Low temperature synthesis and structures of alkaline earth metal chalcogenides Ba₃Cu₄SbS₆OH, BaCuSbS₃ and BaCu₂S₂

Abstract

For the first time, alkaline earth metal multinary chalcogenides Ba₃Cu₄SbS₆OH (1), BaCuSbS₃ (2) and BaCu₂S₂ (3) were synthesized via hydrothermal methods at low temperatures. The structures of these three compounds contain three-dimensional channeled frameworks that are constructed by different types of Cu/S chains.

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Alkaline earth metal multinary chalcogenides feature rich structures, which lead to various unique properties.¹ Compared to the alkali metal chalcogenides, studies on alkaline earth metal chalcogenides are considerably less despite their similar structures. Current synthesis methods for alkaline earth metal chalcogenides mainly rely on high temperature (>700 °C) solid state routes,^1,2 which are different from that for alkali metal chalcogenides. Commonly, for preparing the alkali metal chalcogenides, alkali metal polychalcogenides are adopted as fluxes, which can decrease the reaction temperature to an intermediate range (150–350 °C).³ In fact, the intermediate temperature range favors the formation of metastable phases.⁴ Therefore, to expand the number of alkaline earth metal chalcogenide products, it is important to obtain metastable phases by employing intermediate temperature ranges. On the other hand, hydrothermal methods using typically intermediate temperature synthesis techniques have proven to be successful for the preparation of multinary chalcogenide single crystals, containing mostly quaternary ammonium cations, amine ions, alkali metal ions, and transition metal complexes.⁵ To date, however, alkaline earth metal multinary chalcogenide syntheses using hydrothermal methods have not been reported. A successful route for the preparation of alkali metal chalcogenides by hydrothermal synthesis is by the addition of a proper counter ion source.⁶ The typical examples of counter ion sources are alkali metal carbonates. Nevertheless, it is difficult to imagine that alkaline earth metal carbonates, usually slightly soluble or insoluble in aqueous media, are suitable counter ion sources. Thus, in the absence of a good counter ion source, the hydrothermal preparation of alkaline earth metal chalcogenides has been hindered, and only a few compounds of this type have been reported so far.

Recently, we utilized KOH as a counter ion source and mineralizer for the synthesis of KCuMS₂ (M = Mn, Fe, Zn) with considerable success.⁷ Such successful preparation inspired us to use Ba(OH)₂ (soluble) as a Ba²⁺ source for the synthesis of some barium chalcogenides. As a result, single crystals of three barium chalcogenides, Ba₃Cu₄SbS₆OH, BaCuSbS₃ and BaCu₂S₂, were obtained. To the best of our knowledge, this is the first time that the preparation of alkaline earth metal multinary chalcogenides is realized by hydrothermal synthesis.

Compounds 1, 2 and 3 were originally obtained by identical reactions. Crystals with different morphologies were found by SEM (Fig. S1†) and optical microscope (Fig. S2†). EDS analysis showed the dark red cubic slab-like crystals and bright red rod-like crystals with different stoichiometric ratios. For details, see Fig. S3 and Table S1.† Single crystal X-ray diffraction data demonstrated that the slab-like and rod-like crystals were 1 and 2, respectively. The bundle-like shape phase was identified to be compound 3 (Fig. S4†). Compound 3 was purified when no Sb powder was added as the reagent. However, we failed to purify 1 and 2 by adding reagents according to the stoichiometric ratios of Cu and Sb. The powder XRD pattern of a relatively pure sample of 1 is shown in Fig. S5.†

Compound 1 crystallizes in the space group Pnma (no. 62).‡ This compound features a three-dimensional channeled framework with Ba atoms and OH⁻ ions located in the channels. Viewing along the b axis, the [Cu₄SbS₆]⁴⁻ network (Fig. 1a) is constructed by Cu₈S₁₂ chains interconnected by SbS₃ pyramids. The Cu₈S₁₂ chain (Fig. 1b) is made of butterfly-like Cu₄S₈ (Fig. 1c) units connected with each other along the b axis. Each butterfly-like unit is assembled by two edge-sharing CuS₄ tetrahedrons. One edge of each CuS₄ tetrahedron is bridged by a CuS motif (Fig. 1c with the Cu–S dumbbell pointing up). The butterfly-like units are aligned nearly linearly to form the main Cu₈S₁₂ chain, where one unit is pointing up and the next unit is alternately pointing down. The bridging Cu–S dumbbells are connected to the SbS₃ pyramids.

Fig. 1 (a) Three-dimensional channeled framework [Cu₄SbS₆]⁴⁻ of 1 viewed down from the b axis. (b) The Cu₈S₁₂ chains extended along the b axis. (c) Butterfly-like motif Cu₄S₈ in which the Cu(2)–Cu(2) bond length was marked.

The distances of Cu(1)–Cu(1), Cu(1)–Cu(2) and Cu(2)–Cu(2) are 3.0499 Å, 2.6279 Å and 2.7298 Å, respectively, indicating d¹⁰–d¹⁰ interactions between the Cu(1)–Cu(2) and Cu(2)–Cu(2) atoms. The bond lengths of the Cu–S bonds range from 2.2603 Å to 2.6812 Å compared to the sum of the covalent radii of Cu⁺ (1.32 Å) and S²⁻ (1.05 Å). The Sb–S bond lengths of 2.4619 Å and 2.4709 Å are close to the sum of the covalent radii of Sb³⁺ (1.39 Å) and S²⁻ (1.05 Å).

The channels where Ba atoms and OH⁻ ions are located are star-like objects if viewed along the [100] direction and form an S shape when viewed along the [010] direction (Fig. S6†). Channel shapes are related to the multi-membered rings in the framework. There are 20-membered rings and 24-membered rings in the structure (Fig. S7†). Unlike O atoms in other known barium oxysulfides, where O atoms substitute some S atoms in the coordination sphere of metal atoms to form the skeleton of the structure (examples include Ba₂Ge₂ZnOS₆ (ref. 8a) and Ba₆Ti₅OS₁₅ (ref. 8b)), the O atoms in 1 are bonded only to Ba atoms and are distant from other metal atoms. The treatment of H atoms was performed by difference Fourier synthesis. No hydrogen bonding interactions between the OH⁻ and the framework was found. FT-IR spectrum for compound 1 (Fig. S8†) showing no obvious hydroxyl peak indicated a weak vibrational mode of the O–H bond.

It is worth noticing that the bond angle of Cu(2)–Cu(2)–Cu(2) in the Cu(2)–Cu(2) chain is 169.1°, suggesting an approximately linear alignment. Similar chains are found in K_1.5Dy₂Cu_2.5Te₅ (ref. 9) and 3, where Cu–Cu–Cu angles are 104.2° and 96.7°, respectively. The Cu–Cu chain in 1 extends in [010], suggesting advantageous charge transportation alongside [010], while the three-dimensional channeled framework can imply poor thermal conductivity. If true, these factors could make 1 a potential thermoelectric material.

Compound 2 crystallizes in the space group Pbam (no. 55) possessing a three-dimensional channeled framework (Fig. 2a).§ Ba²⁺ ions are located near the edge of the channels, and are arranged alternately almost in line with the Sb atoms. The three-dimensional [CuSbS₃]²⁻ framework is mainly composed of vertex-shared CuS₄ tetrahedron chains and SbS₃ pyramid linkers along the c axis. The SbS₃ pyramids share vertices with the CuS₄ tetrahedra in the CuS₄ chains. In this manner, the CuS₄ chains are interconnected (Fig. 2b). The CuS₄ chain is an “Einer-einfachketten” in Liebau's nomenclature.¹⁰ Such a one-dimensional chain can also be found in Zintl phases. As expected, the Cu–S distances range from 2.3286 Å to 2.4768 Å. The Sb–S bond lengths of 2.4368 Å to 2.4733 Å are also rational.

Fig. 2 (a) Three-dimensional channeled framework [CuSbS₃]²⁻ of 2 viewed down from the c axis. (b) CuS₄ chains extended along the c axis. (c) Three-dimensional channeled framework of 3 viewed down from the b axis. (d) Two different types of CuS chains in the structure of 3 and the link between them.

The known compounds KZrCuS₃,^11a BaLnCuS₃ (ref. 11b) and NaCdSbS₃ (ref. 11c) are related to 2 in their stoichiometry but have different structures. All of them have a layered structure but crystallize into two kinds of structures. KZrCuS₃ and BaLnCuS₃ are of the same type but NaCdSbS₃ is different. The compound with the same anionic part, K₂CuSbS₃,¹² also features as a layered structure.

BaCu₂S₂ crystallizes in a low temperature orthorhombic form (α)^13a and a high-temperature tetragonal form (β).^13b In this work, hydrothermal synthesis, a low temperature method, was successfully used to obtain compound 3. Compound 3 crystallizes in the space group Pnma (no. 62),¶ which agrees with the low temperature orthorhombic form mentioned in Prof. Park's work,^13c as demonstrated by the single crystal data. Compound 3 holds a three-dimensional network with Ba²⁺ ions residing in the channels (Fig. 2c). The [Cu₂S₂]²⁻ network is established with arrays of two different CuS chains linked together. Both the CuS chains are formed by edge-sharing CuS₄ tetrahedrons extending along the b axis (Fig. 2d). One of the CuS chains consists of Cu(1) and S(2) atoms, while the other one consists of Cu(2) and S(1) atoms. The distances of Cu(1)–Cu(1) and Cu(2)–Cu(2) are 3.2364 Å and 2.6990 Å, respectively. The short distance is a reflection of the strong bonding interaction between the Cu(2) atoms in the adjacent Cu(2)S₄ tetrahedrons, resulting in a zigzag formation of Cu(2) atoms, similar to that of the Cu(2) atoms in compound 1.

XRD analysis revealed that a pure phase of 3 was obtained (Fig. 3a). The single crystal morphology of 3 was rod-like, the same as 2, as shown in the insert of Fig. 3a. UV-vis spectrum (Fig. 3b) and the plot of (αhν)² as a function of hν (Inset, Fig. 3b) indicated that the band gap of 3 was 2.25 eV, which was in agreement with the orange-yellow color of the crystal.

Fig. 3 (a) Experimental and simulated powder XRD patterns of 3. The insert shows the single crystal SEM image of 3. (b) UV-vis absorption spectrum of 3. The insert shows the plot of (αhν)² as a function of hν.

The shortest Cu–Cu distances are 2.6279 Å and 2.6990 Å observed in 1 and 3, respectively. These can be compared to the Cu–Cu distance of 2.56 Å observed for elemental Cu. All the three compounds feature three-dimensional channeled frameworks built by different forms of Cu/S chains. The SbS₃ pyramids interconnect the chains to form the framework. Although elements of 1 and 2 are similar, their Raman spectra are very different, indicating different bond vibrational modes of the compounds (Fig. S9†). The density of states (DOS) and band structure calculation results obtained by the self-consistent linear muffin-tin orbital local density approximation method (LDA) showed 2 to be a semiconductor (Fig. S10†).

In summary, by employing soluble alkaline earth metal hydroxides under hydrothermal conditions at low temperatures (200 °C), three alkaline earth metal multinary chalcogenides Ba₃Cu₄SbS₆OH, BaCuSbS₃ and BaCu₂S₂ have been successfully synthesized. The results demonstrate that it is feasible to prepare alkaline earth metal multinary chalcogenides more readily at low temperatures.

This work is financially supported by the NSF of China (Grant no. 91122034, 51125006, 51102263, 61076062, 21101164, 61106088, 11274328, 61205177, 21201012), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant no. XDB04040200) and the Science and Technology Commission of Shanghai (Grant no. 12XD1406800, 12JC1409000).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experiment details, SEM&EDS analysis, powder XRD patterns, Raman spectra, DOS calculation results and additional figures. CCDC 957372–957374. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ra46878j

‡ The crystal data were collected on a Bruker Smart-1000 CCD diffractometer with graphite-monochromatic Mo Kα radiation (λ = 0.71073 Å) at 293 K. An empirical absorption correction was applied using the SADABS program. The structure was solved with SHELXS-97 and the refinement was done using SHELXL-97. Crystal data for Ba₃Cu₄SbS₆OH: Pnma, a = 9.0609(4) Å, b = 10.8497(5) Å, c = 13.9398(6) Å, V = 1370.39(11) ų, Z = 4, D_c = 4.834 g cm⁻³, μ = 17.372 mm⁻¹, θ = 3.27–28.78°, 1871 total reflections, 1186 unique reflection, refinement on F², GOF = 1.010, 78 parameters, R₁ = 4.70%, wR₂ = 8.26% for I > 2σ(I). CCDC 957373.

§ Crystal data for BaCuSbS₃: Pbam, a = 11.6427(6) Å, b = 12.3716(5) Å, c = 8.4326(4) Å, V = 1214.62(10) ų, Z = 8, D_c = 4.581 g cm⁻³, μ = 15.179 mm⁻¹, θ = 3.41–28.78°, 1695 total reflections, 1078 unique reflection, refinement on F², GOF = 0.946, 64 parameters, R₁ = 4.05%, wR₂ = 5.84% for I > 2σ(I). CCDC 957374.

¶ Crystal data for BaCu₂S₂: Pnma, a = 9.2458(9) Å, b = 4.0389(5) Å, c = 10.3523(10) Å, V = 386.58(7) ų, Z = 4, D_c = 5.646 g cm⁻³, μ = 21.806 mm⁻¹, θ = 3.94–29.01°, 581 total reflections, 433 unique reflection, refinement on F², GOF = 1.087, 31 parameters, R₁ = 2.80%, wR₂ = 6.15% for I > 2σ(I). CCDC 957372.

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