Synthesis and Characterization of Ammonium Potassium Tellurium Polyoxomolybdate: (NH 4 ) 2 K 2 TeMo 6 O 22 · 2H 2 O with One-Dimensional Anionic Polymeric Chain [TeMo 6 O 22 ] 4 −

: A new tellurium polyoxomolybdate hydrate (NH 4 ) 2 K 2 TeMo 6 O 22 · 2H 2 O was synthesized via the hydrothermal reaction method at 190 ◦ C. The compound crystallizes in a one-dimensional tellurium polymolybdate [TeMo 6 O 22 ] 4 − chain structure. The anionic polymeric chain is composed of Mo 6 O 22 hexamers bridged together through sharing four corner oxygen atoms on the electron lone-paired TeO 4 group. The Mo 6 O 22 hexamer cluster is assembled from six distorted MoO 6 octahedra in an edge-sharing manner. The ammonium and potassium cations distribute around the [TeMo 6 O 22 ] 4 − chains and separate them from each other and maintain the charge balance. The thermal stability and optical properties of the compound were also investigated. The optical absorption data reveal that the compound is a wide band semiconductor with an optical band gap of 3.4 eV.


Introduction
Molybdate materials have attracted considerable research interests in recent years owing to their diverse crystal structures and promising applications in the field of nonlinear optics [1,2], catalysis [3][4][5], medicine [6][7][8], and photochromism [9,10]. In most cases, molybdenum atoms can be coordinated with either four or six oxygen atoms into distorted tetrahedron or octahedron structural units, which usually makes molybdates possess large local polarizability. The six-coordinated molybdenum-oxygen octahedron may further be condensed into polyoxomolybdate clusters, based on which a large number of polyoxomolybdates have been synthesized. Tellurite units containing Te 4+ lone pairs have flexible coordination patterns with oxygen atoms, such as TeO 3 pyramid, TeO 4 sphenoid, and TeO 5 square pyramid. The highly distorted acentric structural units of TeO n (n = 3, 4, 5) cooperate with MoO 4 or MoO 6 polyhedra capable of forming new tellurium molybdates with interesting crystal structures and useful properties, such as piezoelectricity and nonlinear optics [11][12][13][14][15][16].

Properties Characterization
The powder XRD patterns were recorded on a PANalytical Empyrean diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å) at room temperature in the 2θ range of 10−70 • with a step size of 0.026 • . Figure S1 shows the experimental powder XRD patterns, which are consistent with the simulated ones from single crystal structure of (NH 4 ) 2 K 2 TeMo 6 O 22 ·2H 2 O using the Powder Cell software [35]. The thermogravimetric (TG) and differential scanning calorimetric (DSC) measurements were carried out on a NETZSCH STA 449F5 instrument under a flowing nitrogen atmosphere. The powder sample of the compound with mass about 16 mg was enclosed in an Al 2 O 3 crucible, and heated from room temperature to 800 • C with the heating rate of 15 K/min. The infrared spectra were recorded on a Nicolet 470 FT-IR spectrometer in the wavenumber range of 4000-400 cm −1 . Several small crystal samples were ground with KBr and pressed into a transparent pellet for FT-IR absorption experiments. UV-Vis-NIR optical diffuse reflectance spectra were measured on a PerkinElmer Lambda 950 spectrophotometer. The spectral range was set from 2000 to 200 nm at room temperature. A BaSO 4 plate was used as the standard material (100% reflectance) for background correction. Optical absorption spectra were obtained through spectral conversion with the Kubelka-Munk function α/S = (1-R) 2 /2R [36].

Synthesis
The title compound was synthesized via hydrothermal reactions. The stoichiometric mixture of TeO 2 (0.0798 g), KCl (0.1491 g), and (NH 4 ) 6 Mo 7 O 24 ·4H 2 O (0.5297 g) was loaded into a 15-ml Teflon liner and added to 2 ml of water. The above components were thoroughly mixed and sealed raw materials were heated to 190 • C, held for 72 h, and then cooled to room temperature in 24 h. A lot of colorless prism-shaped crystals with single phase were obtained after several washes with deionized water. The overall yield ratio is around 60% based on TeO 2 in general.

Crystal Structure Determination
A transparent prism-shaped crystal with suitable size was selected for single crystal X-ray analysis. The diffraction data were collected at room temperature on a Bruker APEX II CCD X-ray diffractometer using graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. Lorentz and polarization corrections were applied, and absorption corrections were performed by multi-scan method. Lattice type and space group C2/c were first selected according to systematic absence conditions. The initial model of the structure was solved by the direct method with SHELXS-97 and subsequently refined by full-matrix least-squares on F 2 using SHELXL-2014 [37]. Hydrogen atoms on crystalline water were placed by residual peaks around oxygen atoms and further refined with DFIX and DANG restraint instructions with the O-H bond length of 0.85 Å and H-H distance of 1.35 Å.
Hydrogen atoms on nitrogen atoms were not designated. The K and N atoms were refined with substitutional disorder at the same site occupied by K/N atoms under the restraint of the total occupation factor equaling to one. The final refined structures were checked on PLATON and no higher symmetry operation was suggested [38]. The final refined crystallographic data and details of structural refinement are summarized in Table 1. Some selected important bond lengths and angles are listed in Table 2. The atomic coordinates and equivalent isotropic displacement parameters are shown in Table S1. The crystal data has been deposited into the Cambridge Crystallographic Data Centre via www.ccdc.cam. ac.uk/structures (accessed on 3 April 2021) with the CCDC 2069363.

Crystal Structure
Single crystal X-ray diffraction revealed that (NH 4 ) 2 K 2 TeMo 6 O 22 ·2H 2 O crystallizes in the centrosymmetric C2/c space group of monoclinic system with the cell parameters a = 21.172 (2)

Dipole Moments and Local Distortion
The local dipole moments for TeO4 and MoO6 structural units were calculated in order to better understand the distortion of the electron lone-paired Te 4+ and the second-

Dipole Moments and Local Distortion
The local dipole moments for TeO4 and MoO6 structural units were calculated in order to better understand the distortion of the electron lone-paired Te 4+ and the secondorder Jahn-Teller (SOJT) distorted Mo 6+ cations in the compound, as all the above cations   [46,47]. The calculated results give the BVSs of 4.28, 5.94, 5.98, and 6.07 for Te1, Mo1, Mo2, and Mo3, respectively, in agreement with their oxidation states suggested from the single crystal structure.

Dipole Moments and Local Distortion
The local dipole moments for TeO 4 and MoO 6 structural units were calculated in order to better understand the distortion of the electron lone-paired Te 4+ and the second-order Jahn-Teller (SOJT) distorted Mo 6+ cations in the compound, as all the above cations are in acentric coordination environments. The methodology for dipole moment has been reported in previous literature [48,49]. As the TeO 4 units contain lone-pair electrons, the lone pair is given a charge of −2, and the Te-E (lone pair) distance of 1.25 Å from the Te 4+ cation is adopted in the calculation [50]. The total and component dipole moments for cations on one position along the crystallographic axis are listed in Table 3. The dipole moments for the same cations on other positions can be obtained by using the symmetry operation of the C2/c space group, and are not listed here. In fact, all the dipole moments for all the cations are cancelled out, as the crystal structure is centrosymmetric. The calculation for TeO 4 units gives a dipole moment of 9.6 Debye along the positive b-axis direction. The result is intrinsically determined by the two-fold symmetrical axis site for tellurium atoms. The relatively large dipole moment of TeO 4 units is attributed to their high geometric distortion in this structure. The calculated dipole moments for Mo1, Mo2, and Mo3 are 7.0, 9.3, and 5.9 Debye, respectively, which are similar with previously reported values [11,51]. The magnitudes of out-of-center distortions of three distorted MoO 6 octahedra were investigated according to the method proposed by Halasyamani [52], giving the ∆d = 1.24, 1.43, and 1.15 for Mo1, Mo2, and Mo3, respectively. It is seen that the out-of-center distortion of Mo3 is the largest among the three MoO 6 octahedra, consistent with the result of local dipole moment calculation.

Thermal Stability
The TG-DSC measurement was carried out to test the thermal stability of the title compound. The TG-DSC curves versus temperature are shown in Figure S2. From the thermogravimetric curve, the weight loss process occurs in a relatively broad temperature range of 200-450 • C with several minor endothermic peaks, corresponding to the dehydration of crystalline water from around 190 to 300 • C (~3.1%) and the decomposition of ammonium from around 300 to 450 • C (~3.0%). It should be mentioned that the minor weight gains along with the flat exothermic band that occurred in the range of RT (room temperature)-200 • C may be caused by the N 2 absorption of the material. Two main endothermic peaks are observed at 290 and 330 • C, both lower than the corresponding decomposition temperatures of 310 and 378 • C of single ammonium phase (NH 4 ) 4 Mo 6 TeO 22 ·2H 2 O [34]. The total weight loss ratio was consistent with the theoretical one (total water and ammonium) of~6.0%. No further weight loss occurred above 450 • C, although two high and strong endothermic peaks were observed, which may be related to the decomposition or melting of the remaining materials. In order to identify the decomposition product after heating, we handled another amount of the compound and heated it at 450 • C under flowing N 2 atmosphere. X-ray diffraction pattern indicated that the remains mainly contained two phases: K 2 Mo 4 O 13 and TeMo 5 O 16 (see Figure S3) [53,54].

Optical Absorption Properties
UV-Vis-NIR optical absorption spectra converted from diffuse reflectance were plotted in Figure 3. It is seen that (NH 4 ) 2 K 2 TeMo 6 O 22 ·2H 2 O is transparent from the 2000 to 400 nm band range. From 400 nm, the optical absorption edge gradually increases, and at around 360 nm the absorption abruptly rises to the maximum, which corresponds to the electron band transition. The optical band gap of 3.4 eV can be derived through extrapolating the tangent line with the largest slope value to the abscissa. Therefore, compound (NH 4 ) 2 K 2 TeMo 6 O 22 ·2H 2 O is a wide bandgap semiconductor. The infrared absorption spectra from 4000 to 400 cm −1 range are shown in Figure 4. The several peaks in the band range of 3201−2767 cm −1 can be ascribed to the symmetric stretching vibrations of the tetrahedral ammonium ion, and the two sharp peaks at 1437 and 1396 cm −1 are related to the asymmetric stretching modes [18,22,34]. The strong and The infrared absorption spectra from 4000 to 400 cm −1 range are shown in Figure 4. The several peaks in the band range of 3201−2767 cm −1 can be ascribed to the symmetric stretching vibrations of the tetrahedral ammonium ion, and the two sharp peaks at 1437 and 1396 cm −1 are related to the asymmetric stretching modes [18,22,34]. The strong and broad absorption around 3440 cm −1 is due to the stretching modes of water molecules, and the peaks at 1590 and 1637 cm −1 are caused by the H-O-H bending modes of crystalline water [55,56].  The infrared absorption spectra from 4000 to 400 cm −1 range are shown in Figure 4. The several peaks in the band range of 3201−2767 cm −1 can be ascribed to the symmetric stretching vibrations of the tetrahedral ammonium ion, and the two sharp peaks at 1437 and 1396 cm −1 are related to the asymmetric stretching modes [18,22,34]. The strong and broad absorption around 3440 cm −1 is due to the stretching modes of water molecules, and the peaks at 1590 and 1637 cm −1 are caused by the H-O-H bending modes of crystalline water [55,56]. The two strong peaks at 935 and 920 cm −

Conclusions
A new tellurium polyoxomolybdate hydrate (NH4)2K2TeMo6O22·2H2O was synthesized from the hydrothermal reaction method and structurally characterized from single crystal X-ray diffraction at room temperature. The compound can be viewed as the structural evolution from (NH4)4TeMo6O22·2H2O through half replacement of ammonium with

Conclusions
A new tellurium polyoxomolybdate hydrate (NH 4 ) 2 K 2 TeMo 6 O 22 ·2H 2 O was synthesized from the hydrothermal reaction method and structurally characterized from single crystal X-ray diffraction at room temperature. The compound can be viewed as the structural evolution from (NH 4 ) 4 TeMo 6 O 22 ·2H 2 O through half replacement of ammonium with potassium cations. The crystal structure contains a novel one-dimensional anionic polymeric chain [TeMo 6 O 22 ] 4− formed of Mo 6 O 22 clusters with TeO 4 units linked alternatively. The local dipole moments and the magnitudes of out-of-center distortions for the electron lone-paired TeO 4 and the distorted MoO 6 octahedra were calculated, indicating that they are in highly distorted coordination environments. Thermal analysis showed that the compound decomposes in the temperature range of 200-450 • C, corresponding to the removal of crystalline water and ammonium in the structure. The infrared absorption spectra in the range of 4000−400 cm −1 and the UV-Vis-NIR optical spectroscopy were investigated to understand the structure-properties relationship. It indicated that the optical bandgap is 3.4 eV, belonging to a wide band semiconductor. This work demonstrates an example of the synthesis of a new tellurium polyoxomolybdate through half substitution of ammonium with potassium cations.
Author Contributions: L.G. performed project design, synthesis, and crystallography; Y.W. performed data analysis, manuscript editing, implemented properties characterization. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.