LI Long ZHUANG Rong-Chun MI Jin-Xio HUANG Y-Xi

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A New Modification of Tellurite Phosphate:-Te3O3(PO4)2①

LI LongaZHUANG Rong-ChuanbMI Jin-XiaoaHUANG Ya-Xia②

a(()361005)b(364200)

A new modification of tellurite phosphate,-Te3O3(PO4)2, has been synthesized under hydrothermal conditions and its crystal structure was determined by single-crystal X-ray diffraction.It crystallizes in the monoclinic space group21/(No.14), with= 11.115(5),= 4.7033(19),= 17.287(7) ?,= 106.086(5)°,= 868.3(6) ?3,= 4, P2Te3O11,M= 620.74,D= 4.748 g/cm3,(Mo) = 10.438 mm–1and F(000) = 1096.The final full-matrix least-squares refinement converged to= 0.0227,= 0.0534 for 1984 observed reflections with> 2(), and= 0.0240,= 0.0540 for all data (2070) and= 1.117.-Te3O3(PO4)2is polymorphic with the known-Te3O3(PO4)2(Weil H.M.et al.Z.Anorg.Allg.Chem.2003, 629, 1068-1072).The crystal structure of-Te3O3(PO4)2features a three-dimensional (3D) network composed of Te6O2220?hexanuclear clusters interconnected by PO4groups.Te6O22hexanuclear cluster is built from three Te2O8dimers (edge-sharing TeO5square pyramids) linked to each via sharing O-corners.The structure difference between- and-forms of Te3O3(PO4)2lies in the polymerization of tellurite oxides TenOm.1D infinite1∞{[Te3O11]10?} single chains are presented in-Te3O3(PO4)2, while 0D discrete Te6O22hexanuclear clusters are observed for-Te3O3(PO4)2.Moreover, thermal analyses, infrared spectra and UV-Vis-NIR diffuse reflectance are also presented.

polymorphism, crystal structure, phosphate, tellurite;

1 INTRODUCTION

Tellurite phosphate materials have received considerable attention owning to their rich structural topologies and potential applications in nonlinear optics[1-6].The rich structural chemistry is attributed to the various coordination building blocks of tellurium atoms, that is, TeO(= 3, 4 or 5), e.g., trigonal pyramidal (TeO3)2?, seesaw (TeO4)4?, and square pyramidal (TeO5)6?groups[4, 7].Furthermore, the TeOpolyhedra can be polymerized into a variety of discrete polynuclear anionic clusters, chains, layers, or even three-dimensional frame- work structures via both corner-sharing and/or edge-sharing[8].In addition, Te4+cation often con- ducts an asymmetric coordination environment attributable to the expulsion of its stereoactive lone pair electron which is proven to easily crystallize in non-centrosymmetric (NCS) crystal structures[5].These NCS materials display a variety of useful physical properties such as second-harmonic generation (SHG), piezoelectricity, pyroelectricity, and ferroelectricity[8, 9].Consideration on the rich crystal structural properties of tellurium oxide, as well as the perfect binding group PO4tetrahedra, together with the possibilities to obtain non-centro- symmetric crystal structure with nonlinear optical (NLO) properties, we believe that tellurite phos- phates could be a very promising type of NLO materials.Up to date, although only dozens of tellurite phosphates[2-3, 10-19]have been observed, two of them, Te2O3(HPO4)[3, 10]and Te2O(PO4)2[2], have been reported crystallizing in non-centro- symmetric space groups and exhibiting NLO pro- perties.Moreover, tellurite phosphates have already exhibited rich structure chemistry with crystal struc- tures ranging from 1D chains[12], 2D layers[10-11, 13, 16, 20], or even 3D frameworks[2, 14-15, 17-18, 20].

In this paper, we report on the synthesis, crystal structure, and characterizations of-Te3O3(PO4)2which is the polymorphism of tellurite phosphate-Te3O3(PO4)2.Structure comparisons between-Te3O3(PO4)2and-Te3O3(PO4)2have also been discussed in the text.This is the first time to find polymorphism in the tellurite phosphate system.

2 EXPERIMENTAL

2.1 Synthesis

-Te3O3(PO4)2has been synthesized via hydro- thermal method.Reactants of NaCl (0.118 g, 2 mmol), TeO2(0.16 g, 1 mmol), H3PO4(85 wt%, 0.5 mL, 7.3 mmol), and HF (40 wt%, 0.05 mL, 1.12 mmol) were successively loaded into a PTFE-lined stainless-steel autoclave (15 mL in volume) along with 2 mL of deionized water without stirring.The autoclaves were sealed and heated at 240 °C for 4 days, and then naturally cooled down to room temperature.The resulting products were washed and filtered with deionized water, and dried in air.Colorless crystals of-Te3O3(PO4)2were obtained in about 80% yield on the basis of starting TeO2.The presence of Te and P has been confirmed by EDX results with a molar ratio of Te:P ≈ 3: 2.5.Although the P content is a little bit more than that derived from the single crystal results, which is quite common since it is often to observe more P contents in tellurite phosphates.

Since Na and F elements are absent from the formula, we did a couple of comparison experi- ments to confirm whether these elements are necessary in the reaction condition or not.First, we excluded NaCl or HF from reactants while keeping other conditions unchanged.Secondly, NaCl and HF were replaced by NaF.Experimental results showed that without the addition of F?ions (either HF or NaF), it was impossible to synthesize the title compound under studied conditions.Instead, TeO2was obtained as solid product.On the other hand, NaCl can be removed from the reactant list without changing the final product.All these results indicate that F?ions play a key role in the formation of the title compound, which most likely act as mineralizer to facilitate the formation and crystallization of the title compound.

2.2 Characterization methods

Powder X-ray diffraction (PXRD) The PXRD measurements were carried out on a Bruker D8 Advance powder X-ray diffractometer with Curadiation (= 1.54056 ?, Ni filter, 40 kV/40 mA) at room temperature.

Scanning electron microscopy/energy-dispersive analysis by X-ray (SEM/EDX) A field emission scanning electron microscope (FE-SEM, LEO-1530 and SU70) was utilized to perform SEM/EDX.

Infrared spectroscopy The FTIR spectra were recorded on a Nicolet iS10 FT-IR spectrometer in the range from 4000 to 400 cm?1.The well-ground samples were mixed homogeneously with KBr and further pressed into pellets for the measurements.

UV?Vis?NIR diffuse reflectance spectrum The diffuse reflectance spectra were collected at room temperature on a Cary 5000 spectrophoto- meter, using BaSO4as the standard in the spectral range of 200～2000 nm.

Thermal analysis Thermal analyses were carried out on a SDTQ600 V20.9 Build 20 thermo- gravimetric/differential scanning calorimetry (TG/ DSC) instrument at a heating rate of 10oC·min?1in a nitrogen atmosphere (100 mL/min) from room temperature to 800oC.

2.3 Crystal structure determination

A suitable single crystal (0.34 × 0.08 × 0.04 mm3) of the title compound has been selected and checked first under polarized microscope prior to single- crystal X-ray diffraction.The single crystal diffrac- tion data were collected on a Bruker Apex charge- coupled device (CCD) diffractometer with graphite- monochromatic Moradiation (= 0.71073 ?, 45 kV/30 mA) at 203(2) K.In the range of 1.96≤≤28.82o, a total of 4987 reflections were collected and 2070 reflections were independent withint= 0.022, of which 1984 were observed with> 2().Absorption correction has been performed.The crystal structure was solved by direct methods and refined by full-matrix least-squares technique using the SHELX programs[21]included in the WinGX package[22], and further checked for missing symmetry elements by using PLATON[23].The structure has been solved in the monoclinic space group21/(No.14), with= 11.115(5),= 4.7033(19),= 17.287(7) ?,= 106.086(5)°,= 868.3(6) ?3,= 4.Te, P and some coordinated O atoms were determined from direct methods.The rest O atom sites were located from difference Fourier maps.The final full-matrix least-squares refinement converged to1= 0.0227,2= 0.0534 for 1984 observed reflections with> 2(), and1= 0.0240,2= 0.0540 for all data (2070),= 1.117,D= 4.748 g/cm3,(Mo) = 10.438 mm–1and(000) = 1096.The largest diffraction peak and hole are 1.050 and –1.031 e·?–3, respectively.The chemical formula were defined to be Te3O3(PO4)2, which possesses the same chemical formula as the known-Te3O3(PO4)2[15]in the database (21/,= 4,= 12.375(2),= 7.317(1),= 9.834(1) ?,= 98.04(1)o,= 881.7(2) ?3).However, the PXRD patterns shown in Fig.1 clearly indicated that two compounds are not of identical structures, sugges- ting that they should be two polymorphisms of Te3O3(PO4)2.Therefore, the title compound was defined as the-phase of Te3O3(PO4)2, named with-Te3O3(PO4)2.Selected bond distances and bond angles of-Te3O3(PO4)2are given in Table 1.

Fig.1. Observed and calculated PXRD patterns of-Te3O3(PO4)2compared with the PXRD pattern known as-Te3O3(PO4)2

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

Symmetry codes: (i),+1,; (ii) ?+1, ?+1, ?+1; (iii)+1/2, ?+1/2,+1/2; (iv) ?+1/2,?1/2, ?+3/2;

(v) ?+1/2,+1/2, ?+3/2; (vi)?1/2, ?+1/2,?1/2; (vii),?1,

3 RESULTS AND DISCUSSION

3.1 Crystal structure description

-Te3O3(PO4)2is a new tellurite phosphate material crystallizing in the monoclinic space group21/(No.14).The structure contains asymmetric TeO5polyhedra and regular PO4tetrahedra.Both P(1) and P(2) are in a rather regular tetrahedral coordination environment with the P?O lengths ranging from 1.520(3) to 1.549(3) ?.Three crystal- lographic unique Te4+cations (Fig.2a) are observed in the title compound.Te(1), Te(2) and Te(3) are all in distorted square pyramidal environments, bonded to five oxygen atoms and one lone pair electrons located at the apex of octahedron when counting lone pair electrons.Te(1) has a comparable long Te?O bond (2.553 ?) according to the definition of TeIV?O bond.The Te?O bond lengths range from 1.886(3) to 2.553 (3) ? and the O?Te?O bond angles range from 68.38(11) to 176.20(12)o.These bond distances and bond angles of tellurium poly- hedra are consistent with those previously reported tellurite phosphates[7, 10].Bond valence calcula- tions[24]on-Te3O3(PO4)2resulted in the values of 3.98～4.21 for Te4+and 4.78～4.83 for P5+, respecti- vely, both of which are the expected values for Te and P.

Te(1)O5square pyramid shares one edge with one Te(3)O5forming Te(1)–Te(3) dimer.Meanwhile, two Te(2)O5square pyramids also share edge to form the Te(2)–Te(2) dimer.Two Te(1)–Te(3) di- mers are interconnected by one Te(2)–Te(2) dimer via the common O corners of Te(1) and Te(2), thus forming a-hexanuclear cluster parallel to the-axis (Fig.2b).The resulting Te6O22hexamers are further interconnected by PO4tetrahedra via sharing common O-atom corners along the- and-axes, thus leading to a neutral 3D network structure (Fig.2c).Within the network, 3-, 6-, and 8-membered ring channels are observed running along the-axis.Note that it is the first time to observe the hexa- nuclear cluster of TenOmpartial structure in tellurite phosphates, or even in the tellurites.

Fig.2. Coordination environments of three crystallographic unique Te atoms (a), Te6O2220?hexanuclear cluster (b), and the polyhedral presentation of the crystal structure of-Te3O3(PO4)2viewed along the-axis (c).TeO5square pyramids: light gray, PO4tetrahedra: dark gray, Te atoms: light gray spheres, O atoms: medium gray spheres.Note that the balloon shape in the coordination environment around Te presents the lone pair electrons

Although-Te3O3(PO4)2and-Te3O3(PO4)2exhibit the same chemical formula composition, their crystal structures show dramatic difference which can be obviously identified from the PXRD patterns shown in Fig.1.Therefore,-Te3O3(PO4)2and-Te3O3(PO4)2are not identical but should be two modifications.Their structural difference (shown in Fig.3) lies in the TenOmpartial structure motifs.Te6O22hexanuclear clusters are observed in-Te3O3(PO4)2while 1D1∞{[Te3O11]10?}-single chains are presented in- Te3O3(PO4)2.As shown in Fig.3(a’), the1∞{[Te3O11]10?} single chain is built from a central Te(2)–Te(3) dimer chain open branched by the additional Te(1)O5square pyramids via3-O atoms.1D1∞{[Te3O11]10?} single chains interconnected by PO4groups result in a 3D network structure.Dif- ferent from-Te3O3(PO4)2containing 3-, 6-, and 8-membered ring channels within the framework, only 4- and 6-membered ring channels are present in the framework of-Te3O3(PO4)2.On considering the same formula with different crystal structures between-Te3O3(PO4)2and-Te3O3(PO4)2, we define that-Te3O3(PO4)2should be a new modi- fication of tellurite phosphate Te3O3(PO4)2.

3.2 Infrared spectroscopy

The infrared spectrum (Fig.4) of-Te3O3(PO4)2reveals that it consists of P?O, Te?O, and Te?O?P vibrations.The absorption bands at 959～1102 and 460～578 cm-1can be assigned to the P?O vibrations.The vibrations at 629～740 cm-1and 415 cm-1belong to the stretching mode of Te?O.A band occurring around 594 cm-1is attributed to the Te?O?P vibration.Similar absorption bands have b een observed in those previously reported related compounds[2, 12, 20, 25].Weak broad absorption bands at 3432 and 1627 cm-1are caused by the water absorption from air[26].

Fig.3. Crystal structure comparisons between-Te3O3(PO4)2(a, b) and-Te3O3(PO4)2(a’, b’).(a) 0D Te6O2220?hexanuclear cluster in-Te3O3(PO4)2, (a’) 1D1∞{[Te3O11]10-} single chains in-Te3O3(PO4)2, (b) and (b’) Polyhedral representation of the crystal structures of-Te3O3(PO4)2and-Te3O3(PO4)2viewed along the-axis.TeO5square pyramids: light gray, PO4tetrahedra: dark gray, Te atoms: light gray spheres, O atoms: medium gray spheres

Fig.4. Infrared spectrum of-Te3O3(PO4)2

3.3 Thermal analysis

Fig.5 shows the thermal behavior of- Te3O3(PO4)2.No obvious weight loss was observed in the TG curve while an endothermic peak is found at 633oC.These data may indicate that the title compound shows a melting process at 633oC.In order to identify whether it melted at 633oC, powder sample of-Te3O3(PO4)2was heated to 640oC for an hour.A glassy product, which stuck to the alumina crucible firmly, was observed after heat treating.This result may indicate that-Te3O3(PO4)2melts at 633oC.

Fig.5. TG-DSC curves of-Te3O3(PO4)2

3.4 UV-Vis-NIR diffuse reflectance spectrum

To calculate the band gap and absorption edge of the materials, UV-Vis-NIR diffuse-reflectance spectrum was utilized.Absorption (/) data were calculated from the Kubelka-Munk function[27, 28]:

Fig.6. Diffuse-reflectance spectrum of-Te3O3(PO4)2

where,andrepresents the reflectance, absorption coefficient and the scattering factor, respectively.In the()--plot (Fig.6 inset), extrapolating the linear part of the rising curve to zero, the optical band gap ofTe3O3(PO4)2is observed at approximately 4.04 eV related to a relatively short absorption edge of ~248 nm (Fig.6).

4 CONCLUSION

In summary, we have synthesized a new modification of tellurite phosphate,-Te3O3(PO4)2under hydrothermal condition.The crystal structure of-Te3O3(PO4)2features a three-dimensional framework composed of 0D Te6O2220?hexanuclear clusters interconnected by PO4groups.The structure difference between- and-forms of Te3O3(PO4)2lies in the polymerization of Te oxide partial structure.Te6O2220?hexanuclear clusterspresent in-Te3O3(PO4)2, whereas 1D1∞{[Te3O11]10?} single chains appear in- Te3O3(PO4)2.This is the first time to find poly- morphism in the tellurite phosphate system.Note that Te6O2220?hexanuclear cluster is the first time to be found in tellurites.

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16 January 2018;

27 February 2018 (ICSD 434333)

the National Natural Science Foundation of China (No.21201144) and the Natural Science Foundation of Fujian Province (No.2018J07006)

.E-mail: yaxihuang@xmu.edu.cn

10.14102/j.cnki.0254-5861.2011-1965