Polar Noncentrosymmetric ZnMoSb2O7 and Nonpolar

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Polar Noncentrosymmetric ZnMoSb2O7 and Nonpolar Centrosymmetric CdMoSb4O10: d10 Transition Metal Size Effect Influencing the Stoichiometry and the Centricity Hongil Jo and Kang Min Ok* Department of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea S Supporting Information *

ABSTRACT: Two new quaternary molybdenum(VI) antimony(III) oxides, ZnMoSb2O7 and CdMoSb4O10, have been synthesized in phase-pure form. The title compounds consist of highly polarizable cations, i.e., d0 (Mo6+) and d10 (Zn2+ or Cd2+), and lone-pair cations (Sb3+). ZnMoSb2O7 exhibits a three-dimensional framework with ZnO4, MoO4, and SbO4 polyhedra in the polar space group P21, whereas CdMoSb4O10 exhibits onedimensional tubule structures with CdO6, MoO4, and SbO3 polyhedra in the space group P21/m. Several synthetic efforts suggest that the the dissimilar radii of Zn2+ and Cd2+ that can accommodate polyhedra of Sb3+ cations influence the stoichiometry as well as the centricity for the reported materials. Spectroscopic, thermal, and elemental analyses are reported along with dipole moment calculations. Nonlinear optical properties and their structural origin are examined for polar ZnMoSb2O7 as well.

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cations (Mo6+), and lone-pair cations (Sb3+). We found that most of the known antimony molybdates crystallize in centrosymmetric (CS) structures.6 Herein we demonstrate that the size of the d10 cation and the subsequent capability to accommodate other polyhedra influence the stoichiometry and macroscopic centricity of the new mixed-metal antimony oxides.

INTRODUCTION With their wide variety of potential applications in catalysts, electronics, electromagnets, and optical devices, functional metal oxides have drawn gigantic attention to date.1 Among many, oxide materials crystallizing in noncentrosymmetric (NCS) space groups have drawn immense attraction because of the fascinating symmetry-dependent physical properties such as pyroelectricity, optical activity, nonlinear optical (NLO) properties, ferroelectricity, piezoelectricity, etc.2 A representative approach for the crystallization of NCS oxide materials is combining asymmetric building units in initial synthesis steps. A family of wellestablished asymmetric building blocks includes ions liable to second-order Jahn−Teller (SOJT) distortions, specifically ions possessing lone pairs (Tl+, Sn2+, Sb3+, Te4+, etc.) and highvalent d0 metals in an octahedral environment (Zr4+, Ta5+, Mo6+, etc.).3 Other proven NCS chromophores found in solidstate oxide materials encompass highly polarizable d10 transition metal cations (Zn2+, Cd2+, etc.) and anions with trigonal-planar geometry containing π-conjugated molecular orbitals (BO33−, CO32−, NO3−, etc.).4 However, all of those approaches combining the asymmetric groups during the synthesis are based on experience rather than theoretical deductions. To increase the possibility of overall NCS structures more systematically, a few crucial factors determining the macroscopic centricity have been suggested through closer structural examinations, including hydrogen bonding between organic templates and ligands in the frameworks, framework flexibility of the asymmetric polyhedra generated from rich coordination moieties, and cation size effects influencing the alignment of other constituent polyhedra.5 In this paper, we report the synthesis, structure determination, and characterization of the novel mixed-metal oxides ZnMoSb2O7 and CdMoSb4O10. Interestingly, both of the reported materials are composed of three different cations with asymmetric environments, i.e., d10 cations (Zn2+ and Cd2+), d0 © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Synthesis. MoO3 (Aldrich, 99.5%), Sb2O3 (Aldrich, 99%), ZnO (Waco, 99.0%), and CdO (Aldrich, 99.5%) were employed as obtained. Single crystals of the title materials were grown hydrothermally. For ZnMoSb2O7, 0.2 mmol of ZnO (0.016 g), 0.2 mmol of MoO3 (0.029 g), 0.4 mmol of Sb2O3 (0.117 g), and water (5 mL) were combined. For CdMoSb4O10, 0.2 mmol of CdO (0.026 g), 0.2 mmol of MoO3 (0.029 g), 0.4 mmol of Sb2O3 (0.117 g), and water (5 mL) were mixed. The combined reagents were transferred into the respective stainless steel autoclaves with Teflon liners. After the reactors were tightly closed, the temperature of the reactors was increased to 200 °C. After 3 days of heating, the autoclaves were cooled to room temperature and opened. Colorless plates of ZnMoSb2O7 (52% yield) and colorless rods of CdMoSb4O10 (68% yield) were isolated along with small amounts of unreacted polycrystalline Sb2O3. After determination of the crystal structures, pure polycrystalline samples of ZnMoSb2O7 and CdMoSb4O10 were synthesized via solid-state reactions. Stoichiometrically added oxides (ZnO or CdO, MoO3, and Sb2O3) were mixed intimately, packed down into pellets, and placed in quartz tubes. To prevent oxidation of Sb3+ during the reactions, the tubes were flame-sealed under vacuum. Heating of the reaction mixtures at 500 °C for 24 h afforded pure phases (see the powder X-ray diffraction patterns in Figure 1). NCS ZnMoSb2O7 was put in the Noncentrosymmetric Materials Bank (http://ncsmb.knrrc.or.kr). Characterization. A colorless plate (0.03 mm × 0.06 mm × 0.18 mm) of ZnMoSb2O7 and a colorless rod (0.02 mm × 0.03 mm × 0.30 mm) Received: April 15, 2016

A

DOI: 10.1021/acs.inorgchem.6b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 2. Selected Bond Distances (Å) for ZnMoSb2O7 and CdMoSb4O10 ZnMoSb2O7 Sb(1)−O(1) Sb(1)−O(2) Sb(1)−O(3) Sb(1)−O(4) Sb(2)−O(1) Sb(2)−O(2) Sb(2)−O(3) Sb(2)−O(4) Mo(1)−O(4) Mo(1)−O(5) Mo(1)−O(6) Mo(1)−O(7) Zn(1)−O(1) Zn(1)−O(2) Zn(1)−O(3) Zn(1)−O(6)

CdMoSb4O10 2.010(6) 2.025(7) 2.085(6) 2.282(6) 2.119(6) 2.025(7) 2.004(5) 2.299(7) 1.872(6) 1.719(5) 1.780(7) 1.727(8) 1.967(7) 1.968(5) 1.965(7) 2.013(7)

Sb(1)−O(1) Sb(1)−O(2) Sb(1)−O(3) Sb(2)−O(1) × 2 Sb(2)−O(4) Sb(3)−O(3) × 2 Sb(3)−O(4) Mo(1)−O(5) Mo(1)−O(6) × 2 Mo(1)−O(7) Cd(1)−O(1) × 2 Cd(1)−O(3) × 2 Cd(1)−O(5) × 2

1.988(4) 1.965(2) 1.997(3) 1.992(4) 1.950(5) 1.996(4) 1.955(5) 1.818(5) 1.759(4) 1.779(5) 2.501(4) 2.419(3) 2.278(3)

Figure 1. Calculated and experimental powder X-ray diffraction patterns of (a) ZnMoSb2O7 and (b) CdMoSb4O10. of CdMoSb4O10 were selected for single-crystal X-ray diffraction. The diffraction data (Mo Kα radiation) were obtained at room temperature with a Bruker SMART BREEZE diffractometer and integrated with the program SAINT.7 The program SADABS8 was used for absorption corrections. The structure solution and refinement were obtained using SHELXS-97 and SHELXL-97,9 respectively, in the software package WinGX-98.10 Important crystallographic data are compiled in Table 1, and representative bond lengths are shown in Table 2.

Table 1. Crystallographic Data for ZnMoSb2O7 and CdMoSb4O10 fw space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z T (K) λ (Å) ρcalcd (g cm−3) Flack parameter R(F)a Rw(Fo2)b

ZnMoSb2O7

CdMoSb4O10

516.85 P21 (No. 4) 5.1697(10) 8.4910(10) 7.4849(10) 99.330(10) 324.21(9) 2 298.0(2) 0.71073 5.294 0.15(3) 0.0236 0.0489

855.39 P21/m (No. 11) 7.1048(10) 7.6798(10) 9.0713(10) 94.561(10) 493.39(11) 2 298.0(2) 0.71073 5.758 N/A 0.0294 0.0632

R(F) = ∑||Fo| − |Fc||/∑|Fo|. ∑w(Fo2)2]1/2.

a

b

Figure 2. ORTEP (50% probability ellipsoids) drawing of the structural building units in ZnMoSb2O7. The framework is composed of MoO4, ZnO4, and SbO4 polyhedra. UV−vis diffuse-reflectance spectra were collected on a Varian Cary 500 scan UV−vis−NIR spectrophotometer. The absorbance data were obtained from the measured reflectance spectra using the Kubelka− Munk Function.11 Infrared spectra were obtained with a Thermo Scientific Nicolet iS10 FT-IR spectrometer. Thermal analysis was performed with a SCINCO TGA N-1000 thermal analyzer up to 900 °C (flow rate, 10 °C min−1; flowing gas, argon). Energy-dispersive X-ray analysis was carried out to obtain approximate elemental ratios of the synthesized materials using a Horiba Energy EX-250 instrument attached to a Hitachi S-3400N scanning electron microscope. The observed elemental analysis data for ZnMoSb2O7 and CdMoSb4O10 matched well to the stoichiometric ratios of the title compounds [(Zn or Cd):Mo:Sb = 1.0:1.0:2.1 and 1.0:1.0:4.1, respectively]. Second-harmonic generation (SHG) properties were measured on NCS ZnMoSb2O7 using a DAWA Q-switched Nd:YAG laser (1064 nm

Rw(Fo2) = [∑w(Fo2 − Fc2)2/

Phase purities for the reported compounds were evaluated by powder X-ray diffraction using a Bruker D8 Advance diffractometer (Cu Kα radiation). B

DOI: 10.1021/acs.inorgchem.6b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry radiation operating at 20 Hz).12 Ground ZnMoSb2O7 was graded into different particle sizes with sieves to resolve the phase-matching capacity (type I). Standard samples of α-SiO2 and KH2PO4 (KDP) were also measured in order to make suitable comparisons. The SHG light reflected from the graded samples in distinct capillary tubes was detected with a Hamamatsu photomultiplier tube. The SHG signal was read with a Tektronix TDS1032 oscilloscope. A comprehensive SHG measurement illustration was reported earlier.13

phase-pure forms. Several synthetic efforts were also made to obtain CdMoSb2O7 and ZnMoSb4O10 using similar solidstate reactions. However, every attempt resulted in mixtures of CdMoSb4O10, Sb2MoO6, CdO, and Sb2O3 rather than CdMoSb2O7 and mixtures of ZnMoSb2O7 and Sb2O3 instead of ZnMoSb4O10, possibly because of the size difference between Zn2+ and Cd2+ that can accommodate different numbers of polyhedra of Sb3+ in the coordination environments. Structures. ZnMoSb2O7. ZnMoSb2O7 is a novel quaternary mixed-metal oxide with a three-dimensional (3D) framework comprising d0 (Mo6+), d10 (Zn2+), and lone-pair (Sb3+) cations. The polar 3D framework of ZnMoSb2O7 (space group P21) is composed of ZnO4 tetrahedra, MoO4 tetrahedra, and SbO4 seesaws (see Figure 2). The Zn2+ cation in a distorted tetrahedral environment with four oxide ligands exhibits O−Zn−O

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RESULTS AND DISCUSSION Synthesis. As described in the Experimental Section, single crystals of ZnMoSb2O7 and CdMoSb4O10 were obtained through hydrothermal reactions under the same reaction conditions using the same ratios of the respective starting reagents. After the structures of ZnMoSb2O7 and CdMoSb4O10 were determined, bulk samples of the title compounds were successfully synthesized in

Figure 3. Ball-and-stick representations showing (a) an infinite zigzag chain of edge-sharing SbO4 polyhedra along the [010] direction, (b) a layer structure in the ab plane formed by the linking of ZnO4 tetrahedra, and (c) the complete three-dimensional framework obtained by the connection of MoO4 tetrahedra in ZnMoSb2O7. Colors: cyan, Zn; blue, Mo; green, Sb; red, O. Six-membered ring (6-MR) channels running along the [100] direction are observed in the 3D framework of ZnMoSb2O7. C

DOI: 10.1021/acs.inorgchem.6b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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bond angles and Zn−O bond lengths of 93.7(3)−119.1(3)° and 1.965(7)−2.013(7) Å, respectively. The Mo6+ cation is also connected to four oxide ligands in a deformed tetrahedral geometry in which two terminal Mo−O bond lengths are notably shorter than the others (see Table 2 and the Supporting Information). The observed O−Mo−O bond angles found from the distorted MoO4 tetrahedra vary from 100.3(3) to 121.1(3)°. Within an asymmetric unit, two unique Sb3+ cations exist, and they are bonded to four oxide ligands in an unsymmetrical seesaw geometry as a result of the lone pairs; the obtained Sb−O bond distances and O−Sb−O angles are 2.004(5)−2.299(7) Å and 75.4(3)−150.2(2)°, respectively. The SbO4 polyhedra share edges through their oxygen atoms, forming infinite zigzag chains along the [010] direction (see Figure 3a). The zigzag chains of edge-sharing SbO4 polyhedra are linked by ZnO4 tetrahedra, resulting in a layer in the ab plane (see Figure 3b). Within the layer, the corners of uncoordinated ZnO4 tetrahedra linkers alternately point in approximately the [010] and [01̅0] directions. Each layer is further connected by MoO4 tetrahedra through O(4) and O(6). As shown in Figure 3c, a complete 3D structure is formed. Thus, ZnO4 and MoO4 tetrahedra serve as interchain and interlayer linkers, respectively. Six-membered ring (6-MR) channels are observed

Figure 4. ORTEP (50% probability ellipsoids) drawing exhibiting the backbone of CdMoSb4O10 comprising MoO4, CdO6, and SbO3 polyhedra.

Figure 5. (a) Ball-and-stick and polyhedral models of unidimensional zigzag chains of corner-sharing CdO6 octahedra running along the [010] direction in CdMoSb4O10. (b) Ball-and-stick and wire representations of zigzag ladders composed of SbO3 groups that run parallel to the [010] direction. (c) Combining the antimony oxide zigzag ladders, the chains of corner-sharing CdO6, and the MoO4 tetrahedra completes onedimensional rods that run in the [010] direction. (d) The lone pairs on the asymmetric Sb3+ cations are directed toward the outside of the 1D rods. Colors: orange, Cd; blue, Mo; green, Sb; red, O. D

DOI: 10.1021/acs.inorgchem.6b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry along the [100] direction in the 3D framework of ZnMoSb2O7. The lone pairs of Sb3+ cations pointing inward in the 6-MRs. As shown in Figure 3c, a 21 screw axis along the [010] direction is found, with all of the corners of the MoO4 tetrahedra, i.e., O(7), pointing in the [010̅ ] direction, confirming the polar NCS structure of ZnMoSb2O7. The connectivity of the backbone of ZnMoSb2O7 can be recorded as a neutral framework of {[ZnO3/3O1/2]−1 [MoO2/1O1/2O1/3]+0.333 2[SbO4/3]+0.333}0. The bond valence sums14 for Zn2+, Mo6+, and Sb3+ in ZnMoSb2O7 are calculated to be 1.90, 5.78, and 2.88−2.95, respectively. CdMoSb4O10. The other new quaternary oxide, CdMoSb4O10, crystallizes in a CS structure (space group P21/m). As shown in Figure 4, the backbone of CdMoSb4O10 consists of CdO6, MoO4, and SbO3 polyhedra. Each Cd2+ is surrounded by six oxide ligands with Cd−O bond lengths of 2.278(2)− 2.501(4) Å. The d0 cation Mo6+ exhibits a distorted tetrahedral coordination moiety with O−Mo−O angles and Mo−O distances of 104.5(3)−118.4(2)° and 1.759(4)−1.818(5) Å, respectively. All three unique Sb3+ cations existing in an asymmetric unit exhibit trigonal-pyramidal SbO3 geometries with three oxide ligands in which the Sb−O bond lengths and O−Sb−O bond angles are 1.950(5)−1.997(3) Å and 82.15(15)−97.9(2)°, respectively. The CdO6 octahedra form 1D zigzag chains along the [010] direction by sharing their corners via O(3) (see Figure 5a). Meanwhile, the SbO3 groups share all of their oxygen atoms to construct extraordinary zigzag ladders parallel to the [010] direction (see Figure 5b). Three-membered rings (3-MRs) and 6-MRs exist parallel to the [001] and [010] directions, respectively, within the zigzag ladders. In fact, the wiggly ladderlike framework composed of only SbO3 polyhedra has not been observed previously. The antimony oxide zigzag ladders wrap around the chains of vertex-shared CdO6 octahedra, and the connections of MoO4 tetrahedra through O(3) complete one-dimensional (1D) rods that run along the b axis (see Figure 5c). The lone pairs on the asymmetric Sb3+ cations are directed toward the outside of the 1D rods (see Figure 5d). Similar tubules containing lone pairs pointing outward have been observed in Sb16Cd8O25Cl14.15 However, the morphologies are quite different: the infinite tubes found in Sb16Cd8O25Cl14 consist of SbO4 polyhedra and wrap around edge-sharing double chains of CdO6 octahedra. The framework of CdMoSb4O10 may be represented as a neutral rod of {[CdO6/3]−2 [MoO3/1O1/3]−0.667 4[SbO2/3O1/2]+0.667}0 in connectivity terms. Bond valence sum calculations14 on CdMoSb4O10 yielded values of 1.62, 5.67, and 2.92−2.96 for Cd2+, Mo6+, and Sb3+, respectively. Spectroscopic Characterization. Band gaps for ZnMoSb2O7 and CdMoSb4O10 can be calculated from the data obtained from reflectance spectra using the Kubelka−Munk function.11 Specifically, the band gaps were extracted by extrapolating the straight line in the plots of K/S versus E. The band gaps estimated from the plots are 3.3 and 3.4 eV for ZnMoSb2O7 and CdMoSb4O10, respectively (see Figure 6a). The IR spectra of the title compounds reveal the Mo−O, Sb−O, and Zn−O vibrations (see Figure 6b). The peaks occurring at ca. 800−950 cm−1 should be due to Mo−O symmetric stretching vibrations, while those for 650−750 cm−1 should be attributable to Sb−O vibrations. The peaks found at ca. 400−450 cm−1 must be due to Mo−O bending. Although the peaks due to Zn−O were found at ca. 420 cm−1, those for Cd−O were not observed because the vibrational bands normally occur below 400 cm−1. Assignments for all of the observed bands are compatible with those for known oxides containing the specific bonds.16

Figure 6. (a) UV−vis diffuse-reflectance spectra and (b) IR spectra of ZnMoSb2O7 and CdMoSb4O10.

Thermal Analyses. The thermal properties of ZnMoSb2O7 and CdMoSb4O10 were studied by thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD) performed at different temperatures (see Figure 7). Both materials are quite stable at higher temperatures: no weight losses were found up to 600 and 700 °C for ZnMoSb2O7 and CdMoSb4O10, respectively. Above these temperatures, ZnMoSb2O7 decomposes to ZnMoO4 by losing Sb2O3, whereas CdMoSb4O10 collapses to a mixture of CdMoO4 and Sb2O4 by 1000 °C through a disproportionation, as confirmed by PXRD patterns measured for the samples heated to higher temperatures (see the insets in Figure 7). Also, the PXRD patterns reveal that no phase transformations occur for either material at higher temperatures. Second-Harmonic Generation (SHG) and Its Structural Origin. The SHG properties of polar ZnMoSb2O7 were investigated using 1064 nm radiation. Powder SHG measurements on the graded sample revealed that ZnMoSb2O7 exhibits an SHG efficiency of about 10 times that of α-SiO2, which compares well to that of ZnO.12 Further SHG measurements on the graded samples with distinct particle size ranges indicated that ZnMoSb2O7 is a type-I non-phase-matchable SHG material (see Figure 8a). The observed SHG phenomenon of ZnMoSb2O7 can be explained by analyzing the arrangement of the constituent polyhedra in the framework structure. As we explained earlier, three different kinds of polyhedra of polarizable cations, i.e., Zn2+, Mo6+, and Sb3+, exist in ZnMoSb2O7. The coordination environment of the d10 cation Zn2+ with four oxygen atoms is a E

DOI: 10.1021/acs.inorgchem.6b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. TGA diagrams and (insets) PXRD patterns of (a) ZnMoSb2O7 and (b) CdMoSb4O10.

distorted tetrahedron. Specifically, as shown in Figure 8b, the ZnO4 tetrahedra contain one long and three short Zn−O bonds, resulting in polarizations pointing in approximately the [001̅] and [001] directions. Therefore, the moment arising from the two ZnO4 tetrahedra within a unit cell effectively cancel. Two unique Sb3+ lone-pair cations are also found within the unit cell. The lone pairs on Sb3+ in the SbO4 polyhedra point in approximately the [001] and [001̅] directions, and thus, the overall moment originating from Sb3+ cations is also negligible. Finally, the d0 cation Mo6+ exhibits another distorted tetrahedral moiety with two long and two short Mo−O bonds, which create moments in the [1̅1̅0] and [11̅0] directions. Thus, a net polarization occurs parallel to the [01̅0] direction, which should account for the observed SHG signal of ZnMoSb2O7 (see Figure 8b). Also, the existence of a partial inversion twinning may have an influence on the relatively weaker SHG efficiency. The dipole moments of the asymmetric polyhedra in the title compounds were calculated in order to obtain a better understanding of their local moieties.17 Especially with SbO3 and SbO4 polyhedra, a localized distance of 1.08 Å between Sb3+ and the lone pair was adopted from the previously reported work, and the charge of the lone pair was assigned as −2.2d By means of this method, the dipole moments of ZnO4, CdO6, MoO4, SbO3, and SbO4 groups in the reported materials

Figure 8. (a) Phase-matching curve (type 1) for ZnMoSb2O7. The curve is to guide the eye and is not a fit to the data. (b) Ball-and-stick representation revealing moments occurring from ZnO4, SbO4, and MoO4 polyhedra in ZnMoSb2O7 (s and l denote short and long, respectively). When these are taken as a whole, a net moment is found along the [01̅0] direction.

were approximately calculated to be 2.6, 0, 4.4−7.5, 9.7−11.4, and 10.0−10.3 D, respectively, which are quite similar to the values of formerly reported polyhedra.15 With NCS polar ZnMoSb2O7, the calculated net dipole moment along the [01̅0] direction for a unit cell is ca. 9.7 D, which agrees well with the structural analysis mentioned above. The dipole moments for each group are shown in Table 3. While ZnMoSb2O7 containing the smaller d10 transition metal cation Zn2+ is NCS polar, CdMoSb4O10 with the larger d10 cation Cd2+ is CS nonpolar. Because of the different ionic radii, the smaller Zn2+ cation prefers to have a small coordination environment whereas the larger Cd2+ cation can interact F

DOI: 10.1021/acs.inorgchem.6b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 3. Calculated Dipole Moments of ZnO4, CdO6, MoO4, SbO3, and SbO4 Polyhedra compound

polyhedron

dipole moment (D)a

ZnMoSb2O7

ZnO4 MoO4 Sb(1)O4 Sb(2)O4 CdO6 MoO4 Sb(1)O3 Sb(2)O3 Sb(3)O3

2.6 7.5 10.0 10.3 0 4.4 9.7 10.7 11.4

CdMoSb4O10

a

with more oxide ligands in the polyhedra of Mo6+ and Sb3+ around the coordination sphere. To maintain the framework in a restricted space, the coordinated polyhedra around the small Zn2+ cation tend to be lined up, which leads to the polar NCS structure. However, the larger cation Cd2+ can accommodate more polyhedra in the coordination environment, in which the interacting groups are lined up in an antiparallel manner. In other words, to achieve a more effective packing, the material crystallizes in a CS structure. Similar cation size effects influencing the macroscopic centricity have been reported previously.5f,g

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CONCLUSIONS Two novel quaternary mixed-metal oxides consisting of polarizable cations, namely, ZnMoSb2O7 and CdMoSb4O10, were successfully synthesized in phase-pure form. While ZnMoSb2O7 reveals a polar structure with a 3D framework, CdMoSb4O10 exhibits a nonpolar structure with infinite unidimensional tubules. The band gaps estimated from the UV−vis spectral data for ZnMoSb2O7 and CdMoSb4O10 are 3.3 and 3.4 eV, respectively. ZnMoSb2O7 and CdMoSb4O10 exhibit thermal stability to 600 and 700 °C, respectively, without any phase transitions. Powder SHG measurements revealed that NCS ZnMoSb2O7 is a non-phase-matchable material (type I) with an SHG efficiency comparable to that of ZnO. Detailed structural analyses suggest that the SHG observed in NCS ZnMoSb2O7 is due to the moment originating from the distorted MoO4 tetrahedra. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00944. X-ray crystallographic file for ZnMoSb2O7 and CdMoSb4O10 (CIF)

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D = debye.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Grant 2014M3A9B8023478). G

DOI: 10.1021/acs.inorgchem.6b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b00944 Inorg. Chem. XXXX, XXX, XXX−XXX