Synergetic Influence of Alkali-Metal and Lone-Pair Cations on

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Synergetic Influence of Alkali-Metal and Lone-Pair Cations on Frameworks of Tellurites Yi-Gang Chen,† Nan Yang,† Xiao-Ni Yao,† Cheng-Bo Li,‡ Yao Guo,*,‡ and Xian-Ming Zhang*,† †

Key Laboratory of Magnetic Molecules and Magnetic Information Material of Ministry of Education, School of Chemistry and Material Science, Shanxi Normal University, Linfen 041004, China ‡ Anyang Institute of Technology, Anyang 455000, China S Supporting Information *

ABSTRACT: Four new tellurites were hydrothermally synthesized by the adjustment of different alkali-metal ions, and all of the compounds demonstrate 2D layer structures. Rb2Te4O9·2H2O in centrosymmetric space group Pccn 12− features a new [Te4O9]2− groups. ∞ anion sheet consisting of rare [Te8O22] RbNaTe8O14(OH)6·8H2O in CS space group P1̅ exhibits a [Te4O9]2− ∞ anion layer by linkage of the TeO3 and TeO4 units. NaPb4Te4O12F located in CS space group P4/n displays an unusual [PbTeO3]∞ neutral layer made up of PbO3 and TeO3 trigonal pyramids. RbK3Te8O18·5H2O lying in noncentrosymmetric (NCS) space group Cc shows a [Te4O9]2− ∞ anion layer composed of the TeO3 and TeO4 units; its second-harmonic-generation response is about 0.2 times that of KH2PO4; structure analysis and local dipole moment calculation verify that the weak polarization mostly from the [Te4O9]2− ∞ layer results from the inverse arrangement of TeOn units, and further theoretical calculation confirms that TeOn groups dominate the band gap of RbK3Te8O18·5H2O and optical properties. Meanwhile, systematic analyses of a series of metal tellurites reveal that the alkali-metal cations exert a considerable impact on polarization of the crystal structures, which puts forward a feasible idea about the design of new NCS materials.

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INTRODUCTION Inorganic materials with lone-pair cations like Te4+ and Pb2+ are raising extensive attention, which is attributable to their diverse structural chemistry, resulting in various functional properties such as second-order nonlinear optical behavior, pyroelectricity, and ferroelectricity.1 The lone pairs giving rise to an asymmetric structural unit or spontaneous polarization are deemed to be the outcome of second-order Jahn−Teller distortion.2 For tellurites with variable structural backbones, a Te4+ cation has the ability to bind to a variable number of O atoms to generate a TeO3 trigonal pyramid, a TeO4 seesaw, and a TeO5 square pyramid;3 polymerization of these Te−O blocks can create 1D chains and ribbons, 2D layers, and 3D frameworks, demonstrating fruitful structural chemistry.4 In the extended structures, when the arrangement direction of the local asymmetric units is aligned in a parallel manner, noncentrosymmetric (NCS) compounds with very intriguing characteristics like second-harmonic generation (SHG) have been obtained. Anionic group theory has been developed to assign the SHG response to the anionic group.5 By virtue of inherent local distortion with nonbonded electron pairs, many NCS metal tellurites have been discovered by chemists’ constant efforts.4h,6 Alkali or alkali-earth cations are often introduced to the metal tellurites in order to maintain a charge balance.7 In fact, large cations are prone to asymmetry of the structure units owing to variable coordination numbers inducing asymmetry, and ionic polarization or distortion of © XXXX American Chemical Society

the cation contributes to the increase of the SHG response. Sometimes, distortion of the cation plays a key role in the formation of NCS materials such as K2Te4O9·3.2H2O,8 where incorporation of the K+ cation is believed to break the inversion symmetry. Our efforts here are directed toward the discovery of new metal tellurites composed of Te4+ and alkali-metal cations. However, because most ternary tellurites have already been developed such as K2Te4O9,3d K2Te4O9·3.2H2O,8 Na2TeO3,9 Na2TeO3·5H2O,10 Na2Te2O5·2H2O,11 α- and β-Na2Te4O9,12 Na4Te4O10,13 Na2Te2O6·1.5H2O,12 and so forth,14 we focus on exploring quaternary tellurites via tuning of the alkali-metal and lone-pair ions. Through convenient hydrothermal reactions, three new alkaline tellurites, Rb 2 Te 4 O 9 ·2H 2 O, RbNaTe8O14(OH)6·8H2O, and RbK3Te8O18·5H2O, were obtained via tuning of the alkali-metal ions. The Pb2+ cation with a lonepair and large ionic radius exhibits asymmetric coordination geometries,15 and incorporation in the system produced the addional quaternary compound NaPb4Te4O12F. It is interesting that RbK3Te8O18·5H2O crysallizes in NCS Cc space group. Herein, structures, IR spectra, elemental and thermal analyses, optical properties, band structure, SHG coefficients, and dipole moment calculations are explored. The effect of alkali-metal Received: February 10, 2018

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DOI: 10.1021/acs.inorgchem.8b00266 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

function: F(R) = (1 − R)2/2R = K/S, where R is the reflectance, K is the absorption, and S is the scattering. SHG Measurements. The SHG measurements depending on the Kurtz−Perry method at 298 K were carried out using a Q-switched Nd:YAG laser at a wavelength of 1064 nm. Polycrystalline RbK3Te8O18·5H2O was ground and sieved into the particle size range 125−150 μm. Polycrystalline KH2PO4 (KDP) as the reference was sieved into the above-mentioned particle size range, and the SHG signal responses of RbK3Te8O18·5H2O and KDP were separately obtained. Thermogravimetric Analysis (TGA). TGA and differential scanning calorimetry (DSC) measurements were done on a Netzsch STA 449 F3 unit. The sample of about 11.0 mg was placed in a Pt crucible and heated from 30 to 800 °C at a rate of 10 °C/min under a N2 atmosphere. Calculations of the Electronic Structures and SHG Coefficients. On the basis of density functional theory (DFT), firstprinciples calculations on RbK3Te8O18·5H2O were done by using the MedeA-VASP 5.3 package.19 The plane-wave pseudopotential method was adopted. The total energies were achieved within the generalized gradient approximation of Perdew−Burke−Ernzerhof (GGA-PBE) functional under the convergence criterion of 1 × 10−6 eV/atom. The plane-wave cutoff energy was located at 600 eV. The Gaussian smearing (0.2 eV) scheme was done for integration of the Brillouin zones in the electronic structure. The ion−electron interaction was obtained by the projector augmented wave method. The slab model was repeated under periodic boundary conditions with Cc symmetry. The k-point sampling of the Brillouin zone was performed using Monkhorst−Pack meshes with a spacing of 0.25 Å−1. The SHG coefficients were gained using the so-called length-gauge formalism of Aversa and Sipe.20 At a zero-frequency limit, the secondorder nonlinear susceptibility could be obtained. Because of the limitation of the DFT method, a scissor of 0.66 eV was applied in the optical property calculations.

ions on distortion of the structures is systematically analyzed and researched.

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EXPERIMENTAL SECTION

Reagents. TeO2 (99.99%), PbO (99.0%), KOH (AR), and RbF (99.8%) were purchased from Aladdin Chemistry Co. Ltd., and NaOH (96.0%) was purchased from Xilong Chemical Factory. The reagents were used as received. Synthesis. All crystals (Figure S1) were achieved by hydrothermal synthesis. For Rb2Te4O9·2H2O, 0.638 g of TeO2 (4.0 mmol), 0.186 g of KOH (3.0 mmol), 0.418 g of RbF (4.0 mmol), and 6.0 mL of deionized water were combined. For RbNaTe8O14(OH)6·8H2O, 0.638 g of TeO2 (4.0 mmol), 0.240 g of NaOH (6.0 mmol), 0.209 g of RbF (2.0 mmol), and 4.0 mL of deionized water were mixed. For RbK3Te8O18·5H2O, 0.638 g of TeO2 (4.0 mmol), 0.336 g of KOH (6.0 mmol), 0.312 g of RbF (3.0 mmol), and 4.0 mL of deionized water were combined. Each reaction mixture was sealed in a stainless steel bomb equipped with a Teflon liner (25 mL). The mixtures were heated at 210 °C for 4 days. When the mixture was cooled to 30 °C at a rate of 3 °C/h, the transparent crystals were manually collected and washed with ethanol. For NaPb4Te4O12F, 0.319 g of TeO2 (2.0 mmol), 0.446 g of PbO (3.0 mmol), 0.252 g of NaF (6.0 mmol), and 4.0 mL of deionized water were combined, the reaction mixture was sealed in a stainless steel bomb equipped with a Teflon liner (10 mL), heated at 230 °C for 3 days, and cooled to 30 °C at a rate of 6 °C/h, and the transparent block crystals were manually collected and washed with deionized water. Single-Crystal X-ray Diffraction (XRD). Single-crystal XRD data were collected on an Agilent Technologies Gemini EOS diffractometer with an EOS CCD detector at 293 K using Mo Kα radiation (λ = 0.71073 Å) and integrated with the CrysAlisPro program. The structures were solved by direct methods with the program SHELXS and refined by the full-matrix least-squares program SHELXL.16 The O10−O12 atoms in Rb2Te4O9·2H2O, O10, O11, O13, and O14 atoms in RbNaTe8O14(OH)6·8H2O, and O18−O20, O22, and O23 atoms in RbK3Te8O18·5H2O are taken as H2O molecules based on charge balance and bond valence sums (BVSs),17 and their calculated bond valences are in the ranges 0.10−0.35, 0−0.12, and 0.20−0.37, respectively. The H atoms in the compounds Rb2Te4O9·2H2O, RbNaTe8O14(OH)6·8H2O, and RbK3Te8O18·5H2O were not located. According to the program PLATON/TWINROTMAT tool18 for twinning detection, RbK3Te8O18·5H2O shows a twinning structure, and the structure was refined on the basis of a twinning matrix (HKLF 5), giving the Flack parameter (absolute structure parameter) a value of 0.437(12). All of the structures were further examined through the program PLATON. Details of the crystallographic data and structural refinements are dispalyed in Table S1. The important bond distances and angles are listed in Table S2. Powder X-ray Diffraction (PXRD). PXRD patterns of the four compounds were collected on a Rigaku MiniFlex II X-ray diffractometer using Cu Kα radiation (λ = 1.540598 Å) in the 2θ angular range 4−70° at 293 K with a step size of 0.02°. The measured PXRD patterns are in accordance with the calculated XRD patterns on the basis of single-crystal XRD analysis (Figures S2−S5). Elemental Analysis. Microprobe elemental analysis on several crystals was done with field-emission scanning electron microscopy (JSM6700F) with energy-dispersive X-ray spectroscopy (EDS; Oxford INCA). All of the average atomic ratios are consistent with the results from single-crystal X-ray structure analyses (Table S3 and Figures S6− S9). IR Spectroscopy. IR spectra of the four compounds were obtained by a Nicolet 5DX spectrometer in the range 400−3600 cm−1 at room temperature. The compounds and KBr were mixed, ground, and then pressed into pellets. UV−Vis−Near-IR Diffuse-Reflectance Spectrum. A Varian Cary 5000 Scan spectrophotometer was used to record the spectra of the powder samples over a 200−900 nm wavelength range at 293 K, and a BaSO4 sample was the standard (100% reflectance). Reflectance spectra were converted into absorbance spectra by the Kubelka−Munk

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RESULTS AND DISCUSSION Crystal Structure. Rb2Te4O9·2H2O. Rb2Te4O9·2H2O (Pccn, No. 56) demonstrates a layer structure, and the adjacent layers are separated by Rb+ cations and occluded H2O molecules (Figure 1a). It features a novel [Te4O9]2− ∞ layer consisting of

Figure 1. (a) Layer structure of Rb2Te4O9·2H2O separated by Rb+ cations and occluded H2O molecules. (b) Te2O7 dimer of the TeO4 seesaw. (c) Te4O14 tetramer of the TeO5 square pyramid. (d) New Te8O22 structural unit. (e) 2D [Te4O9]2− ∞ layer in the bc plane.

TeO4 and TeO5 units via sharing edges and corners. Within the layer, two TeO4 units are interconnected via sharing O atoms to form the Te2O7 dimer (Figure 1b); four TeO5 units are interconnected via sharing edges to form the Te4O14 tetramer (Figure 1c); two Te2O7 dimers and one Te4O14 tetramer generate a new Te8O22 fundamental unit with an inversion center located at (−1/2, 1 + y, 1 + z) (Figure 1d). The Te8O22 unit by itself spreads wavily in the bc plane, which gives rise to B

DOI: 10.1021/acs.inorgchem.8b00266 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the structural layer of [Te4O9]2− ∞ , and is interconnected through a b-glide plane (e.g., x, y, 3/4) and a c-glide plane (e.g., x, 1/4, z) (Figure 1e). Similarly, the 2D layer possesses an inversion center at (1/2, 1/2, 1/2), and it is centrosymmetric (CS). The Rb+ cations between the layers retain the charge balance, and the cations within asymmetric units interact with O atoms in oxide ligands as well as in H2O molecules. In an asymmetric unit, there are 4 Te, 2 Rb, and 11 O atoms. The Te−O bond distances from TeO4 and TeO5 polyhedra range from 1.859(5) to 2.462(6) Å, and the Rb−O bond lengths with asymmetric RbO8 and RbO9 polyhedra vary between 2.808(5) and 3.548(5) Å. BVSs for the Rb+ and Te4+ ions have values of 0.92−1.11 and 3.98−4.17, respectively. RbNaTe8O14(OH)6·8H2O. It crystallizes in a CS space group P1̅, which exhibits a [Te4O9]2− ∞ anionic sheet by a cornersharing linkage of TeO3 and TeO4 units. The Na+ and Rb+ cations as well as H2O molecules fill with the layers (Figure 2a).

Figure 3. (a) Layer structure of RbK3Te8O18·5H2O filled by K+ and Rb+ cations and H2O molecules. (b) [Te6O18]12− 6-MR with a star motif. (c) [Te6O18]12− 6-MR with a ship motif. (d) [Te4O9]2− ∞ sheet along the c-axis direction.

RbK3Te8O18·5H2O, two types of 6-MRs have no inversion center, the [Te4O9]2− ∞ layer (Figure 3d) has no symmetry operation except the identity operation, and the lone pairs on the Te4+ centers point toward the interlayer. In other words, inversion symmetry of the similar [Te4O9]2− ∞ layer is broken up. Although the [Te4O9]2− ∞ layer is distorted, by and large, the Te polyhedra are arranged alternately in an opposite direction (Figure 3b,c). Therefore, polarization from the [Te4O9]2− ∞ layer is mostly balanced off. In its asymmetric unit, there are 8 Te, 3 K, 1 Rb, and 23 O atoms. Each Te4+ cation is surrounded by three or four O atoms, resulting in TeO3 and TeO4 units with bond lengths in the range of 1.796(15)−2.186(16) Å. There are two kinds of TeO3 unit coordination (Te1O3 and Te5O3), and the others are TeO4 units. The K+ and Rb+ cations are separately connected with O atoms to form K1O6, K2O5, K3O5, and RbO8 asymmetric polyhedra with bond distances varying between 2.575(19) and 3.137(4) Å for K−O bonds and between 2.99(3) and 3.462(15) Å for Rb−O bonds. The BVSs of K+, Rb+, and Te4+ have values of 0.84−1.11, 0.71, and 3.59− 4.26, respectively. NaPb4Te4O12F. The compound (P4/n, No. 85) features an unusual [PbTeO3]∞ neutral layer made up of PbO3 and TeO3 trigonal pyramids, and the F− anions are between the layers to keep the charge balance and link the adjacent layers together (Figure 4a,b). Along the c-axis direction, the sheet produces [Pb4Te4O16]8− 8-MRs by alternate corner-sharing linkages of PbO3 and TeO3 trigonal pyramids (Figure 4b), and the Na+ cations reside discontinuously in the centers of the 8-MRs (Figure 4c). Interestingly, the centers of the 8-MRs with the Na+ cations possess a C4 axis, while the 8-MRs without the Na+ cations have a S4 inversion axis. Distribution of the Na+ cations results from the repulsion of lone pairs, and in the centers of the 8-MRs with the Na+ cations, the Na+ cation is surrounded by four O atoms, whereas the centers of the 8-MRs without the Na+ cation are surrounded by lone pairs from four TeO3 trigonal pyramids. Each Na+ cation as well as two F atoms attached to the Na+ cation lies on the C4 axis, and clearly the F−Na−F angle is 180° (Figure 4d). In the neutral sheet, there is a center of symmetry at (0, 0, 0), and thus the sheet is CS. It should be pointed out that the F− anions on the C4 axis are surrounded by four Pb atoms with a Pb−F bond length of 2.815 Å. According to the coordination sphere of the PbII

Figure 2. (a) Layer structure of RbNaTe8O14(OH)6·8H2O filled by Na+ and Rb+ cations and H2O molecules. (b) [Te6O18]12− 6-MR with a star motif. (c) [Te6O18]12− 6-MR with a ship motif. (d) [Te4O9]2− ∞ sheet along the (110) direction.

Along the (110) direction, the layer generates [Te6O18]12− sixmembered rings (6-MRs) with a “star” configuration and the other [Te6O18]12− 6-MRs with a “ship” configuration, and two types of the 6-MRs separately contain inversion centers (Figure 2b−d). The asymmetric unit of the structure contains 4 Te, 0.5 Na, 0.5 Rb, and 14 O atoms. There are two kinds of TeO3 and TeO4 polyhedra, with the Te−O bond lengths varying between 1.835(4) and 2.196(4) Å. The Na+ ions interact with six O atoms, while the Rb+ ions contact with 12 O atoms, with the Na−O and Rb−O distances ranging from 2.250(6) to 2.969(7) Å and from 2.925(4) to 3.5204 Å, respectively; thus, the polyhedral units are evidently distorted. Note that the anionic layer [Te4O9]2− ∞ (Figure 2d) possesses a center of symmetry at (0, 0, 0). The O12 atom from the Na−O12 bonds [bond length: 2.250(6) Å] and the terminal O8 and O9 atoms from the Te−O bonds are assigned as the hydroxyl groups. BVSs for the Na+, Rb+, and Te4+ ions have values of 1.04, 1.02, and 3.65−4.05, respectively. RbK3Te8O18·5H2O. The compound lies in a NCS space group (Cc, No. 9) and displays a [Te4O9]2− ∞ anionic sheet by a cornersharing linkage of TeO3 and TeO4 units (Figure 3a), and along the c-axis direction, [Te6O18]12− 6-MRs with a “star” configuration and [Te6O18]12− 6-MRs with a “ship” configuration are discovered (Figure 3b−d), similar to that of RbNaTe8O14(OH)6·8H2O. However, the difference is that, in C

DOI: 10.1021/acs.inorgchem.8b00266 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

tellurites are analyzed (Table 1 and Figure S11).8,11,12,23 The increase in the radii of the alkali-metal ions (Table 1) makes their asymmetric coordination number tend to increase owing to the intrinsic ionicity of alkali metals, consistent with the reported literature.24,25 For RbNaTe8O14(OH)6·8H2O, the Rb+ and smaller Na+ cations as well as partial H2O molecules are placed in the center of the anion layers (Figure 2a), favorable for minimizing the internuclear repulsion between the cations as well as H2O molecules and the framework layers, and the partial H2O molecules near the anion layers have a small bond with the framework O atoms, which results in the CS structure. Although RbK3Te8O18·5H2O has almost the same framework layer as RbNaTe8O14(OH)6·8H2O and their interlayer spacing is very close (Table 1), RbK3Te8O18·5H2O crystallizes in a NCS space group. In RbK3Te8O18·5H2O, larger and more K+ cations (ionic standard radii for the Na+ and K+: 0.97 and 1.33 Å, respectively) as well as the Rb+ cations (radius: 1.48 Å) and H2O molecules are filled between the layers. However, in the center of the layers, no sufficient positions can accommodate so many large cations. So, the partial K+ cations with slightly smaller radii occupy the positions near the anion layers to meet the requirement of their own coordination environment and bond to the framework O atoms, which can exert a stronger influence on distortion of the framework structures and result in the NCS space group. Also, the K+ ions in Ke2Te4O9·3.2H2O and Cs+ in Cs2Te4O9 (Figure S12) are also similar. For Rb2Te4O9·2H2O, K2Te4O9, and RbNH4Te4O9·2H2O, their formed framwork layers (Figure S11) are fairly rigid in comparison with that of RbK3Te8O18·5H2O. As a result, their layer spacing becomes bigger (Table 1) in order to accommodate the larger cations and H2O molecules, which reduces the interaction of the cations as well as H2O molecules and the rigid framework structures, generating the CS structures. Note that the Na+ cations in β-Na2Te4O9 and the Cs+ cations in Cs2Te4O9 reside in channels with the 3D structures, and α-Na2Te4O9 exhibits 1D polymeric sheets. Optical Characterization. The four compounds were determined by IR spectroscopy (Figure S13). For Rb2Te4O9· 2H2O, RbNaTe8O14(OH)6·8H2O, and RbK3Te8O18·5H2O, the intense peaks at about 3440 and 1630 cm−1 are assigned to the stretching vibrations of O−H and H−O−H, respectively, when the peak near 1386 cm−1 is considered to be from the O−H bending vibration. Bands for Te−O stretching vibrations are observed at about 760, 668, and 607 cm−1. The results agree with those previously reported.8,12 The UV−vis diffuse-reflectance spectral data of the four compounds were obtained. Approximate band-gap energies were calculated to be 4.15, 4.17, 3.82, and 3.13 eV for Rb2Te4O9·2H2O, RbNaTe8O14(OH)6·8H2O, RbK3Te8O18·

Figure 4. (a) Layer structure of NaPb4Te4O12F filled by F− cations. (b) Unusual [PbTeO3]∞ neutral layer showing [Pb4Te4O16]8− 8-MRs by alternate corner-sharing linkages of PbO3 and TeO3 trigonal pyramids. (c) Octahedral chart of NaO4F2 residing discontinuously in the center of [Pb4Te4O16]8− 8-MRs (dark color, out-of-plane; light color, in-plane). (d) F−Na−F chains lying on the C4 axis with a F− Na−F angle of 180°.

atom,21 there are weak bonds between the PbII and F atoms. Note that the reported PbTeO3 (Figure S10) is composed of PbO3, PbO4, and TeO3 polyhedra,22 which is completely different from the [PbTeO3]∞ neutral layer. In NaPb4Te4O12F, its asymmetric unit is composed of 0.25 Na, 1 Pb, 1 Te, 0.25 F, and 3 O atoms, in which the Na and F atoms occupy the special position. The Pb2+ and Te4+ cations are both linked to three O atoms, bringing separately about PbO3 and TeO3 trigonal pyramids with bond lengths of 2.323(4)−2.541(5) Å for the Pb−O bonds and 1.855(5)−1.880(4) Å for the Te−O bonds, respectively. The Na+ cations are coordinated by the F and O atoms to form a slightly distorted octahedron of NaO4F2 with Na−O and Na−F bond lengths of 2.502(4) and 2.264(6)− 2.352(6) Å, respectively. The BVSs for Na+, Pb2+, Te4+, and F− have values of 0.97, 1.70, 3.91, and 0.84, respectively. Role of Alkali-Metal Cations. Among these four compounds, RbNaTe8O14(OH)6·8H2O and RbK3Te8O18· 5H2O exhibit the same [Te4O9]2− ∞ anionic sheet by a cornersharing linkage of TeO3 and TeO4 units. However, the difference is that one anionic sheet possesses a CS structure and the other does not. The effects of alkali-metal cations and H2O molecules as well as the bonding of framework O atoms with alkali cations on the centric−acentric symmetry option along with the other known [Te4O9]2−-containing alkali-metal

Table 1. Asymmetric Units and Features of Tellurites Containing the Anion Group [Te4O9]2− compound

space group

anion group

α-Na2Te4O9 β-Na2Te4O9 RbNaTe8O14(OH)6·8H2O K2Te4O9·3.2H2O RbK3Te8O18·5H2O Cs2Te4O9 K2Te4O9 RbNH4Te4O9·2H2O Rb2Te4O9·2H2O

P1,̅ CS Pccn, CS P1̅, CS P1̅, NCS Cc, NCS I42̅ d, NCS P21/c, CS C2/c, CS Pccn, CS

Te4O9, CS Te4O9, CS Te4O9, CS Te4O9, CS Te8O18, NCS Te4O9, NCS Te4O9, CS Te4O9, CS Te4O9, CS

asymmertic units TeO4, TeO4 TeO3, TeO3, TeO3, TeO4, TeO3, TeO3, TeO4, D

TeO5 TeO4 TeO4 TeO4 TeO5 TeO4 TeO4, TeO5 TeO5

alkaline metal units NaO5, NaO6 NaO6 NaO6, RbO12 KO6 KO5, KO6, RbO8 CsO9, CsO12 KO6, KO7 RbO9, RbO10 RbO8, RbO9

interlayer size, nm

7.221 7.151 7.106 7.575 7.712 10.234 DOI: 10.1021/acs.inorgchem.8b00266 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

asymmetric polyhedra is performed by a BVS approach.23 As for the TeO3 and TeO4 polyhedra, the lone pair is given a charge of 2− and localized 1.25 Å from the Te4+ cation. The results are summarized in Table 2. The magnitude of the dipole

5H2O, and NaPb4Te4O12F, respectively, by extrapolating the linear part of K/S versus E plots (Figures S14−S17). Thermal Behavior. TGA and DSC (Figures S18−S21) indicate that the three compounds Rb2Te4O9·2H2O, RbNaTe8O14(OH)6·8H2O, and RbK3Te8O18·5H2O experience obvious weight loss at 100−200 °C, which can be ascribed to the gradual volatilization of H 2 O, in agreement with the endothermic peak at 120−150 °C in the DSC diagrams. The weight losses are about 3.85%, 8.32%, and 5.32%, close to the calculated losses of 4.17%, 8.95%, and 5.62% for Rb2Te4O9· 2H2O, RbNaTe8O14(OH)6·8H2O, and RbK3Te8O18·5H2O, respectively. After dehydration, the three compounds have no obvious weight loss in the range of 200−800 °C. Also, the corresponding endothermic peaks at 432.42, 459.01, and 449.04 °C may be attributed to the melting temperatures of the dehydrated products. NaPb4Te4O12F displays no obvious weight loss and has endothermic peaks in the DSC curve at a temperature of 427 °C, corresponding to the melting point. Theoretical Calculation. To further demonstrate the relationship between the crystal structure and optical properties, the band structures and densities of states (DOSs) stemming from DFT methods were calculated. Band structure calculation gives RbK3Te8O18·5H2O an indirect band-gap compound with a value of 3.16 eV (Figure 5a), slightly smaller

Table 2. Dipole Moments of the TeOn, KOn, and RbO8 Units and Total Polarization of the Asymmetric Unit dipole moment (D) polar unit

x

y

z

total magnitude

Te1O3 Te2O4 Te3O4 Te4O4 Te5O3 Te6O4 Te7O4 Te8O4 ∑(TeOn) K1O6 K2O5 K3O5 Rb1O8 asymmetric unit unit cell, Z = 2

−1.029 1.332 6.945 1.291 −3.478 −1.647 −3.277 −0.698 −0.562 1.050 1.440 −0.062 1.261 3.126 6.252

−0.803 2.943 1.031 0.585 0.327 6.227 −1.263 1.484 10.530 1.024 0.668 −0.322 −0.011 12.890 25.780

8.495 −7.916 4.606 −6.435 −8.370 4.515 −5.884 5.671 −5.317 1.290 0.282 −1.603 1.293 −3.055 −6.111

8.595 8.550 8.397 6.589 9.070 7.866 6.852 5.904 11.810 1.953 1.440 1.636 1.806 13.611 27.222

moments for TeO3 and TeO4 ranges between 5.904 and 9.070 D, and the total dipole moment is merely 11.810 D, which is, nonetheless, in line with the asymmetric unit arrangement of the [Te4O9]2− ∞ anion layer. In other words, polarization from the TeOn group mostly cancels out. As to the KOn (n = 6 or 5) and RbO8 polyhedra, the weak dipole moment is established. Overall, the net dipole moment in the unit cell is mainly induced by the TeOn group. Because RbK3Te8O18·5H2O in the space group Cc belongs to class CS, considering the restriction of Kleinman symmetry, there are six independent SHG tensors (d11, d12, d13, d31, d32, and d33). The calculated values for d11, d12, d13, d31, d32, and d33 are −0.527, −0.273, 0.081, 0.356, 0.167, and 0.063 pm/V, respectively, substantially smaller than that of KDP (d36: 0.39 pm/V).

Figure 5. Energy bands (a) and DOSs (b) of RbK3Te8O18·5H2O.

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than that of the experimental result (3.82 ev) ascribed to the limitation of the DFT method. Figure 5b shows the total and partial DOSs of RbK3Te8O18·5H2O. Because electron transfer mostly results from the outer-shell electrons, the upper sections of the valence (VB; −7.5 eV to the VB maximum) and conduction (CB; the CB minimum to 8 eV) bands are mainly analyzed. Clearly, the VB consists mostly of O 2p orbitals mixing with Te 5p orbitals, and, likewise, the CB is composed largely of Te 5p orbitals and minor O 2p orbitals. Close to the Fermi level, O 2p orbitals overlapping Te 5p orbitals occupy the top section of the VB, while Te 5p and O 2p orbitals occupy the bottom section of the CB. As a result, the bonding interactions between O and Te, or TeOn groups, dominate the band gap of RbK3Te8O18·5H2O, and thus the optical properties depend on the TeOn groups. SHG Response and Dipole Moment Analysis. Because RbK3Te8O18·5H2O is in a NCS space group, its SHG properties are analyzed. The powder SHG intensities of RbK3Te8O18·5H2O at 1064 nm reveal that RbK3Te8O18· 5H2O shows a weak SHG intensity (about 0.2KDP; Figure S22). To further explore the SHG origin of RbK3Te8O18·5H2O, calculation of the local dipole moments for different

CONCLUSIONS We synthesized the four new metal tellurites through a facile hydrothermal method. Each compound exhibits a 2D layer. Because of the cooperation of the alkaline-metal ions and lonepair cations, Rb2Te4O9·2H2O, RbNaTe8O14(OH)6·8H2O, and NaPb4Te4O12F are in the CS space groups, while RbK3Te8O18· 5H2O lies in the NCS space group. Rb 2Te4O 9·2H 2O demonstrates a novel [Te8O22]12− group consisting of TeO4 and TeO 5 units. RbNaTe 8 O 14 (OH) 6 ·8H 2 O features a [Te4O9]2− ∞ anion sheet obtained from a corner-sharing linkage of TeO3 and TeO4 units. NaPb4Te4O12F exhibits a rare [PbTeO3]∞ neutral layer composed of PbO3 and TeO3 trigonal pyramids. RbK3Te8O18·5H2O possesses a [Te4O9]2− ∞ anion layer composed of TeO3 and TeO4 units. Because RbK3Te8O18· 5H2O belongs to the NCS space group, the SHG measurement indicates that its SHG response is about 0.2 times that of KDP, and further structure analysis and local dipole moment calculation reveal that weak polarization mainly from the [Te4O9]2− ∞ anion layer is the result of the inverse arrangement of TeOn units; theoretical calculation confirms that TeOn groups determine the band gap of RbK3Te8O18·5H2O. E

DOI: 10.1021/acs.inorgchem.8b00266 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(2) (a) Daub, M.; Krummer, M.; Hoffmann, A.; Bayarjargal, L.; Hillebrecht, H. Synthesis, Crystal Structure, and Properties of Bi3TeBO9 or Bi3(TeO6)(BO3): A Non-Centrosymmetric Borate− Tellurate (VI) of Bismuth. Chem. - Eur. J. 2017, 23, 1331−1337. (b) Zhang, M.; Su, X.; Mutailipu, M.; Yang, Z.; Pan, S. Bi3OF3(IO3)4: Metal Oxyiodate Fluoride Featuring a Carbon-Nanotube-like Topological Structure with Large Second Harmonic Generation Response. Chem. Mater. 2017, 29, 945−949. (c) Phanon, D.; Gautier-Luneau, I. Promising Material for Infrared Nonlinear Optics: NaI3O8 Salt Containing an Octaoxotriiodate (V) Anion Formed from Condensation of [IO3]− Ions. Angew. Chem., Int. Ed. 2007, 46, 8488−8491. (d) Shen, Y.; Yang, Y.; Zhao, S.; Zhao, B.; Lin, Z.; Ji, C.; Li, L.; Fu, P.; Hong, M.; Luo, J. Deep-Ultraviolet Transparent Cs2LiPO4 Exhibits an Unprecedented Second Harmonic Generation. Chem. Mater. 2016, 28, 7110−7116. (e) Zhao, S.; Yang, Y.; Shen, Y.; Zhao, B.; Li, L.; Ji, C.; Wu, Z.; Yuan, D.; Lin, Z.; Hong, M.; Luo, J. Cooperation of Three Chromophores Generates the Water-Resistant Nitrate Nonlinear Optical Material Bi3 TeO6 OH(NO3)2. Angew. Chem., Int. Ed. 2017, 56, 540−544. (3) (a) Lin, J.; Diefenbach, K.; Fu, J.; Cross, J. N.; Clark, R. J.; Albrecht-Schmitt, T. E. LnV3Te3O15(OH)3·nH2O (Ln = Ce, Pr, Nd, Sm, Eu, Gd; n = 1−2): A New Series of Semiconductors with MixedValent Tellurium (IV, VI) Oxoanions. Inorg. Chem. 2014, 53, 9058− 9064. (b) Hou, J.; Huang, C.; Zhang, H.; Tu, C.; Sun, R.; Yang, Q. A New Noncentrosymmetric Tellurite: BaMo2Te2O11(H2O) with {Mo(1)Mo(2)O10}n Spiral Chains. J. Mol. Struct. 2006, 785, 37−42. (c) Feng, J. H.; Hu, C. L.; Xu, X.; Kong, F.; Mao, J. G. Na2RE2TeO4(BO3)2 (RE = Y, Dy−Lu): Luminescent and Structural Studies on a Series of Mixed Metal Borotellurates. Inorg. Chem. 2015, 54, 2447−2454. (d) Becker, C. R.; Tagg, S. L.; Huffman, J. C.; Zwanziger, J. W. Crystal Structures of Potassium Tetratellurite, K2Te4O9, and Potassium Ditellurite, K2Te2O5, and Structural Trends in Solid Alkali Tellurites. Inorg. Chem. 1997, 36, 5559−5564. (4) (a) Kim, Y.; Lee, D. W.; Ok, K. M. Rich Structural Chemistry in New Alkali Metal Yttrium Tellurites: Three-Dimensional Frameworks of NaYTe4O10, KY(TeO3)2, RbY(TeO3)2, and a Novel Variant of Hexagonal Tungsten Bronze, CsYTe3O8. Inorg. Chem. 2015, 54, 389− 395. (b) Kim, Y. H.; Lee, D. W.; Ok, K. M. Strong Second Harmonic Generation (SHG) Originating from Combined Second-Order Jahn− Teller (SOJT) Distortive Cations in a New Noncentrosymmetric Tellurite, InNb(TeO4)2. Inorg. Chem. 2014, 53, 5240−5245. (c) Kim, M. K.; Kim, S. H.; Chang, H. Y.; Halasyamani, P. S.; Ok, K. M. New Noncentrosymmetric Tellurite Phosphate Material: Synthesis, Characterization, and Calculations of Te2O(PO4)2. Inorg. Chem. 2010, 49, 7028−7034. (d) Kong, F.; Hu, C. L.; Hu, T.; Zhou, Y.; Mao, J. G. Explorations of New Phases in the GaIII/InIII-MoVI-SeIV/TeIV-O Systems. Dalton Trans. 2009, 4962−4970. (e) Mayer, H.; Weil, M. Synthese und Kristallstruktur von Te3O3(PO4)2, einer Verbindung mit fünffach koordiniertem Tellur (IV). Z. Anorg. Allg. Chem. 2003, 629, 1068−1072. (f) Alcock, N. W.; Harrison, W. D. Refinement of the Structure of Tellurium Phosphate Te2O3.HPO4. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1982, 38, 1809−1811. (g) Porter, Y.; Ok, K. M.; Bhuvanesh, N. S. P.; Halasyamani, P. S. Synthesis and Characterization of Te2SeO7: A Powder Second-Harmonic-Generating Study of TeO2, Te2SeO7, Te2O5, and TeSeO4. Chem. Mater. 2001, 13, 1910−1915. (h) Ra, H. S.; Ok, K. M.; Halasyamani, P. S. Combining Second-Order Jahn−Teller Distorted Cations to Create Highly Efficient SHG Materials: Synthesis, Characterization, and NLO Properties of BaTeM2O9 (M = Mo6+ or W6+). J. Am. Chem. Soc. 2003, 125, 7764−7765. (5) Ye, N.; Chen, Q.; Wu, B.; Chen, C. Searching for New Nonlinear Optical Materials on the Basis of the Anionic Group Theory. J. Appl. Phys. 1998, 84, 555−558. (6) (a) Vidyavathy; Vidyasagar, K. Low-Temperature Syntheses and Characterization of Novel Layered Tellurites, A2Mo3TeO12 (A = NH4, Cs), and “Zero-Dimensional” Tellurites, A4Mo6Te2O24·6H2O (A = Rb, K). Inorg. Chem. 1998, 37, 4764−4774. (b) Almond, P. M.; McKee, M. L.; Albrecht-Schmitt, T. E. Unusual Uranyl Tellurites Containing [Te2O6]4‑ Ions and Three-Dimensional Networks. Angew. Chem., Int.

Research of different tellurites discovers that alkaline-metal cations play a considerable role in the crystal structure, which generates prospects for the design of new NCS materials. In the future, a systematic study of metal tellurites by the adjustment of alkali-earth and Pb2+ cations will be further carried out to explore the design of new NCS materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00266. Crystal data and structure refinement, selected bond lengths and angles, photographs of crystals, EDS plots, experimental and calculated XRD pattern data, IR spectra, UV−vis diffuse-reflectance spectra, DSC curves, SHG signals, and structure and interlayer distances of metal tellurites (PDF) Accession Codes

CCDC 1823032−1823035 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +86 357 2051402. *E-mail: [email protected]. Fax: +86 372 2909732. ORCID

Yao Guo: 0000-0002-3964-8002 Xian-Ming Zhang: 0000-0002-8809-3402 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The subject was endowed financially by the Plan for 10000 Talents in China, National Science Fund for Distinguished Young Scholars (Grant 20925101), and the 1331 Project of Shanxi Province.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b00266 Inorg. Chem. XXXX, XXX, XXX−XXX