Darstellung,Kristallstruktur und Eigenschaften des Kupfer(II)‐Indium(III)‐Orthophosphats Cu3In2[PO4]4

Contributions on Crystal Chemistry and Thermal Behaviour of Anhydrous Phosphates. XXIV. Preparation, Crystal Structure, and Properties of Copper(II) Indium(III) Orthophosphate Cu₃In₂[PO₄]₄ Crystals of Cu₃In₂[PO₄]₄ were grown by chemical vapour transport (temperature gradient 1273 K → 1173 K) using chlorine as transport agent. The mixed metal phosphate forms a new structure type (P2₁/c, Z = 2, a = 8.9067(6), b = 8.8271(5), c = 7.8815(5) Å, β = 108.393(5)°, 13 atoms in asymmetric unit, 2549 unique reflections with F_o > 4σ, 116 variables, R(F²) = 0.065). The crystal structure shows a hexagonal closest packing of [PO₄]^3– tetrahedra. Close‐packed layers parallel (1 0 –1) are stacked according to the sequence A, B, A′, B′, A. The octahedral interstices in this packing are completely occupied by two In³⁺, one (Cu1)²⁺ and a “dumb bell” (Cu2)₂⁴⁺. In the latter case four of the six phosphate groups that belong to this octahedral void act as bi‐dentate ligands, thus forming dimers [(Cu2)₂O₁₀] with d_Cu–Cu = 3.032 Å. Cu₃In₂[PO₄]₄ is paramagnetic (μ_eff = 1.89 μ_B, θ_P = –16.9 K). The infrared and UV/Vis reflectance spectra are reported. The observed d‐electron levels of the Cu²⁺ cations agree well with those obtained from angular overlap calculations.     

Phase relations and structural properties of the ternary narrow gap semiconductors Zn5Sb4In2−δ (δ=0.15) and Zn9Sb6In2

A systematic study of the Zn-rich corner of the ternary system Zn-Sb-In revealed the presence of two ternary compounds: stable Zn₅Sb₄In_2−δ (δ=0.15) and metastable Zn₉Sb₆In₂ with closely related crystal structures. Their common motif is a tetragonal basic structure of 3²434 nets formed by the Sb atoms. The nets are stacked in antiposition to yield layers of square antiprisms sharing edges plus intervening tetracapped tetrahedra (tetreadersterns). The majority of Zn atoms occupy peripheral tetrahedra of such tetraedersterns, which produces frameworks with a composition “ZnSb”. These frameworks represent orthorhombic superstructures: (2×1×1) for Zn₅Sb₄In_2−δ (Z=4) and (2×3×1) for Zn₉Sb₆In₂ (Z=8) with respect to the tetragonal arrangement of Sb atoms. The In and remaining Zn atoms are distributed in the channels formed by the square antiprisms. Phase relations in the Zn-Sb-In system are complex. Crystals of metastable Zn₉Sb₆In₂ are regularly intergrown with various amounts of Zn₅Sb₄In_2−δ. Additionally, a monoclinic variant to orthorhombic Zn₉Sb₆In₂ could be identified. Zn₉Sb₆In₂ decomposes exothermically into a mixture of Zn₅Sb₄In_2−δ, Zn₄Sb₃ and elemental Zn at around 480 K. Both Zn₅Sb₄In_2−δ and Zn₉Sb₆In₂ are poor metals with resistivity values that are characteristic of heavily doped or degenerate semiconductors (0.2−3 m Ω cm at room temperature).     

Structure and Bonding in [Sb@In8Sb12]3− and [Sb@In8Sb12]5−

We report the characterization of the compound [K([2.2.2]crypt)]₄[In₈Sb₁₃], which proves to contain a 1:1 mixture of [Sb@In₈Sb₁₂]^3? and [Sb@In₈Sb₁₂]^5?. The tri‐anion displays perfect T_h symmetry, the first completely inorganic molecule to do so, and contains eight equivalent In³⁺ centers in a cube. The gas‐phase potential energy surface of the penta‐anion has eight equivalent minima where the extra pair of electrons is localized on one In⁺ center, and these minima are linked by low‐lying transition states where the electron pair is delocalized over two adjacent centers. The best fit to the electron density is obtained from a model where the structure of the 5? cluster lies close to the gas‐phase transition state.     

Dipotassium Sodium Diantimonidoindate,K2Na[InSb2], a compound with the polyanion [In2Sb2Sb4/2]6−

The novel compound K₂Na[InSb₂] was synthesized from the elements at 900 K in sealed niobium ampoules. The compound forms plate-like crystals with silver metallic luster, which are very unstable in air and moisture. The crystal structure of K₂NaInSb₂ has been determined using single-crystal X-ray diffraction methods (space group Cmca (No. 64); a = 14.032(2), b = 16.399(3), c = 7.009(1) Å; Z = 8; Pearson symbol oC48). The structure contains pairs of edge-sharing InSb₄ tetrahedra which are linked to four other pairs via common vertices and form a two-dimensional [In₂Sb₂Sb_4/2]^6? anionic partial structure. The resulting pairs of tetrahedral holes are filled by Na⁺ cations. These [In₂Sb₂Sb_4/2]^6? layers are stacked along the b-axis and are interconnected by K⁺ cations. The whole structure can be considered as an ordered derivative of the KMnP structure (PbFCl type).     

A New Compound in the Cu‐In System – the Synthesis and Structure of Cu10In7

A new binary phase, Cu₁₀In₇, was found during the investigation of the η‐phase field in the Cu‐In system. Single crystals of Cu₁₀In₇ were grown from a melt under an inert atmosphere. The compound crystallizes in the monoclinic space group C2/m with cell parameters a = 13.8453(2) Å, b = 11.8462(1) Å, c = 6.7388(1) Å and β = 91.063(1). The structure is based on a unit of face‐sharing octahedra consisting of five Cu₄In₂ octahedra terminated by Cu₅In octahedra at both ends. The crystal structure is closely related to the Cu₁₁In₉ structure type.     

Reaction mechanism and thermoelectric properties of In0·22Co4Sb12 prepared by magnesiothermy

The magnesioreduction synthesis of In_0·22Co₄Sb₁₂ with high In rattler concentration from Sb₂O₄ and In-doped Co₃O₄ precursors is reported. This process directly yields a submicronic powder in a single step of 96 h at 810 K. The reaction mechanism has been investigated by stopping the reaction every 12 h and quantifying the existing phases by X-ray diffraction and Rietveld refinements. The precursors are first reduced in CoO and Sb₂O₃ lower oxides, then form CoSb₂O₆ and CoSb₂O₄ intermediates which are finally reduced in In_xCo₄Sb₁₂. A powder with 350 nm average size and mostly composed of In-filled skutterudite phase with composition close to In_0·17Co₄Sb₁₂ is obtained. Upon spark plasma sintering, small residual amount of InSb reacts with the skutterudite matrix to form a single-phase densified pellet with composition close to In_0·22Co₄Sb₁₂. The resulting densified material with 1.8 μm average grain size shows a figure of merit ZT_max of 0.95 at 750 K.     

Preparation and properties of two indium antimony selenides

Crystals of antimony-doped In₂Se₃ were grown by the Bridgeman method. This compound, whose composition is In_1.8Sb_0.2Se₃, appears to be isostructural with In_1.9As_0.1Se₃. The refined unit cell parameters are a = 3.97(1), c = 18.87(1) Å. Orthorhombic crystals of InSbSe₃ were grown from an isothermal melt. The refined unit cell parameters are a = 9.43(1), b = 14.02(5), and c = 3.96(1) Å. These parameters agree with those determined for α-InSbSe₃ by other studies. The observed densities measured by a hydrostatic technique are 5.98(3) g/cm³ for In_1.8Sb_0.2Se₃ and 6.07(2) g/cm³ for InSbSe₃. The room temperature dc resistivity for In_1.8Sb_0.2Se₃ has been found to be 4.4 × 10⁴ Ω-cm, whereas that of InSbSe₃ has been found to be 15.2(1) Ω-cm. A resistivity versus temperature study has beenn carried out for InSbSe₃ between 230 and 400°K. Optical studies indicate that In_1.8Sb_0.2Se₃ is an n-type semiconductor with a band gap of 1.1 eV and InSbSe₃ is a p-type semiconductor with a band gap of 0.92 eV.     

Celluloseaustauscher mit Pyrogallol als Ankergruppe; Abtrennung von Antimon(III)

Zusammenfassung Die Eigenschaften eines Celluloseaustauschers mit Pyrogallol als Ankergruppe werden beschrieben. Der Austauscher eignet sich für die Trennung von Sb^(III) und Sb^(V). Verteilungskoeffizienten für Cu²⁺, Fe³⁺, Sb³⁺, UO ₂ ²⁺ , und Zn²⁺ als Funktion des pH-Wertes in Abwesenheit und in Anwesenheit von 0,5 M NaCl werden angegeben. Der Austauscher eignet sich auch für die Abtrennung anderer Schwermetallionen (Cr³⁺, Cu²⁺, Fe³⁺, Ni²⁺, Pb²⁺, UO ₂ ²⁺ , Zn²⁺) aus Wasser. Dies wird durch die Analyse von Leitungswasser belegt.

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Cellulose exchangers with pyrogallol as Anchor Group. Separation of antimony(III)

Summary The properties of a cellulose exchanger containing pyrogallol as anchor group are described. This exchanger permits separation of Sb^(III) and Sb^(V). Distribution coefficients for Cu²⁺, Fe³⁺, Sb³⁺, UO ₂ ²⁺ and Zn²⁺ as function of pH in absence and in presence of 0.5 M NaCl are given. The exchanger can also be used for separation of other heavy metal ions (Cr³⁺, Cu²⁺, Fe³⁺, Ni²⁺, Pb²⁺, UO ₂ ²⁺ , Zn²⁺) from water. This is checked by the analysis of tap water.

Frau Dr. Tscholakowa dankt dem DAAD für die Gewährung eines Stipendiums. Außerdem danken wir dem BMFT für die finanzielle Unterstützung der Forschungsarbeit.     

K21–δNa2+δIn39 (δ = 2.8): A Cluster-Replacement Clathrate-II Structure with an Alkali Metal M136-Network

K_21–δNa_2+δIn₃₉ with δ = 2.82 was synthesized (melted at 973 K, annealed at 623 K) from the elements in sealed niobium ampoules. The compound forms prismatic crystals with silver metallic lustre and is unstable in air and moisture. The crystal structure of K_21–δNa_2+δIn₃₉ (orthorhombic; space group Pnma, No. 62; a = 17.844(5) Å, b = 17.192(3) Å, c = 25.078(7) Å; Z = 4; Pearson code oP248; δ = 2.82, obtained from the structure refinement) contains eight empty In icosahedra of two types, A (12 exo-bonds) and B (7 exobonds), and four open In₁₅ clusters (15 exo-bonds). The latter are centered by K atoms and belong to C units, which are defined as [K(Na₂M₃In₁₅)] heteroatomic clusters (M = K + Na). The spatial distribution of the In icosahedra A, B and heteroatomic clusters C is that of the atoms in the cubic Laves phase MgCu₂: MgCu₂ ? [K(Na₂M₃In₁₅)][In]₂. All the In_n clusters are interconnected by exo-bonds forming a covalent three-dimensional framework (d(In? In) = 2.832 to 3.301 Å). The remaining alkali metal atoms build up a three-dimensional M₁₃₆ network of the clathrate-II type with (16 + 8) cages, which envelopes the In icosahedra and [K(Na₂M₃In₁₅)] clusters. This structure can be described as a cluster-replacement derivative of the clathrate-II structure: (H₂S)₁₆(CCl₄)₈ · (H₂O)₁₃₆ ? [In]₁₆[K(M₅In₁₅)]₈M₁₃₆, and is one member of a novel hierarchical structure family, based upon cluster-replacement. The bonding as well as the structural relationships to other phases are discussed.     

Composition‐Tunable Antiperovskite CuxIn1−xNNi3 as Superior Electrocatalysts for the Hydrogen Evolution Reaction

A group of newly reported antiperovskite nitrides Cu_xIn_1?xNNi₃ (0≤x≤1) with tunable composition are employed as electrocatalysts for the hydrogen evolution reaction (HER). Cu_0.4In_0.6NNi₃ shows the highest intrinsic performance among all developed catalysts with an overpotential of merely 42 mV at 10 mA cm_geo^?2. Stability tests at a high current density of 100 mA cm_geo^?2 show its super‐stable performance with only 7 mV increase in overpotential after more than 60 hours of measurement, surpassing commercial Pt/C (increase of 170 mV). By partial substitution, the derived antiperovskite nitride achieves a smaller kinetic barrier of water dissociation compared to the unsubstituted InNNi₃ and CuNNi₃, revealed by first‐principle calculations. It is found that the partially substituted Cu_xIn_1?xNNi₃ possesses a thermal neutral and desirable Gibbs free energy of hydrogen for HER, ascribed to the tailoring of the energy of d‐band center arose by the A‐site (A=Cu or In) substitution and a resulting optimization of adsorbate interactions.     

Cd13−xInySb10 (x≈2.7, y≈1.5): An Interstitial‐Free Variant of Thermoelectric β‐Zn4Sb3

Although free from structural disorder , the new intermetallic compound Cd_13?xIn_ySb₁₀ (see figure) displays similarly low thermal conductivity values as disordered thermoelectric β‐Zn₄Sb₃ with an isostructural framework.

     

Synthesis and crystal structure analysis of Sr8Cu3In4N5 and Sr0.53Ba0.47CuN

Single crystals of new quaternary compounds Sr₈Cu₃In₄N₅ and Sr_0.53Ba_0.47CuN were prepared, respectively, from a Sr–Cu–In–Na melt under 7 MPa of N₂ and from a Sr–Ba–Cu–In–Na melt under 0.5 MPa of N₂ by slow cooling from 1023 to 823 K. The crystal structures were determined by single-crystal X-ray diffraction. Sr₈Cu₃In₄N₅ has an orthorhombic structure (space group, Immm, Z=2, a=3.8161(5) Å, b=12.437(2) Å, c=18.902(2) Å), and is isostructural with Ba₈Cu₃In₄N₅. It contains nitridocuprates of isolated units ⁰[CuN₂] and one-dimensional linear chains _∝¹[CuN_2/2] and one-dimensional indium clusters _∝¹[In₂In_2/2]. Sr_0.53Ba_0.47CuN crystallizes in an orthorhombic cell, space group Pbcm, Z=4, a=5.4763(7) Å, b=9.2274(12) Å, c=9.0772(12) Å. The structure contains infinite zig-zag chains _∝¹[CuN_2/2] which kink at every second nitrogen atom.     

Solid-state reactions in the system Cu—Sb—O; Formation of a new copper(I) antimony oxide

The solid-state reactions in the system Cu—Sb—O were investigated by thermogravimetry and X-ray diffraction. Equimolar mixtures of CuO and Sb₂O₃ form Cu(II)Sb₂O₆ when slowly heated in air up to 1000°C. The firt step in this reaction is the oxidation of Sb₂O₃ to Sb₂O₄ at 380–500°C, followed by further oxidation of Sb₂O₄ and the formation of CuSb₂O₆ at 500–1000°C. Thermal decomposition of CuSb₂O₆ in a flowing nitrogen atmosphere occurs in three stages; the first, with an activation energy of 356 kJ mole^?1, results in the formation of a new copper(I) antimony oxide, with a composition of Cu₄SbO_4.5, as determined by atomic absorption analysis and X-ray fluoresecence. Confirmation of predominantly monovalent copper and pentavalent antimony in the new compound was by ESR and ESCA, respectively. Two forms of Cu₄SbO_4.5 have been distinguished; one of these (form II) has a structure of lower symmetry, and decomposes when heated in air at 600°C to a mixture of CuO and another new copper antimony oxide, as yet uncharacterized. On further heating to 1100°C in air, Cu₄SbO_4.5 (form I) gradually reforms. Details of these reactions are summarized and X-ray powder data presented for Cu₄SbO_4.5.     

ChemInform Abstract: Experimental and Theoretical Investigations for Site Preference and Anisotropic Size Change of RE11Ge4In6‐xMx (RE: La,Ce; M: Li,Ge; x = 1, 1.96).

La₁₁Ge₄In_5.00Li_1.00 and Ce₁₁Ge_5.96In_4.04 are prepared from the elements (Nb tubes, 1080 °C for 5 h and 750 °C for 2 d) and characterized by powder and single crystal XRD, and TB‐LMTO‐ASA computations.     

Gd3Pt4In12 and Tb3Pt4In12 — New Ternary Indides with Condensed Distorted [PtIn6] Trigonal Prisms

The ternary indium compounds Gd₃Pt₄In₁₂ and Tb₃Pt₄In₁₂ were synthesized from an indium flux. Arc‐melted precursor alloys with the starting compositions ∼GdPtIn₄ and ∼TbPtIn₄ were annealed with a slight excess of indium at 1200 K followed by slow cooling (5 K/h) to 870 K. Both compounds were investigated by X‐ray powder diffraction: a = 990.5(1), c = 1529.5(3) pm for Gd₃Pt₄In₁₂ and a = 988.65(9), c = 1524.0(1) pm for Tb₃Pt₄In₁₂. The structure of the gadolinium compound was solved and refined from single crystal X‐ray data: P3¯m1, wR2 = 0.0470, 1469 F² values and 62 variable parameters. Both crystallographically different platinum sites have a slightly distorted trigonal prismatic indium coordination. These [PtIn₆] prisms are condensed via common edges and corners forming a complex three‐dimensional [Pt₁₂In₃₂] network. The gadolinium, In1 and In7 atoms fill cavities within this polyanion. Tb₃Pt₄In₁₂ is isotypic with the gadolinium compound.     

ChemInform Abstract: Barium Dierbium(III) Tetrasulfide.

Orange needles of BaEr₂S₄ are obtained during an investigation of the Ba/Er/U/S system by solid state reaction of a mixture of ²³⁸U, Er, BaS, and S in the ratio 1:1:3:3 (evacuated C‐coated fused silica tube, 1273 K, 8 d, cooling rate 3 K/h) together with black crystals of Sb₂S₃ and US₂.     

High ammonia sensitive ability of novel Cu12Sb4S13 quantum dots@reduced graphene oxide nanosheet composites at room temperature

In the work, rGO nanosheet is synthesized using the typical Hummer’s method, then Cu₁₂Sb₄S₁₃ quantum dots@rGO composites are prepared by solvent thermal method, and Cu₁₂Sb₄S₁₃ quantum dots with the average size of 5 nm are densely distributed on the surface of rGO sheet. NH₃ gas response of Cu₁₂Sb₄S₁₃ quantum dots@rGO nanosheet composites at room temperature of 25 °C is enhanced compared with the pure Cu₁₂Sb₄S₁₃ quantum dots and rGO nanosheet, and the composites possess an excellent stability during the humidity range of 45%–80% with a low detection limit of 1 ppm, which is related with the intrinsic hydrophobicity characteristic of Cu₁₂Sb₄S₁₃ quantum dots. It also proves that Cu₁₂Sb₄S₁₃ quantum dots@rGO nanosheet composites have a quite high selectivity towards ammonia compared with ethanol, methanol, acetone, toluene, hydrogen sulfide and nitrogen dioxide at room temperature. The gas sensing mechanism of the composites is discussed primarily.     

Synthesis and Structure of Ba8Cu3In4N5 with Nitridocuprate Groups and One-Dimensional Infinite Indium Clusters

Single crystals of the quaternary compound Ba₈Cu₃In₄N₅ were prepared by heating Ba, Cu, and In in a Na flux at 1023 K under 7 MPa of N₂, and by slow cooling from this temperature. The crystal structure was analyzed by single-crystal X-ray diffraction. It crystallizes in an orthorhombic cell (space group Immm (No. 71), Z=2) with a=4.0781(6), b=12.588(2), and c=19.804(3) Å at 298 K. The structural formula is expressed as Ba₈[CuN₂]₂ [CuN]In₄. Nitridocuprates of one-dimensional chains ¹_∞[CuN_2/2] and isolated units ⁰[CuN₂], and one-dimensional indium clusters ¹_∞[In₂In_4/2] are contained in the structure. A split-site model applied for the arrangement of ¹_∞[CuN_2/2] chains suggested that there is a short-bond, long-bond alternation of the Cu-N bondings. The electrical resistivity of Ba₈Cu₃In₄N₅ was 3.44 mΩ·cm at 298 K. A metallic temperature dependence of the resistivity was observed down to 10 K.     

Ternary Indides RE4Pd10In21 (RE = La,Ce, Pr,Nd, Sm) — Synthesis,Structure, and Physical Properties

The ternary indium compounds RE₄Pd₁₀In₂₁ (RE = La, Ce, Pr, Nd, Sm) were synthesized from the elements in glassy carbon crucibles in a high‐frequency furnace. Single crystals of Sm₄Pd₁₀In₂₁ were obtained from an indium flux. An arc‐melted precursor alloy of the starting composition ～SmPd₃In₆ was annealed with a slight excess of indium at 1200 K followed by slow cooling (5 K/h) to 870 K. All compounds were investigated by X‐ray powder diffraction and the structures were refined from single crystal diffractometer data. The RE₄Pd₁₀In₂₁ indides are isotypic with Ho₄Ni₁₀Ga₂₁, space group C2/m: a = 2314.3(2), b = 454.70(7), c = 1940.7(2) pm, β = 133.43(2)°, wR2 = 0.0681, 1678 F² values for La₄Pd₁₀In₂₁, a = 2308.2(1), b = 452.52(4), c = 1944.80(9) pm, β = 133.40(1)°, wR2 = 0.0659, 1684 F² values for Ce₄Pd₁₀In₂₁, a = 2303.8(2), b = 450.78(4), c = 1940.6(1) pm, β = 133.39(1)°, wR2 = 0.0513, 1648 F² values for Pr₄Pd₁₀In₂₁, a = 2300.2(2), b = 449.75(6), c = 1937.8(2) pm, β = 133.32(1)°, wR2 = 0.1086, 1506 F² values for Nd₄Pd₁₀In₂₁, and a = 2295.6(2), b = 447.07(4), c = 1935.7(1) pm, β = 133.16(1)°, wR2 = 0.2291, 2350 F² values for Sm₄Pd₁₀In₂₁, with 108 variables per refinement. All palladium atoms have a trigonal prismatic coordination. The strongest bonding interactions occur for the Pd—In and In—In contacts. The structures are composed of covalently bonded three‐dimensional [Pd₁₀In₂₁] networks in which the rare earth metal atoms fill distorted pentagonal channels. The crystal chemistry and chemical bonding in these indides is briefly discussed. Magnetic susceptibility measurements show diamagnetism for La₄Pd₁₀In₂₁ and Curie‐Weiss paramagnetism for Ce₄Pd₁₀In₂₁, Pr₄Pd₁₀In₂₁, and Nd₄Pd₁₀In₂₁. The neodymium compound orders antiferromagnetically at T_N = 4.5(2) K and undergoes a metamagnetic transition at a critical field of 1.5(2) T. All the RE₄Pd₁₀In₂₁ indides studied are metallic conductors.     

Refinement of the crystal structure of as-schwatzite Cu<Subscript>6</Subscript>(Cu<Subscript>5.26</Subscript>Hg<Subscript>0.75</Subscript>)(As<Subscript>2.83</Subscript>Sb<Subscript>1.17</Subscript>)S<Subscript>13</Subscript> (Aktash,Altai mountaines)

The crystal structure of As-schwatzite Cu₆(Cu_5.26Hg_0.75)(As_2.83Sb_1.17)S13 (Aktash deposit, Altai mountains) is refined. Tetrahedrally shaped dark-gray single crystals of the mineral belong to the cubic crystal system: I4¯3m space group, a = 10.2890(1) Å, V = 1089.2(1) ų, d = 4.99 g/cm³, Z = 2 for the composition Cu_11.26Hg_0.75As_2.83Sb_1.17S₁₃, R = 0.0177. The structure is based on the sphalerite-like framework comprising identically oriented (Cu,Hg)S₄ tetrahedra ((Cu,Hg)-S 2.3452(8) Å) and (As,Sb)S₃ pyramids ((As,Sb)-S 2.311(1) Å) sharing their vertices. The centers of [Cu₆] octahedra in the (000) and (1/2 1/2 1/2) positions coinciding with the centers of the “cluster” anionic vacancies [□]₄ are occupied by the so-called “thirteenth” sulfur atom. Quantum chemical calculations of the electron density are carried out for the [As₄S₁₃Cu₆]⁶ fragment. The calculation results confirm the presence of strain in the [As₄S₁₃Cu₆]⁶ moiety, which exists due to the support of the surrounding symmetric framework including the external sulfur atoms of the fragment. The possibility of inclusion of mercury into the framework, which is much richer in arsenic than in antimony, is demonstrated. High stability of the framework determines significant compression of the S-centered [SCu₆] octahedron in its interstices, bringing together copper atoms to 3.145(1) Å and shortening the Cu-S distances to 2.224(1) Å