A Novel Mineral-like Copper Phosphate Chloride with a Disordered Guest Structure: Crystal Chemistry and Magnetic Properties

Novel copper phosphate chloride has been obtained under middle-temperature hydrothermal conditions. Its crystal structure was established based on the low-temperature X-ray diffraction data: Na2Li0.75(Cs,K)0.5[Cu5(PO4)4Cl]·3.5(H2O,OH), sp. gr. C2/m, a = 19.3951(8) Å, b = 9.7627(3) Å, c = 9.7383(4) Å, β = 99.329(4)°, T = 150 K, MoKα (λ = 0.71073 Å), R = 0.049. The crystal structure includes tetrameric copper clusters as the main building blocks, which are built of four CuO4Cl pyramids sharing apical Cl vertices. The clusters are combined through phosphate groups and additional copper-centered polyhedra to form two mostly ordered periodic layers. Between the layers and inside the framework channels, alkali ions, H2O molecules, or OH groups are statistically distributed. Na2Li0.75(Cs,K)0.5[Cu5(PO4)4Cl]·3.5(H2O,OH) is a synthetic modification of a sampleite-polymorph of the lavendulan mineral group and represents a new member in a mero-plesiotype series of copper phosphates and arsenates, for which the crystal structures contain two-periodic [Cu4X(TO4)4]∞ modules (T = As, P; X = Cl, O). Magnetically, this phase exhibits the phase transition at TC = 6.5 K, below which it possesses a weak ferromagnetic moment.


Introduction
Open-framework materials comprise a wide range of crystalline compounds of different chemical classes [1,2]. Along with the zeolites comprised by more than 255 unique framework types [3], metal-organic frameworks, permeable polymers, and hybrid organicinorganic framework materials, the inorganic metal phosphates, oxyfluorides, nitrides, and sulphides with three-periodic porous structures are well known [4,5]. The interest in these materials is due to their multiple applications in catalysis, gas and ion separations, ion exchange, gas adsorption and storage, waste treatment, anion recognition, etc. [6]. The amazing variety of such properties is related to their structure peculiarities: robust 3D framework structures enclose pores of different sizes that may comprise extra-framework molecules and atoms.
Among inorganic materials with an open framework, transition metal phosphates are scarce and mainly include Mo, V, Fe, Co, Mn, Zr, and Ti cations, and thus only three zeolites with the copper phosphate composition are known [3]. Presumably, metal cations prefer to be arranged in polyoxometalates (POMs) that are composed of oxometal polyhedra and polyanion clusters with structural diversity [7]. Among them, polyoxocuprates (POCus) have been distinguished, showing cross-structural topological transformations related to their multiple physical properties in homogeneous photocatalysis, medical chemistry, molecular magnetism, and quantum computing [8]. Recent discoveries of new rare and complex minerals demonstrate the presence of natural POMs and POCus in geochemical systems [9,10]. Here, we report the hydrothermal synthesis, crystal structure, and physical properties of new complex copper phosphate chloride, Na 2 Li 0.75 (Cs,K) 0.5 [Cu 5 (PO 4 ) 4 Cl]·3.5(H 2 O,OH), which is a new member of the polysomatic series of the lavendulan mineral group. As seen from the complex formula, the new compound is capable of capturing and accumulating various alkali metal ions, from a small to large radii. It is promising to study its potential in the field of waste processing and ion-exchange technologies.

Hydrothermal Synthesis and Crystallization
It is well known that natural geothermal systems are multicomponent. In particular, typical alkali metal cations of chloride geothermal waters are Na + , Li + and K + [11]. Following the synthetic approach, the presence of various alkali metal cations impacts the crystallization process under hydrothermal conditions. For example, the presence of smaller Li + can reduce the crystallization time [12], whereas the larger one, such as Cs + , plays a key role in the formation of microporous phases in the absence of organic directing agents [13,14]. To obtain mineral-like phases with open framework structures, we provided our exploratory synthesis in a system enriched with alkali metal salts. The main chemical agents were CsCl (0.5 g; 3 mmol), CuCl 2 (1 g; 7.4 mmol), and Na 3 PO 4 (1 g; 6.1 mmol). To simulate the composition of the natural geothermal medium, we added a small amount of KCl (0.1 g; 1.3 mmol) and LiCl (0.05 g; 1.2 mmol) to the initial mixture.
All reagents were stirred in an agate mortar, put into a 6 mL standard Teflon-lined stainless-steel pressure vessel, and filled with distilled water up to 83% of its volume. The autoclaves were heated to 523 K in a furnace, isothermed for 20 days (for completion of the reaction), and followed by cooling to room temperature for over 24 h. Crystallization products were washed with hot distilled water and dried at room temperature. Bright turquoise slim plates up to 0.2 mm were selected under a binocular microscope ( Figure 1). Few green column crystals were found in close intergrowth with the main phase ( Figure  S2). According to the powder X-ray diffraction analysis (see Figure S1), this impurity crystalline phase was identified as Cu 3 (PO 4 ) 2 [15]. Chemical analysis was performed on a JEOL JSM-6480LV Oxford X-Max N energy-dispersive diffraction spectrometer (JEOL, Akishima, Japan) at the Laboratory of Local Methods for Studying Materials (Department of Petrology, Faculty of Geology, Lomonosov Moscow State University). The elemental chemical composition based on EDS (JEOL, Akishima, Japan) was Na, Cs, K, Cu, P, Cl, and O atoms for the main phase.

X-ray Experiment and Crystal Structure Determination
Single crystal X-ray diffraction data were collected at T = 150 K using MoK α radiation with an Oxford Diffraction Gemini single crystal diffractometer (Agilent Technologies, Yarnton, UK) equipped with a CCD detector; Mo Kα radiation (λ = 0.71073 Å). The dataset was corrected for background, Lorentz and polarization effects, and absorption [16]. All of the calculations were performed within the WinGX program system [17]. The crystal structure was solved by direct methods and refined using SHELX programs [18,19]. Crystal data and details of data collection and refinement are presented in Table 1 and in the supporting materials, Table S1

X-ray Experiment and Crystal Structure Determination
Single crystal X-ray diffraction data were collected at T = 150 K using MoKα radiation with an Oxford Diffraction Gemini single crystal diffractometer (Agilent Technologies, Yarnton, UK) equipped with a CCD detector; Mo Kα radiation (λ = 0.71073 Å). The dataset was corrected for background, Lorentz and polarization effects, and absorption [16]. All of the calculations were performed within the WinGX program system [17]. The crystal structure was solved by direct methods and refined using SHELX programs [18,19]. Crystal data and details of data collection and refinement are presented in Table 1 and in the  supporting materials, Table S1-S3. The refined structural formula was (Na0.98Cu0.02)2Li0.74(Cs0.33K0.20H2O0.12)[Cu5(PO4)4ClH2O]Cl0.072.5(H2O,OH).    [20]. Empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm.
The X-ray diffraction pattern at room temperature, photograph of crystals, fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters, interatomic distances, and hydrogen-bond geometry for Na 2 Li 0.75 (Cs,K) 0. 5

Interatomic Distances and Crystal Structure Description
The basic structural units of Na 2 Li 0.75 (Cs,K) 0.5 [Cu 5 (PO 4 ) 4 Cl]·3.5(H 2 O,OH) are shown in Figure 2a. Two phosphate tetrahedra are strongly distorted with P-O bond lengths ranging from 1.519(5) to 1.561(5) Å; the largest P-O1 and P-O2 distances correspond to bridging O atoms, which also participate in the first coordination spheres of two Cu atoms. Four symmetrically independent Cu atoms have four nearest O with Cu-O distances in the interval 1.938(5)-1.975(4) Å. The fifth O atom in the tetragonal-pyramidal coordination of Cu1 at a distance of 2.25 Å belongs to the H 2 O (O9) molecule. Cu2-, Cu3-, and Cu4-centered polyhedra share one Cl atom at a distance of about 2.70 Å, which also completes their coordination to a tetragonal pyramidal shape ( Figure 2a). Therefore, Cu polyhedra possess Jahn-Teller distortion typical for Cu 2+ cations.
Analytical numeric absorption correction using a multifaceted crystal model based on expressions derived by R.C. Clark and J.S. Reid [20]. Empirical absorption correction using spherical h monics, implemented in the SCALE3 ABSPACK scaling algorithm.

Interatomic Distances and Crystal Structure Description
The basic structural units of Na2Li0.75(Cs,K)0.5[Cu5(PO4)4Cl]3.5(H2O,OH) are shown Figure 2a. Two phosphate tetrahedra are strongly distorted with P-O bond lengths ra ing from 1.519(5) to 1.561(5) Å; the largest P-O1 and P-O2 distances correspond to bri ing O atoms, which also participate in the first coordination spheres of two Cu ato Four symmetrically independent Cu atoms have four nearest O with Cu-O distances the interval 1.938(5)-1.975(4) Å. The fifth O atom in the tetragonal-pyramidal coordinat of Cu1 at a distance of 2.25 Å belongs to the H2O (O9) molecule. Cu2-, Cu3-, and C centered polyhedra share one Cl atom at a distance of about 2.70 Å, which also comple their coordination to a tetragonal pyramidal shape ( Figure 2a). Therefore, Cu polyhe possess Jahn-Teller distortion typical for Cu 2+ cations. The CuO4Cl pyramids share common edges and the Cl-vertex to form tetrame clusters, which are additionally connected by orthophosphate tetrahedra in [Cu4Cl(PO4)4] 5− blocks parallel to the bc plane ( Figure 2b). On both sides of the blocks alo the x axis, the Cu1O4(H2O) tetragonal pyramids are attached by vertex-bridge conta with PO4 tetrahedra ( Figure 3). Alkali cations Na + , Li + , Cs + , K + , and H2O molecules located between the [Cu5(PO4)4Cl(H2O)] 3

Interatomic Distances and Crystal Structure Description
The basic structural units of Na2Li0.75(Cs,K)0.5[Cu5(PO4)4Cl]⋅3.5(H2O,OH) are show Figure 2a. Two phosphate tetrahedra are strongly distorted with P-O bond lengths r ing from 1.519(5) to 1.561(5) Å; the largest P-O1 and P-O2 distances correspond to br ing O atoms, which also participate in the first coordination spheres of two Cu ato Four symmetrically independent Cu atoms have four nearest O with Cu-O distance the interval 1.938(5)-1.975(4) Å. The fifth O atom in the tetragonal-pyramidal coordina of Cu1 at a distance of 2.25 Å belongs to the H2O (O9) molecule. Cu2-, Cu3-, and C centered polyhedra share one Cl atom at a distance of about 2.70 Å, which also compl their coordination to a tetragonal pyramidal shape (Figure 2a). Therefore, Cu polyhe possess Jahn-Teller distortion typical for Cu 2+ cations. The CuO4Cl pyramids share common edges and the Cl-vertex to form tetram clusters, which are additionally connected by orthophosphate tetrahedra in [Cu4Cl(PO4)4] 5− blocks parallel to the bc plane ( Figure 2b). On both sides of the blocks al the x axis, the Cu1O4(H2O) tetragonal pyramids are attached by vertex-bridge cont with PO4 tetrahedra (Figure 3). Alkali cations Na + , Li + , Cs + , K + , and H2O molecules located between the [Cu5(PO4)4Cl(H2O)] 3− slabs (Figure 3), and compensate for their n tive charge. Sodium atoms occupy two positions at the centres of the tetragonal bicap pyramid (Na1) (Figure 4a) and the trigonal prism (Na2) (Figure 4b) with Na-O dista  Table S2, and hydrogen-bond geometry in Table S3. ranging between 2.338(8) and 2.632(4) Å (average 2.42 Å). The occupancy factors for Na atoms are 98%; at a close distance from them, additional Cu atoms (Cu5 and Cu6) are distributed in the amount of 2%. Strongly distorted LiO4 tetrahedra, statistically populated by 74% Li atoms, are characterized by two Li-O distances of 1.76(1) Å and two significantly larger ones, equal to 2.44(2) Å (Figure 4c).   Table S2, and hydrogen-bond geometry in Table S3.

Crystal Chemistry
Similar modules A [Cu 4 X(TO 4 ) 4 ] in the crystal structures of copper phosphates and arsenates alternate with the second-type modules B, which vary in composition and topology. According to the classification by Makovicky, the series is merotypic [22]. At the same time, all members are built from modules A slightly differing in chemistry and configuration; this fact denotes the series as plesiotype [22,23]. Considering both characteristics, the series of compounds under discussion should be called mero-plesiotype.
The presence of similar (pseudo)tetragonal slabs [Cu 4 X(TO 4 ) 4 ], assembled from (CuO 3 ) 4 Cl/(CuO 3 ) 4 O clusters and PO 4 /AsO 4 tetrahedra, underlies the "equality" of two unit-cell parameters for all phases (Figure 2b). For example, b and c parameters for Na 2 Li 0.75 (Cs,K) 0.5 Cu 5 (PO 4 ) 4 Cl·3.5(H 2 O,OH) are both close to 9.7 Å, which is distinctive for phosphate members. For arsenates, these values are normally larger and reach 10.0 Å. For richelsdorfite and two synthetic isostructural compounds, RbNa 5 Cu 4 (AsO 4 ) 4 Cl 2 and CsNa 5 Cu 4 (AsO 4 ) 4 Cl 2 , two pseudo-tetragonal parameters are close to 14 Å, which exactly corresponds to the diagonal directions of the 10 Å cell mentioned above. The third parameter of the unit cell varies significantly for the series members from 7.1 Å to 23.7 Å due to the different ways of assembling the two modules A and B.
The title crystal structure, apparently, is closely related to the structure of the polymorphic modification of the mineral sampleite, NaCaCu 5 (PO 4 ) 4 Cl·4.5H 2 O [24]. It is very likely that in both structures, two types of modules alternate in the same sequence (ABA'B' . . . ), where A' and B' are the inverted blocks A and B. This leads to the proximity of their third unit cell parameter being equal to~19.5 Å. For both structures, the monoclinic angles take a value of 99 • -102 • , which is close to the β angle in some other minerals-richelsdorfite, (calico)andyrobertsite, and epifanovite. All other compounds of this polysomatic series exhibit an angle close to 90 • , which is obviously related to a strong (pseudo)tetragonality of their structures.
The main feature of our phase is the presence of exclusively alkali monovalent cations (even of four types), which form the B module, while the other members, including sampleite, always contain divalent alkali-earth elements like Ca, Sr, Ba, Cd, or even Pb. Using four different alkaline cations in our hydrothermal experiment, we tried to simulate a natural hydrothermal process. As a result, the monoclinic Na 2 Li 0.75 (Cs,K) 0.5 Cu 5 (PO 4 ) 4 Cl·3.5(H 2 O,OH) phase was obtained similarly to most monoclinic natural minerals, but was the first case among all synthetic tetragonal or orthorhombic compounds of the series under discussion.
Among natural and synthetic inorganic compounds, the recent review mentions at least 26 types of CuO m Cl n coordination polyhedra with O and Cl ligands [25]. While the [4O+Cl] square pyramid is a typical Cu 2+ -centered polyhedron with Jahn-Teller distortion, a tetrameric cluster of four copper pyramids sharing a common chlorine vertex turned out to be rather rare. As described above, the [(CuO 3 ) 4 Cl] clusters ( Figure 5) are basic structural units for our compound and other members of the lavendulan polysomatic series, which includes secondary arsenate and phosphate copper minerals. Note, the tetrameric clusters of [(CuO 3 ) 4 Cl] were recently recognized in the synthetic phase Na 6 Cu 7 BiO 4 (PO 4 ) 4 [Cl,(OH)] 3 . In its composite crystal structure, the copper tetramers form layers of square-kagomé topology responsible for a possible quantum spin-liquid property [26].

Physical Properties
The temperature dependences of the thermodynamic properties, i.e., magnetization M and heat capacity Cp, in the range 2-300 K under magnetic field B up to 9 T, have been measured on the pressed pellets of crushed tiny crystals using relevant options of the "Quantum Design" Physical Properties Measurement System PPMS-9T.
The temperature dependences of the magnetic susceptibility χ = M/B in Na2Li0.75(Cs,K)0.5[Cu5(PO4)4Cl]3.5(H2O,OH) measured in the field-cooled (FC) and zerofield-cooled regimes at B = 0.1 T are shown in Figure 6. At high temperatures, these curves coincide within experimental error, following the Curie-Weiss law. The fitting in the range 100-300 K gives the Weiss temperature, Θ = −25.7 K, pointing to the predominance

Physical Properties
The temperature dependences of the thermodynamic properties, i.e., magnetization M and heat capacity C p , in the range 2-300 K under magnetic field B up to 9 T, have been measured on the pressed pellets of crushed tiny crystals using relevant options of the "Quantum Design" Physical Properties Measurement System PPMS-9T. The temperature dependences of the magnetic susceptibility χ = M/B in Na 2 Li 0.75 (Cs,K) 0.5 [Cu 5 (PO 4 ) 4 Cl]·3.5(H 2 O,OH) measured in the field-cooled (FC) and zerofield-cooled regimes at B = 0.1 T are shown in Figure 6. At high temperatures, these curves coincide within experimental error, following the Curie-Weiss law. The fitting in the range 100-300 K gives the Weiss temperature, Θ = −25.7 K, pointing to the predominance of the antiferromagnetic interactions in the system. The Curie constant C = 2.27 emu K/mol corresponds to the presence of five Cu 2+ ions with spin S = 1/2 and g-factor g = 2.2. With lowering the temperature, the experimental data deviated downward from the extrapolation of the Curie-Weiss law, which indicates the predominance of the antiferromagnetic correlations in the system. At T*~22 K, a weakly pronounced anomaly is observed in both FC and ZFC curves ascribed to the presence of the Cu 3 (PO 4 ) 2 secondary phase, as evidenced by X-ray diffraction [27]. This was followed by a sharp upturn at T C = 6.5 K in the FC curve and a downward inclination in the ZFC curve. The sharp dichotomy of FC and ZFC curves at T < T C indicates the presence of a weak ferromagnetic component and/or disorder in the magnetic subsystem of the low temperature phase.
Main magnetic units in the structure of Na 2 Li 0.75 (Cs,K) 0.5 [Cu 5 (PO 4 ) 4 Cl] × 3.5(H 2 O, OH) are Cu 5 pentamers organized by four edge-sharing CuO 4 Cl pyramids ( Figure 5), which are connected via four phosphate groups with the fifth CuO 5 pyramid, as shown in the inset to Figure 6. In the basal plane, the angle of Cu-O-Cu bond is equal to 107 o , which corresponds to an antiferromagnetic exchange interaction J between the nearest neighbor basal centers in accordance with the Goodenough-Kanamori-Anderson rules [28]. The magnetic exchange interaction between basal and apical ions passes through the intermediate PO 4 groups and is also usually antiferromagnetic J [29]. The fit of χ(T) curve in the model of S = 1/2 pentamers can be done based on the Hamiltonian with exchange interaction J between two spins expressed asĤ = −2JS 1 ·S 2 [30]. Experimental values of the product of χT vs. T and the fit are shown in the inset of Figure 6. The parameters of magnetic exchange interactions are J = −19 K and J = −24 K for g-factor g = 2.27 close to that obtained χ(T) curve.  Figure 7. At about 200 K, Cp is still far from the Dulong-Petit thermodynamic limit of about 10 3 J/mol K. At TC, a pronounced anomaly in Cp(T) is seen, corresponding to the phase transition in the magnetic subsystem. An additional anomaly at T*~22 K corresponds to the ordering of the secondary phase Cu3(PO4)2 [27]. The lower inset shows the results of the specific heat measurements at several magnetic fields in the range from 0 to 9 T. Under a magnetic field, the phase transition at TC shifts upward in temperature at a rate of 0.085 K/T. An increase in the stability region of the low-temperature phase with magnetic field correlates  Figure 7. At about 200 K, C p is still far from the Dulong-Petit thermodynamic limit of about 10 3 J/mol K. At T C , a pronounced anomaly in C p (T) is seen, corresponding to the phase transition in the magnetic subsystem. An additional anomaly at T*~22 K corresponds to the ordering of the secondary phase Cu 3 (PO 4 ) 2 [27]. The lower inset shows the results of the specific heat measurements at several magnetic fields in the range from 0 to 9 T. Under a magnetic field, the phase transition at T C shifts upward in temperature at a rate of 0.085 K/T. An increase in the stability region of the low-temperature phase with magnetic field correlates with the presence of a weak ferromagnetic component in this phase.
temperature dependence of the product χT and arrangement of Cu5 unit. J and J′ denote magnetic exchange interaction pathways. The dashed line is a fit with S = 1/2 pentamer model. The temperature dependence of the specific heat Cp of Na2Li0.75(Cs,K)0.5[Cu5(PO4)4Cl] ·3.5(H2O,OH) measured in the quasi-adiabatic regime is shown in Figure 7. At about 200 K, Cp is still far from the Dulong-Petit thermodynamic limit of about 10 3 J/mol K. At TC, a pronounced anomaly in Cp(T) is seen, corresponding to the phase transition in the magnetic subsystem. An additional anomaly at T*~22 K corresponds to the ordering of the secondary phase Cu3(PO4)2 [27]. The lower inset shows the results of the specific heat measurements at several magnetic fields in the range from 0 to 9 T. Under a magnetic field, the phase transition at TC shifts upward in temperature at a rate of 0.085 K/T. An increase in the stability region of the low-temperature phase with magnetic field correlates with the presence of a weak ferromagnetic component in this phase.

Discussion
The studied Na 2 Li 0.75 (Cs,K) 0. 5 4 Cl units, as shown in Figure 8. However, magnetic subsystems demonstrate different properties under the variation of halogen. The fluorine compound demonstrates a broad hump at 19 K and antiferromagnetic phase transition at T N = 11.2 K. The obtained parameters of basal and apical magnetic exchange interactions in Na 3 [Cu 5 (PO 4 ) 4 F]·4H 2 O were determined as J = −24 K and J = −14 K. Large difference between J and J led to the manifestation first of the plaquette properties at T > T N , while in the studied chlorine compound, J and J were closer, which made it possible to see the behavior of the copper pentamers.

Conclusions
Copper phosphate chloride is synthesized in the form of single crystals under middle-temperature hydrothermal conditions. Low-temperature X-ray diffraction study made it possible to refine the complex and highly disordered crystal structures of

Conclusions
Copper phosphate chloride is synthesized in the form of single crystals under middletemperature hydrothermal conditions. Low-temperature X-ray diffraction study made it possible to refine the complex and highly disordered crystal structures of Na 2 Li 0.75 (Cs,K) 0.5 [Cu 5 (PO 4 ) 4 Cl] ·3.5(H 2 O,OH), which is a novel synthetic modification of the sampleite-polymorph of the lavendulan mineral group and a new member in the mero-plesiotype series of copper phosphates and arsenates. The unique feature of its monoclinic layered structure is the possibility to accommodate various alkali ions between ordered copper-phosphate blocks.