Mixed anion compounds, especially their unique structures and excellent physical properties, have been extensively studied and have great applications in military and civilization. They have attracted abundant attention since the difference of electronegativities, ionic radii, polarizabilities, and oxidation states between unlike anions can generate the emergence of novel properties that differ from those with mono-type anion^[1,2,3,4]. The active research of metallic mixed-anion materials with potential application in electronics, detectors of moisture, gas sensors, electrodes for solar batteries, etc. has been realized in several types of crystalline and thin film materials^[5]. It is meaningful to search a suitable method to synthesize aforementioned compounds. The main hotpot to research this kind of compounds lies in how to control the arrangement of anions to refine their electronic structures, such as the two-dimensional quantum antiferromagnetism in Sr₂CuO₂Cl₂ with trans-configuration of Cl ions in the CuO₄Cl₂ octahedra^[6].

Recently, it has been reported that incorporation of halides into oxides can strikingly change their electronic structures and modify the physical properties^[7,8]. Several novel transition-metal oxychlorides have been reported to date, such as MnSb₄O₆Cl₂^[9], PbCu₂(SeO₃)₂Cl₂^[10], Cu₃Bi(SeO₃)₂O₂Cl^[11], FeTe₂O₅X (X=Cl, Br)^[12], SrCu₂(SeO₃)₂Cl₂^[13], SmSb₂O₄Cl^[14], and MSb₂O₃(OH)Cl (M=Co, Fe, Mn)^[15]. Above-mentioned materials show novel structures and special magnetic properties due to their diversity structural which was more helpful to generate the magnetic ordering during low temperature. This was especially manifested in layered transition metal oxyhalide FeTe₂O₅X (X=Br, Cl), where the layers are built by [FeO₆] octahedra, and then forming the [Fe₄O₁₆]^20- units which were linked via [Te₄O₁₀X₂]^6- anionic groups. The magnetic properties are reported within a cluster approach of antiferromagnetically coupled tetramers including spin frustration and a ferromagnetic inter-tetramer interaction^[12].

The flux method widely applied for mixed-anion crystal growth is based on metathetical reaction with appropriate metal-salts flux under mild conditions. Experimentally, zone melting and hydrothermal synthesis methods are all comparatively complex and expensive for growing single crystals, and the flux method is at present one of the most economic and convenient methods for mixed anions compounds^[16]. In this study, we report the single crystal growth of tungsten oxychloride Li₂₃CuW₁₀O₄₀Cl₅ in copper chloride (CuCl₂, melting point of 498 ℃) flux, and analyze its crystal in details.

Reagents were used as received: Li₂CO₃ (Macklin, 99.99%), WO₃ (Aladdin, 99.99%), and CuCl₂ (Macklin, 98%).

The tungsten oxychloride Li₂₃CuW₁₀O₄₀Cl₅ was synthesized through flux method in open-end quartz tubes. And the synthesis of Li₂₃CuW₁₀O₄₀Cl₅ adopt a two-step process. Firstly, precursor polycrystalline Li₄WO₅ were synthesized by reacting high-purity reagents in solid-state method. Li₂CO₃ and WO₃ were mixed with the mole ratio of 2:1. Then these raw materials were placed in a covered alumina crucible and calcinated at 890 ℃ for 12 h as described in reference^[17]. Then, single crystal of Li₂₃CuW₁₀O₄₀Cl₅ was grown from CuCl₂ flux and precursor Li₄WO₅. Li₄WO₅ polycrystalline samples and ten times excess CuCl₂ was loaded into an open-end quartz tube, and put into a vertical pit furnace. The raw mixture was heated at 873 K for 2 d, and then cooling down to 573 K with a cooling rate of 5 K/h before shutting down the furnace.

Finally, the reaction products were washed by hot demineralized water to eliminate the fluxing agents. After subsequent drying at 353 K, yellow, block shaped single crystals of the desired products were grown and suitable for subsequent single-crystal X-ray diffraction measurements.

Block-shaped single crystal of Li₂₃CuW₁₀O₄₀Cl₅ was selected for single-crystal diffraction measurements. The R-AXIS Spider CCD diffractometer were used to collect data equipped with the graphite monochromated Mo Kα radiation (λ=0.071073 nm) at 293 K.

The structure of Li₂₃CuW₁₀O₄₀Cl₅ was determined through direct method and refined by full-matrix least-squared methods on F² with SHELXL package^[18]. ADDSYM/PLATON was performed to studied with the final structure for additional symmetry, and no other missed or higher symmetry was found^[19]. Crystallographic data (including structure factors) for the structures in this study have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the depository numbers CCDC-1952905.

The tungsten oxychloride Li₂₃CuW₁₀O₄₀Cl₅ synthesized from CuCl₂ flux in a vertical pit furnace crystallizes hexagonally in space group of P6₃/mcm with the unit cell parameters of a=1.02846(3) nm, c=1.98768(9) nm, and Z=2. There are crystallographically independent five Li, two W, one Cu, two Cl, and five O atoms in the unit cell, respectively, where W(1) atoms are coordinated with one Cl and five O atoms in a distorted octahedra geometry, while W(2) atoms are connected with four O atoms in a tetrahedral coordination. The Cu atoms are connected with six O atoms forming [CuO₆] octahedra.

Every two W(2)O₄ tetrahedra are in reverse symmetry along the c direction (Fig. 1 and 3). Three [W(1)O₅Cl] octahedra are connected with each other via sharing one Cl atom, and three O atoms with [CuO₆] octahedra to form the [W(1)₆CuO₂₄Cl₂] unit (Fig. 2). The three-dimensional (3D) structure of Li₂₃CuW₁₀O₄₀Cl₅ is assembled by the [W(1)₆CuO₂₄Cl₂] units sharing O and Cl atoms with Li. The Li atoms in Li₂₃CuW₁₀O₄₀Cl₅ present four kinds of environments (Fig. 4): Li(1) atoms are connected with two Cl atoms and four O atoms and Li(2) with one Cl atom and five O atoms, while Li(3) and Li(4) atoms are coordinated with six O atoms, and Li(5) atoms are surrounded by three Cl and four O atoms.

In Li₂₃CuW₁₀O₄₀Cl₅, the W-O distances ranging from 0.1778(9) to 0.2154(5) and the bond lengths of W-Cl are 0.2141(5) nm, which are comparable to those in Ba₃WO₅Cl₂^[20], K₂W₃O₁₀^[21], WCl₆^[22], and WOCl₄^[23], the Cu-O distances are 0.1992(8) nm in good agreement with Ba₃Cu₂O₄Cl₂^[23], and BaCuSi₂O₆^[24], from bond valence sums (BVS, Table 3) calculations in Li₂₃CuW₁₀O₄₀Cl₅, Cu displays +2 formal oxidation states according to charge balance.

The Li-O and Li-Cl distances range from 0.2012(6) to 0.249(3) nm and 0.248(3) to 0.295(4) nm, which are closed to those in Li₂MnCl₄^[25], Li₂ZnCl₄^[26], LiWCl₆^[27], Li₂CaTa₂O₇^[28]. The crystallographic data and structural refinements for Li₂₃CuW₁₀O₄₀Cl₅ are summarized in Table 1.

Atomic coordinates and equivalent isotropic displacement parameters are listed in Table 2. Selected bond distances and atomic BVS are displayed in Table 3.

In summary, new single crystals Li₂₃CuW₁₀O₄₀Cl₅(1) have been successfully grown by flux growth method in open-end silica tubes. The crystal structure of Li₂₃CuW₁₀O₄₀Cl₅ has been characterized by single crystal diffraction method. The 3D framework is built by [CuO₆] octahedra, [W(1)O₅Cl] octahedra and [W(2)O₄] tetrahedra. The adjacent [W(1)O₅Cl] octahedra are connected with each other via sharing one Cl atom, and further sharing three O atoms with [CuO₆] octahedra to form the [W(1)₆CuO₂₄Cl₂] unit. The successful synthesis of Li₂₃CuW₁₀O₄₀Cl₅ through flux-growth method is meaningful for explore new mixed anion compounds in the future work.