Syntheses and crystal structures of the quaternary thio­germanates Cu₄FeGe₂S₇ and Cu₄CoGe₂S₇

The isostructural crystal structures of Cu₄FeGe₂S₇ and Cu₄CoGe₂S₇ were solved and refined. All the metal cations form MS₄ tetra­hedra and share corners to create a three-dimensional, non-centrosymmetric structure.

Abstract

The quaternary thio­germanates Cu₄FeGe₂S₇ (tetra­copper iron digermanium hepta­sulfide) and Cu₄CoGe₂S₇ (tetra­copper cobalt digermanium hepta­sulfide) were prepared in evacuated fused-silica ampoules via high-temperature, solid-state synthesis using stoichiometric amounts of the elements at 1273 K. These isostructural compounds crystallize in the Cu₄NiSi₂S₇ structure type, which can be considered as a superstructure of cubic diamond or sphalerite. The monovalent (Cu⁺), divalent (Fe²⁺ or Co²⁺) and tetra­valent (Ge⁴⁺) cations adopt tetra­hedral geometries, each being surrounded by four S²⁻ anions. The divalent cation and one of the sulfide ions lie on crystallographic twofold axes. These tetra­hedra share corners to create a three-dimensional framework structure. All of the tetra­hedra align along the same crystallographic direction, rendering the structure non-centrosymmetric and polar (space group C2). Analysis of X-ray powder diffraction data revealed that the structures are the major phase of the reaction products. Thermal analysis indicated relatively high melting temperatures, near 1273 K.

Chemical context  

The title compounds belong to the family of quaternary thio­germanates, which can be constructed from different [Ge_xS_y]^z- building blocks, such as [GeS₄]⁴⁻ (Aitken et al., 2001 ▸) and [Ge₂S₆]⁴⁻ (Choudhury et al., 2015 ▸). Two GeS₄ tetra­hedra can share a corner to create [Ge₂S₇]⁶⁻ units, which are featured in the title compounds. Cu₄FeGe₂S₇ and Cu₄CoGe₂S₇ also belong to the family of diamond-like semiconductors (DLSs), with structures that can be derived from the cubic or hexa­gonal (Frondel & Marvin, 1967 ▸) forms of diamond. The synthesis of new diamond-like materials is guided by valence electron principles and Pauling’s rules, and the resulting DLSs can be binary, ternary, or quaternary, depending on the number of elements employed in the reaction (Parthé, 1964 ▸; Pamplin, 1981 ▸; Goryunova, 1965 ▸). Increasing the number of elements in the formula allows for greater tunability of the material’s properties; thus, quaternary DLSs are a particularly appealing class of materials. As a result of their technologic­ally relevant properties, these materials are of inter­est for a number of applications, such as solar cells (Ito & Nakazawa, 1988 ▸; Heppke et al., 2020 ▸; Liu et al., 2018 ▸), batteries (Brant, Devlin et al., 2015 ▸; Kaib et al., 2013 ▸) and magnetic devices (Wintenberger, 1979 ▸; Greenwood & Whitfield, 1968 ▸). Furthermore, owing to their inherently non-centrosymmetric structures, DLSs are attractive candidates for infrared non-linear optical (IR–NLO) devices that make use of second-harmonic generation (SHG) crystals (Ohmer & Pandey 1998 ▸): only crystals that lack an inversion center can exhibit SHG.

IR–NLO materials are used to shift the radiation of lasers to more suitable wavelengths for use in military (Hopkins 1998 ▸), medical (Stoeppler et al., 2012 ▸) and industrial applications (Bamford et al., 2007 ▸). Currently, ternary DLSs, most of which are sulfides, dominate the market of SHG crystals for use in the infrared (Ohmer & Pandey, 1998 ▸). Yet the current commercially available IR–NLO materials suffer from serious drawbacks, such as low laser-induced damage thresholds (LIDTs) and multi-photon absorption (Schunemann, 2007 ▸). Turning attention to the discovery of new quaternary DLSs provides a reliable route to next-generation IR–NLO materials that allows for greater control of the material’s properties. Compounds such as Li₂CdGeS₄ (Brant, Clark et al., 2014 ▸), Li₂MnGeS₄ (Brant, Clark et al., 2015 ▸), and Li₄HgGe₂S₇ (Wu, Yang & Pan, 2017 ▸) have shown potential to outperform currently used ternary IR–NLO crystals. These DLSs have shown promising SHG capabilities, as well as resilience to high powered lasers, a necessity to broaden future usage (Hopkins, 1998 ▸). For these reasons, we were motivated to investigate the Cu–Fe–Ge–S and Cu–Co–Ge–S systems for new DLSs.

Structural commentary  

The title compounds, Cu₄FeGe₂S₇ (I) and Cu₄CoGe₂S₇ (II), are isostructural and crystallize in the non-centrosymmetric, monoclinic space group C2 (No. 5) with the Cu₄NiSi₂S₇ structure type (Schäfer et al., 1980 ▸). The structure contains two crystallographically unique Cu⁺ ions, one divalent metal (Fe or Co) sited on a crystallographic twofold axis, one Ge⁴⁺ cation and four S²⁻ anions (one with site symmetry 2) (Fig. 1 ▸). The sulfide anions create a ‘cubic’ close-packed array and the cations reside in one-half of the tetra­hedral holes; these tetra­hedra share corners to form a three-dimensional network. Two GeS₄ tetra­hedra share corners to form (Ge₂S₇)⁶⁻ subunits that are isolated from each other (Fig. 1 ▸). These subunits are separated by isolated FeS₄ tetra­hedra and surrounded by a snaking, three-dimensional network of corner-sharing CuS₄ tetra­hedra that serve to link the (Ge₂S₇)⁶⁻ and FeS₄ subunits. All of the tetra­hedra are aligned along one crystallographic direction, rendering the structure non-centrosymmetric (Fig. 2 ▸). All DLSs exhibit a honeycomb pattern in their crystal structure (Fig. 3 ▸); the various resulting space groups arise from the different possible cation-ordering patterns.

Figure 1.

The structure of Cu₄FeGe₂S₇ with crystallographically unique ions labeled. Ellipsoids are shown at 99% probability. Cu₄CoGe₂S₇ is isostructural and has similar atomic displacement parameters.

Figure 2.

Polyhedral view of Cu₄FeGe₂S₇ showing the polar nature of the structure.

Figure 3.

The ‘honeycomb’ pattern found in Cu₄FeGe₂S₇, a characteristic of DLSs. Highlighted in the bottom left corner is one of the [Ge₂S₇]⁶⁻ subunits that forms in this structure.

Selected geometrical data for (I) and (II) are given in Tables 1 ▸ and 2 ▸, respectively. The average Fe—S (I) and Co—S (II) bond distances are 2.334 (6) and 2.317 (6) Å, respectively. These values align well with other compounds containing iron or cobalt tetra­hedrally coordinated by sulfur. For example, the average Fe—S distance found in Li₂FeGeS₄ is 2.34 (2) Å (Brant, dela Cruz et al., 2014 ▸), while the average Co—S distance found in Li₂CoGeS₄ is 2.31 (3) Å (Brant, Devlin et al., 2015 ▸). The average Ge—S distances are 2.240 (4) and 2.244 (5) Å for (I) and (II), respectively. These distances are also close to those of the lithium-containing DLSs: Li₂FeGeS₄ (Brant, Devlin et al., 2015 ▸) and Li₂CoGeS₄ (Brant, dela Cruz et al., 2014 ▸) possess values of 2.23 (2) and 2.22 (3) Å, respectively. The average tetra­hedral bond angles for all cations in both title compounds is, within uncertainty, ideal. For comparison, the tetra­hedral angular ranges encountered in Cu₂FeGeS₄ (Wintenberger, 1979 ▸) and Cu₂CoGeS₄ (Gulay et al., 2004 ▸) are 109.471–109.484° and 109.473–109.579°, respectively. The sulfur anions also exhibit tetra­hedral coordination. Both S2 and S4 are coordinated by two copper, one germanium and one iron or cobalt cation. S1 is connected to two germanium and two copper cations, while S3 is surrounded by one germanium and three copper cations.

Table 1. Selected bond lengths (Å) for (I).

Cu1—S3i	2.290 (2)	Fe—S4v	2.331 (3)
Cu1—S3	2.302 (2)	Fe—S4	2.331 (3)
Cu1—S2ii	2.303 (2)	Fe—S2iv	2.337 (3)
Cu1—S1iii	2.353 (2)	Fe—S2vi	2.337 (3)
Cu2—S3	2.294 (2)	Ge—S3vii	2.196 (2)
Cu2—S2	2.309 (2)	Ge—S2	2.221 (2)
Cu2—S4iv	2.309 (2)	Ge—S4iii	2.233 (2)
Cu2—S4	2.319 (2)	Ge—S1iii	2.3107 (19)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) ; (vii) .

Table 2. Selected bond lengths (Å) for (II).

Cu1—S3i	2.292 (2)	Co—S4v	2.308 (3)
Cu1—S2ii	2.295 (2)	Co—S4	2.308 (3)
Cu1—S3	2.304 (2)	Co—S2iv	2.325 (3)
Cu1—S1iii	2.343 (3)	Co—S2vi	2.325 (3)
Cu2—S3	2.290 (2)	Ge—S2	2.211 (3)
Cu2—S2	2.298 (3)	Ge—S3vii	2.211 (2)
Cu2—S4iv	2.301 (2)	Ge—S4iii	2.236 (2)
Cu2—S4	2.311 (2)	Ge—S1iii	2.316 (2)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) ; (vii) .

Database survey  

Quaternary DLSs exist with several different formulae; examples that incorporate chalcogenides as the anion are I–II₂–III–VI₄, I₂–II–IV–VI₄, and I₄–II–IV₂–VI₇. In these formulae, the Roman numerals represent the number of valence electrons for each element. Compounds of the formula I–II₂–III–VI₄, such as CuMn₂InS₄ (Delgado & Sagredo, 2016 ▸) and CuFe₂InSe₄ (Delgado et al., 2008 ▸) include trivalent elements, while the other relevant formulae mentioned above, including the title compounds, contain tetra­valent elements. Numerous DLSs of the general formula I₂–II–IV–VI₄ have been reported and crystallize in non-centrosymmetric space groups, such as I 2m and Pmn2₁ with the stannite and wurtz-stannite structure types that are derived from the cubic and hexa­gonal diamond structures, respectively (Brunetta et al., 2013 ▸). The monovalent ions incorporated in these materials include Li (Wu, Zhang, et al., 2017 ▸; Wu & Pan, 2017 ▸), Cu (Parthé et al., 1969 ▸) or Ag (Brunetta et al., 2013 ▸) and the divalent ions include a number of metals, such as Mg (Liu et al., 2013 ▸), Mn (Bernert & Pfitzner, 2005 ▸), Fe (Wintenberger, 1979 ▸), Co (Bernert and Pfitzner 2006 ▸), Zn (Parasyuk et al., 2001 ▸), Cd (Rosmus et al., 2014 ▸) and Hg (Olekseyuk et al., 2005 ▸). The tetra­valent ions found in these compounds are usually Si, Ge, or Sn, while the hexa­valent atoms (i.e., the divalent anions) can be S (Lekse et al., 2009 ▸), Se (Gulay, Romanyuk & Parasyuk, 2002 ▸), or Te (Parasyuk et al., 2005 ▸). Some specific examples include Cu₂MgGeS₄ (Liu et al., 2013 ▸) and Ag₂MnSnS₄ (Friedrich et al., 2018 ▸).

In contrast, considerably fewer compounds of the general formula I₄–II–IV₂–VI₇ have been discovered: only seven of these, which crystallize in either space group C2 or Cc with structures derived from cubic or hexa­gonal diamond, respectively, have been published to date: Li₄MnGe₂S₇ (Cc) (Kaib et al., 2013 ▸), Li₄MnSn₂Se₇ (Cc) (Kaib et al., 2013 ▸), Li₄HgGe₂S₇ (Cc) (Wu, Yang, Pan 2017 ▸), Ag₄HgGe₂S₇ (Cc) (Gulay, Olekseyuk & Parasyuk 2002 ▸), Ag₄CdGe₂S₇ (Cc) (Gulay, Olekseyuk & Parasyuk 2002 ▸), Cu₄NiSi₂S₇ (C2) (Schäfer et al., 1980 ▸), and Cu₄NiGe₂S₇ (C2) (Schäfer et al., 1980 ▸).

X-ray powder diffraction and thermal analysis  

The calculated and observed X-ray powder diffraction patterns match well (Fig. 4 ▸), indicating that the title compounds are the major phases of the respective reactions. An optimization of the synthetic protocol is needed to isolate the desired phases in phase-pure form.

Figure 4.

Comparison of experimental X-ray powder diffraction patterns before and after DTA with those calculated using the single-crystal structures for Cu₄FeGe₂S₇ (left) and Cu₄CoGe₂S₇ (right).

Differential thermal analysis (DTA) reveals that Cu₄FeGe₂S₇ and Cu₄CoGe₂S₇ show relatively high thermal stability and melting and recrystallization events with appropriate hysteresis around 1000°C (Fig. 5 ▸). Multiple heating-cooling cycles for each sample were consistent, suggesting that the thermal events are reversible. X-ray powder diffraction of the DTA residues indicated that the samples were not changed by the thermal analyses, implying that they melt congruently. DTA also suggests that neither compound is a single phase, as there are some small shoulders on the peaks indicative of the thermal events.

Figure 5.

Differential thermal analysis diagrams obtained for the title compounds showing the melting and recrystallization events.

Materials and methods  

All powdered elements were acquired from commercial suppliers and used as obtained with the exception of germanium metal, which was purchased as chunks and ground to a fine powder using a Diamonite™ mortar and pestle prior to use. Powder X-ray diffraction data were recorded from 10–100° 2θ using a PANalytical X’Pert Pro MPD powder X-ray diffractometer operating with Cu K _α radiation (λ = 1.541871 Å), a tube power of 45 kV and 40 mA and a step size of 0.017°. DTA data were obtained using a Shimadzu DTA50 thermal analyzer. Each sample was vacuum-sealed in a fused-silica ampoule, placed alongside an ampoule containing an Al₂O₃ reference of comparable mass, heated from room temperature to 1050°C at a rate of 10°C min⁻¹ and subsequently cooled to room temperature at the same rate. A second heating–cooling cycle was conducted in order to determine the reproducibility of the thermal events.

Synthesis and crystallization  

Cu₄FeGe₂S₇ and Cu₄CoGe₂S₇ were synthesized by combining stoichiometric amounts of Cu (99.999%), Fe (99.99%) or Co (99.99%), Ge (99.999%) and S (99.5%, sublimed) powders. The powders were mixed and placed into 12 mm o.d. fused-silica tubes that were subsequently attached to a vacuum line, evacuated and flame sealed. The reaction vessels were placed upright into ceramic containers inside programmable furnaces, where they were heated to 1000°C in 24 h, held there for 48 h, cooled to 900°C over the course of 50 h, and held there for 96 h, before being allowed to cool to room temperature over a 24 h period. Subsequently, the reaction vessels were cut open and the contents were examined under a light microscope. The products consisted of loose silvery gray microcrystalline powders from which small single crystals were selected for single-crystal X-ray diffraction.

Refinement  

Crystal data, data collection parameters, and structure refinement details are summarized in Table 3 ▸. Extinction parameters were refined for each compound. After the final refinement, the Flack parameter for both structures refined to 0.06 (3), indicating that the absolute structure is correct. In Cu₄FeGe₂S₇, the largest difference peak is located 1.15 Å from Cu2 while the deepest difference hole is 1.50 Å from S3. For Cu₄CoGe₂S₇, the largest difference peak is 0.67 Å from Co while the deepest difference hole is 0.83 Å from Ge.

Table 3. Experimental details.

	(I)	(II)
Crystal data
Chemical formula	Cu4FeGe2S7	Cu4CoGe2S7
M r	679.61	682.69
Crystal system, space group	Monoclinic, C2	Monoclinic, C2
Temperature (K)	296	296
a, b, c (Å)	11.7405 (6), 5.3589 (2), 8.3420 (4)	11.7280 (2), 5.33987 (10), 8.33133 (14)
β (°)	98.661 (3)	98.6680 (12)
V (Å3)	518.86 (4)	515.80 (2)
Z	2	2
Radiation type	Mo Kα	Mo Kα
μ (mm−1)	16.46	16.76
Crystal size (mm)	0.08 × 0.08 × 0.03	0.08 × 0.07 × 0.06
Data collection
Diffractometer	Bruker SMART APEXII	Bruker SMART APEXII
Absorption correction	Multi-scan (SADABS; Sheldrick, 2002 ▸)	Multi-scan (SADABS; Sheldrick, 2002 ▸)
T min, T max	0.246, 0.435	0.356, 0.435
No. of measured, independent and observed [I > 2σ(I)] reflections	2258, 1187, 994	2239, 1177, 1039
R int	0.021	0.017
(sin θ/λ)max (Å−1)	0.649	0.649
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.034, 0.094, 1.06	0.030, 0.075, 1.11
No. of reflections	1187	1177
No. of parameters	67	67
No. of restraints	1	1
Δρmax, Δρmin (e Å−3)	0.75, −0.89	0.88, −0.49
Absolute structure	Flack (1983 ▸)	Flack (1983 ▸)
Absolute structure parameter	0.06 (3)	0.06 (3)

Computer programs: SMART and SAINT (Bruker, 1998 ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELXL2018/3 (Sheldrick, 2015 ▸) and CrystalMaker (Palmer, 2014 ▸).

Supplementary Material

CCDC references: 2009145, 2009144

Additional supporting information: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

Tetracopper iron digermanium heptasulfide (I). Crystal data

Cu4FeGe2S7	F(000) = 636
Mr = 679.61	Dx = 4.350 Mg m−3
Monoclinic, C2	Mo Kα radiation, λ = 0.71073 Å
a = 11.7405 (6) Å	Cell parameters from 4891 reflections
b = 5.3589 (2) Å	θ = 4.2–31.4°
c = 8.3420 (4) Å	µ = 16.46 mm−1
β = 98.661 (3)°	T = 296 K
V = 518.86 (4) Å3	Irregular, grey
Z = 2	0.08 × 0.08 × 0.03 mm

Tetracopper iron digermanium heptasulfide (I). Data collection

Bruker SMART APEXII diffractometer	994 reflections with I > 2σ(I)
φ and ω Scans scans	Rint = 0.021
Absorption correction: multi-scan (SADABS; Sheldrick, 2002)	θmax = 27.5°, θmin = 2.5°
Tmin = 0.246, Tmax = 0.435	h = −15→15
2258 measured reflections	k = −6→6
1187 independent reflections	l = −10→10

Tetracopper iron digermanium heptasulfide (I). Refinement

Refinement on F2	w = 1/[σ2(Fo2) + (0.0315P)2] where P = (Fo2 + 2Fc2)/3
Least-squares matrix: full	(Δ/σ)max < 0.001
R[F2 > 2σ(F2)] = 0.034	Δρmax = 0.75 e Å−3
wR(F2) = 0.094	Δρmin = −0.89 e Å−3
S = 1.06	Extinction correction: SHELXL-2018/3 (Sheldrick 2018), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
1187 reflections	Extinction coefficient: 0.0123 (9)
67 parameters	Absolute structure: Flack (1983)
1 restraint	Absolute structure parameter: 0.06 (3)
Primary atom site location: structure-invariant direct methods

Tetracopper iron digermanium heptasulfide (I). Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refined as a two-component inversion twin

Tetracopper iron digermanium heptasulfide (I). Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cu1	0.64298 (8)	0.2054 (3)	0.93484 (11)	0.0174 (7)
Cu2	0.71147 (8)	0.7098 (2)	0.64229 (10)	0.0165 (6)
Fe	1.000000	0.7200 (6)	0.500000	0.0112 (6)
Ge	0.42524 (6)	0.7285 (3)	0.78371 (7)	0.0092 (3)
S1	1.000000	0.9789 (6)	1.000000	0.0091 (6)
S2	0.56834 (15)	0.9593 (5)	0.71791 (18)	0.0111 (6)
S3	0.78390 (14)	0.4527 (4)	0.85310 (17)	0.0091 (6)
S4	0.85688 (15)	0.9704 (5)	0.58180 (19)	0.0101 (5)

Tetracopper iron digermanium heptasulfide (I). Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
Cu1	0.0171 (6)	0.0169 (15)	0.0184 (6)	−0.0019 (5)	0.0032 (4)	0.0025 (5)
Cu2	0.0171 (6)	0.0142 (13)	0.0185 (6)	0.0005 (8)	0.0037 (4)	0.0017 (4)
Fe	0.0115 (8)	0.0137 (17)	0.0093 (7)	0.000	0.0041 (6)	0.000
Ge	0.0080 (5)	0.0100 (8)	0.0096 (5)	−0.0009 (8)	0.0018 (3)	−0.0007 (4)
S1	0.0095 (13)	0.0093 (16)	0.0087 (11)	0.000	0.0014 (9)	0.000
S2	0.0089 (11)	0.0128 (15)	0.0120 (9)	−0.0029 (10)	0.0030 (7)	0.0009 (9)
S3	0.0081 (11)	0.0076 (14)	0.0122 (9)	0.0004 (8)	0.0033 (8)	−0.0003 (8)
S4	0.0101 (10)	0.0094 (13)	0.0107 (9)	−0.0012 (7)	0.0010 (7)	−0.0004 (10)

Tetracopper iron digermanium heptasulfide (I). Geometric parameters (Å, º)

Cu1—S3i	2.290 (2)	Fe—S4v	2.331 (3)
Cu1—S3	2.302 (2)	Fe—S4	2.331 (3)
Cu1—S2ii	2.303 (2)	Fe—S2iv	2.337 (3)
Cu1—S1iii	2.353 (2)	Fe—S2vi	2.337 (3)
Cu2—S3	2.294 (2)	Ge—S3vii	2.196 (2)
Cu2—S2	2.309 (2)	Ge—S2	2.221 (2)
Cu2—S4iv	2.309 (2)	Ge—S4iii	2.233 (2)
Cu2—S4	2.319 (2)	Ge—S1iii	2.3107 (19)
S3i—Cu1—S3	111.52 (5)	Geviii—S1—Geix	109.23 (13)
S3i—Cu1—S2ii	108.81 (12)	Geviii—S1—Cu1viii	112.37 (4)
S3—Cu1—S2ii	107.60 (6)	Geix—S1—Cu1viii	109.92 (4)
S3i—Cu1—S1iii	112.79 (6)	Geviii—S1—Cu1ix	109.92 (4)
S3—Cu1—S1iii	106.23 (13)	Geix—S1—Cu1ix	112.37 (4)
S2ii—Cu1—S1iii	109.75 (6)	Cu1viii—S1—Cu1ix	102.96 (15)
S3—Cu2—S2	109.84 (6)	Ge—S2—Cu1x	109.71 (8)
S3—Cu2—S4iv	109.30 (11)	Ge—S2—Cu2	110.72 (12)
S2—Cu2—S4iv	111.38 (6)	Cu1x—S2—Cu2	109.87 (7)
S3—Cu2—S4	109.17 (7)	Ge—S2—Fevii	109.96 (9)
S2—Cu2—S4	107.49 (11)	Cu1x—S2—Fevii	108.33 (14)
S4iv—Cu2—S4	109.62 (5)	Cu2—S2—Fevii	108.21 (7)
S4v—Fe—S4	109.72 (19)	Gevi—S3—Cu1viii	108.48 (7)
S4v—Fe—S2iv	107.13 (6)	Gevi—S3—Cu2	109.52 (7)
S4—Fe—S2iv	113.18 (6)	Cu1viii—S3—Cu2	106.84 (11)
S4v—Fe—S2vi	113.18 (6)	Gevi—S3—Cu1	111.62 (11)
S4—Fe—S2vi	107.13 (6)	Cu1viii—S3—Cu1	108.31 (7)
S2iv—Fe—S2vi	106.58 (17)	Cu2—S3—Cu1	111.89 (8)
S3vii—Ge—S2	112.97 (13)	Geix—S4—Cu2xi	107.94 (11)
S3vii—Ge—S4iii	109.74 (6)	Geix—S4—Cu2	113.68 (8)
S2—Ge—S4iii	110.97 (6)	Cu2xi—S4—Cu2	109.46 (7)
S3vii—Ge—S1iii	108.92 (5)	Geix—S4—Fe	112.60 (9)
S2—Ge—S1iii	107.64 (6)	Cu2xi—S4—Fe	105.12 (7)
S4iii—Ge—S1iii	106.34 (12)	Cu2—S4—Fe	107.68 (14)

Symmetry codes: (i) −x+3/2, y−1/2, −z+2; (ii) x, y−1, z; (iii) x−1/2, y−1/2, z; (iv) −x+3/2, y−1/2, −z+1; (v) −x+2, y, −z+1; (vi) x+1/2, y−1/2, z; (vii) x−1/2, y+1/2, z; (viii) −x+3/2, y+1/2, −z+2; (ix) x+1/2, y+1/2, z; (x) x, y+1, z; (xi) −x+3/2, y+1/2, −z+1.

Tetracopper cobalt digermanium heptasulfide (II). Crystal data

Cu4CoGe2S7	F(000) = 638
Mr = 682.69	Dx = 4.396 Mg m−3
Monoclinic, C2	Mo Kα radiation, λ = 0.71073 Å
a = 11.7280 (2) Å	Cell parameters from 4827 reflections
b = 5.33987 (10) Å	θ = 4.2–32.6°
c = 8.33133 (14) Å	µ = 16.76 mm−1
β = 98.6680 (12)°	T = 296 K
V = 515.80 (2) Å3	Irregular, grey
Z = 2	0.08 × 0.07 × 0.06 mm

Tetracopper cobalt digermanium heptasulfide (II). Data collection

Bruker SMART APEXII diffractometer	1039 reflections with I > 2σ(I)
φ and ω Scans scans	Rint = 0.017
Absorption correction: multi-scan (SADABS; Sheldrick, 2002)	θmax = 27.5°, θmin = 2.5°
Tmin = 0.356, Tmax = 0.435	h = −15→15
2239 measured reflections	k = −6→6
1177 independent reflections	l = −10→10

Tetracopper cobalt digermanium heptasulfide (II). Refinement

Refinement on F2	w = 1/[σ2(Fo2) + 3.9921P] where P = (Fo2 + 2Fc2)/3
Least-squares matrix: full	(Δ/σ)max < 0.001
R[F2 > 2σ(F2)] = 0.030	Δρmax = 0.88 e Å−3
wR(F2) = 0.075	Δρmin = −0.49 e Å−3
S = 1.11	Extinction correction: SHELXL2018/3 (Sheldrick 2015), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
1177 reflections	Extinction coefficient: 0.060 (3)
67 parameters	Absolute structure: Flack (1983)
1 restraint	Absolute structure parameter: 0.06 (3)
Primary atom site location: structure-invariant direct methods

Tetracopper cobalt digermanium heptasulfide (II). Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refined as a two-component inversion twin

Tetracopper cobalt digermanium heptasulfide (II). Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cu1	0.64303 (9)	0.2051 (4)	0.93412 (12)	0.0177 (7)
Cu2	0.71154 (9)	0.7108 (2)	0.64143 (12)	0.0158 (6)
Co	1.000000	0.7206 (7)	0.500000	0.0141 (8)
Ge	0.42703 (6)	0.7276 (4)	0.78209 (9)	0.0092 (4)
S1	1.000000	0.9762 (6)	1.000000	0.0090 (6)
S2	0.56898 (15)	0.9604 (5)	0.7167 (2)	0.0118 (6)
S3	0.78425 (15)	0.4533 (4)	0.8520 (2)	0.0092 (6)
S4	0.85742 (15)	0.9682 (5)	0.5803 (2)	0.0094 (5)

Tetracopper cobalt digermanium heptasulfide (II). Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
Cu1	0.0181 (6)	0.0182 (18)	0.0167 (6)	−0.0026 (5)	0.0023 (4)	0.0018 (6)
Cu2	0.0168 (6)	0.0131 (14)	0.0175 (6)	0.0004 (8)	0.0029 (4)	0.0014 (6)
Co	0.0118 (7)	0.021 (2)	0.0096 (7)	0.000	0.0028 (5)	0.000
Ge	0.0090 (5)	0.0100 (9)	0.0086 (5)	−0.0011 (7)	0.0012 (3)	−0.0019 (6)
S1	0.0102 (12)	0.0092 (18)	0.0072 (11)	0.000	−0.0001 (9)	0.000
S2	0.0101 (9)	0.0160 (15)	0.0097 (9)	−0.0022 (11)	0.0028 (7)	−0.0007 (9)
S3	0.0095 (9)	0.0076 (16)	0.0109 (9)	0.0012 (8)	0.0025 (7)	0.0017 (9)
S4	0.0113 (9)	0.0090 (13)	0.0076 (8)	−0.0008 (7)	0.0008 (7)	−0.0012 (10)

Tetracopper cobalt digermanium heptasulfide (II). Geometric parameters (Å, º)

Cu1—S3i	2.292 (2)	Co—S4v	2.308 (3)
Cu1—S2ii	2.295 (2)	Co—S4	2.308 (3)
Cu1—S3	2.304 (2)	Co—S2iv	2.325 (3)
Cu1—S1iii	2.343 (3)	Co—S2vi	2.325 (3)
Cu2—S3	2.290 (2)	Ge—S2	2.211 (3)
Cu2—S2	2.298 (3)	Ge—S3vii	2.211 (2)
Cu2—S4iv	2.301 (2)	Ge—S4iii	2.236 (2)
Cu2—S4	2.311 (2)	Ge—S1iii	2.316 (2)
S3i—Cu1—S2ii	109.37 (12)	Geviii—S1—Geix	109.14 (15)
S3i—Cu1—S3	111.62 (6)	Geviii—S1—Cu1viii	111.56 (4)
S2ii—Cu1—S3	107.25 (7)	Geix—S1—Cu1viii	110.41 (4)
S3i—Cu1—S1iii	112.08 (6)	Geviii—S1—Cu1ix	110.41 (4)
S2ii—Cu1—S1iii	109.73 (7)	Geix—S1—Cu1ix	111.56 (4)
S3—Cu1—S1iii	106.65 (13)	Cu1viii—S1—Cu1ix	103.68 (17)
S3—Cu2—S2	109.96 (7)	Ge—S2—Cu1x	109.59 (8)
S3—Cu2—S4iv	108.79 (11)	Ge—S2—Cu2	110.30 (13)
S2—Cu2—S4iv	111.31 (7)	Cu1x—S2—Cu2	109.95 (8)
S3—Cu2—S4	108.88 (8)	Ge—S2—Covii	109.91 (9)
S2—Cu2—S4	107.97 (11)	Cu1x—S2—Covii	108.57 (14)
S4iv—Cu2—S4	109.90 (5)	Cu2—S2—Covii	108.48 (8)
S4v—Co—S4	110.11 (19)	Gevi—S3—Cu2	109.57 (8)
S4v—Co—S2iv	107.45 (6)	Gevi—S3—Cu1viii	108.46 (8)
S4—Co—S2iv	112.63 (7)	Cu2—S3—Cu1viii	107.17 (11)
S4v—Co—S2vi	112.63 (7)	Gevi—S3—Cu1	111.81 (11)
S4—Co—S2vi	107.45 (6)	Cu2—S3—Cu1	111.81 (8)
S2iv—Co—S2vi	106.59 (18)	Cu1viii—S3—Cu1	107.84 (7)
S2—Ge—S3vii	112.75 (14)	Geix—S4—Cu2xi	107.42 (11)
S2—Ge—S4iii	111.49 (7)	Geix—S4—Co	111.98 (10)
S3vii—Ge—S4iii	109.30 (7)	Cu2xi—S4—Co	105.80 (7)
S2—Ge—S1iii	108.43 (7)	Geix—S4—Cu2	113.66 (9)
S3vii—Ge—S1iii	108.34 (6)	Cu2xi—S4—Cu2	109.24 (7)
S4iii—Ge—S1iii	106.29 (12)	Co—S4—Cu2	108.42 (14)

Symmetry codes: (i) −x+3/2, y−1/2, −z+2; (ii) x, y−1, z; (iii) x−1/2, y−1/2, z; (iv) −x+3/2, y−1/2, −z+1; (v) −x+2, y, −z+1; (vi) x+1/2, y−1/2, z; (vii) x−1/2, y+1/2, z; (viii) −x+3/2, y+1/2, −z+2; (ix) x+1/2, y+1/2, z; (x) x, y+1, z; (xi) −x+3/2, y+1/2, −z+1.

Funding Statement

This work was funded by National Science Foundation, Division of Materials Research grants 1611198, CHE-0234872, and DUE-0511444.