (top) Magnetic interactions in a [Cu3O8]10− trimer; (bottom) magnetic interactions in a 2D QHAF.

J3/kB = 18(1) K. The same models have been used to evaluate the magnetic properties of K2Cu3Ge4O12 and are depicted in Figure 6 (top). For the Heisenberg trimer model, coupling constants of J1/kB = 150(20) K and J2/kB = 240(20) K were obtained. In contrast to the sodium compound (Table S8), both interactions are fairly strong, suggesting no antiferromagnetic coupling of the outer spins. For the low-temperature fit, J3/kB = 4(1) K has been obtained. It becomes readily evident that the two curves of the Heisenberg trimer model (red) and Heisenberg chain (green), in contrast to what has been shown for Na2Cu3Ge4O12, do not intersect. Additionally, the intertrimer coupling seems to be rather strong. Taking all these results and finally the different topology of the magnetic centers (Figure 3, bottom) of the potassium compound into account, the need for a different magnetic model arises. Landee and Turnbull summarized a broad variety of different coupling mechanisms,26 including a model for a two-dimensional S = 1/2 Heisenberg antiferromagnet (2D QHAF). The 2D model can be described as a chain with the coupling constant J1 (depicted in orange in Figure 8), which interacts with a neighboring chain via the second interaction αJ1 (depicted in green in Figure 8). For α = 0, the chains are isolated, while for α = 1, a square is formed. The intermediate cases of 0 < α < 1 correspond to rectangular arrangements.

Figure 9. Interatomic angles in the [Cu3O8]10− trimers in Na2Cu3Ge4O12 (top) and K2Cu3Ge4O12 (bottom).

Na2Cu3Ge4O12 and K2Cu3Ge4O12, showing that the angles in K2Cu3Ge4O12 are more bent, hence reducing the superexchange interaction, in line with what has been reported for 1D Heisenberg antiferromagnets.27

4. CONCLUSION In this work we present the new germanate K2Cu3Ge4O12. Its crystal structure consists of layers of [Ge4O12]8− units and layers of netlike arranged [Cu3O8]10− units. The quasi-netlike arrangement of the [Cu3O8]10− unit gives rise to a unique magnetic behavior, which can be described by a model for a 2D S = 1/2 Heisenberg antiferromagnet (2D QHAF). At hightemperatures, it exhibits a strong intra-trimeric coupling (J1/kB = 56(5) K) and a weak coupling between the trimeric compounds (αJ1/kB = 8.4 K). At low temperatures, the coupling within the trimeric compound decreases (J1/kB = 9(1) K), whereas the inter-trimeric coupling stays in the same

Figure 8. Magnetic topology of the 2D QHAF interactions in K2Cu3Ge4O12. Intratrimer interactions J1 are depicted in orange; intertrimer interactions αJ1 in green. E


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Inorganic Chemistry

(7) Ami, T.; Crawford, M. K.; Harlow, R. L.; Wang, Z. R.; Johnston, D. C.; Huang, Q.; Erwin, R. W. Magnetic susceptibility and lowtemperature structure of the linear chain cuprate Sr2CuO3. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 51, 5994−6001. (8) Völlenkle, H.; Wittmann, A.; Nowotny, H. Zur Kristallstruktur von CuGeO3. Monatsh. Chem. 1967, 98, 1352−1357. (9) Hoppe, R.; Rieck, H. Die Kristallstruktur von Li2CuO2. Z. Anorg. Allg. Chem. 1970, 379, 157−164. (10) Lebernegg, S.; Tsirlin, A. A.; Janson, O.; Rosner, H. Spin gap in malachite Cu2(OH)2CO3 and its evolution under pressure. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 224406. (11) Schmitt, M.; Janson, O.; Golbs, S.; Schmidt, M.; Schnelle, W.; Richter, J.; Rosner, H. Microscopic magnetic modeling for the S = 1/2 alternating-chain compounds Na3Cu2SbO6 and Na2Cu2TeO6. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 174403. (12) Weht, R.; Pickett, W. E. Extended Moment Formation and Second Neighbor Coupling in Li2CuO2. Phys. Rev. Lett. 1998, 81, 2502−2505. (13) Mo, X.; Etheredge, K. M. S.; Hwu, S.-J.; Huang, Q. New Cuprates Featuring Ladderlike Periodic Arrays of [Cu3O8]10− Trimeric Magnetic Nanaostructures. Inorg. Chem. 2006, 45, 3478− 3480. (14) Monge, M. A.; Gutierrez-Puebla, E.; Cascales, C.; Campa, J. A. A Copper Germanate Containing Potassium in Its Two-Dimensional Channel Network. Chem. Mater. 2000, 12, 1926−1930. (15) Sheldrick, G. M. SADABS v2014/5; Bruker AXY Inc.: Madison, WI, 2001. (16) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. SIR92 - a program for automatic solution of crystal structures by direct methods. J. Appl. Crystallogr. 1994, 27, 435. (17) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (18) Sheldrick, G. M. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (19) Farrugia, L. J. WinGX and ORTEP for Windows: an update. J. Appl. Crystallogr. 2012, 45, 849−854. (20) TOPAS - Academic; Coelho Software: Brisbane, Queensland, Australia, 2007. (21) Liu, X.-Q.; Battle, P. D.; Ridout, J.; Xu, D.; Ramos, S. Structural chemistry and magnetic properties of Y2CoGe4O12. J. Solid State Chem. 2015, 228, 183−188. (22) Brese, N. E.; O’Keeffe, M. Bond-Valence Parameters for Solids. Acta Crystallogr., Sect. B: Struct. Sci. 1991, 47, 192−197. (23) Brown, I. D.; Altermatt, D. Bond-Valence Parameters Obtained from a Systematic Analysis of the Inorganic Crystal Structure Database. Acta Crystallogr., Sect. B: Struct. Sci. 1985, 41, 244−247. (24) Hoppe, R.; Voigt, S.; Glaum, H.; Kissel, J.; Müller, H. P.; Bernet, K. A new route to charge distribution in ionic solids. J. LessCommon Met. 1989, 156, 105−122. (25) Zhang, L. Y.; Li, H.; Hu, L. L. Statistical structure analysis of GeO2 modified Yb3+: Phosphate glasses based on Raman and FTIR study. J. Alloys Compd. 2017, 698, 103−113. (26) Landee, C. P.; Turnbull, M. M. Review: A gentle introduction to magnetism: units, fields, theory, and experiment. J. Coord. Chem. 2014, 67, 375−439. (27) Xiang, H.; Tang, Y.; Zhang, S.; He, Z. Intra-chain superexchange couplings in quasi-1D 3d transition-metal magnetic compounds. J. Phys.: Condens. Matter 2016, 28, 276003.

magnitude (αJ1/kB = 8.3 K). Even though the magnetic behavior of K2Cu3Ge4O12 can be described by the 2D QHAF approach, this model is still not sufficient. New models for 2D magnetic structures need to be developed to lead to a better understanding of these systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02594. Wyckoff positions, atomic coordinates, anisotropic displacement parameters, interatomic distances, bond angles, charge distribution calculation, picture of the sample, as well as a comparison of K2Cu3Ge4O12 with Na2Cu3Ge4O12 (PDF) Accession Codes

CCDC 1867313 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +43 512 50757099. E-mail: [email protected]. at. ORCID

Oliver Janka: 0000-0002-9480-3888 Hubert Huppertz: 0000-0002-2098-6087 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Christian Koch (OSRAM Opto Semiconductors) for the EDX analysis. We also thank Dr. M. K. Schmitt for his support with the Rietveld analysis.

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REFERENCES

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