Assisting the Effective Design of Polar Iodates with Early Transition

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Assisting the Effective Design of Polar Iodates with Early Transition Metal Oxide Fluoride Anions Hongwei Yu, Matthew L. Nisbet, and Kenneth R. Poeppelmeier J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04762 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 13, 2018

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Journal of the American Chemical Society

Assisting the Effective Design of Polar Iodates with Early Transition Metal Oxide Fluoride Anions Hongwei Yu,†,‡ Matthew L. Nisbet, ‡ and Kenneth R. Poeppelmeier*,‡ †

College of Functional Crystals, Tianjin University of Technology, No. 391 Bin Shui Xi Dao Road, Xiqing District, Tianjin 300384, China ‡ Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States To whom correspondence should be addressed. E-mail: [email protected] (Kenneth R. Poeppelmeier). KEYWORDS (Word Style “BG_Keywords”). Polar Materials, Iodates, Oxide-fluoride, NLO, Phase Transition

ABSTRACT: Polar materials are of great technical interest but challenging to effectively synthesize. That is especially true for iodates, an important class for visible and mid-IR transparent nonlinear optical (NLO) materials. Aiming at developing new design strategy for polar iodates, we successfully synthesized two sets of polymorphic early transition metal (ETM) oxide-fluoride iodates, ɑ and β-Ba[VFO2(IO3)2], and ɑ and β-Ba2[VO2F2(IO3)2]IO3 based on distinct structure-directing properties of oxide-fluoride anions. ɑ and β-Ba[VFO2(IO3)2] contain the trans-[VFO2(IO3)2]2+ poly-anion and crystallize in the non-polar space groups Pbcn and P212121, in contrast ɑ and β-Ba2[VO2F2(IO3)2]IO3 contain the cis-[VO2F2(IO3)2]3+ Λ-shaped poly-anion and crystallize in the polar space groups Pna21 and P21, respectively. Detailed structural analyses show the variable polar orientation of trans-[VFO2(IO3)2]2+ poly-anions is the main cause of the non-polar structures in ɑ and β-Ba[VFO2(IO3)2]. However, the Λ-shaped configuration of cis[VO2F2(IO3)2]3+ poly-anions can effectively guarantee the polar structures. Further property measurements show that polar ɑ and βBa2[VO2F2(IO3)2]IO3 possess excellent NLO properties, including the large SHG responses (~9 × KDP), wide visible and mid-IR transparent region (~0.5-10.5 µm), and high thermal stability (up to 470 oC). Therefore, combining cis-directing oxide-fluoride anions and iodates is a viable strategy for the effective design of polar iodates.

Introduction Noncentrosymmetric (NCS) compounds, especially those that are polar, are of current academic and technological interest because of their functional properties, such as piezoelectricity, pyroelectricity, ferroelectricity and second harmonic generation (SHG).1-5 However, rationally designing and synthesizing a NCS and polar material remains a challenge because the NCS crystal engineering involves not only creating the specific acentric basic building units (BBUs) but also packing the polar directionality of these BBUs in an additive fashion.6-7 Generally, the acentric BBUs can be effectively created by a variety of strategies. For example, in oxide coordination environments, transition metal cations with d0 electron configurations or cations with stereochemically active lone pairs (SCALP) can be employed to form distorted polyhedra.8-14 Using acentric inorganic π-conjugated systems, particularly planar borate rings, is also ideal for constructing acentric BBUs for UV and deep-UV nonlinear optical (NLO) materials.15-22 For example, Pan, et al have chelated Pb2+ cations and BO3 groups to enhance SHG response of Pb2Ba3(BO3)3Cl.16 In addition, recent research shows that using fluoride to partially substitute for oxygen can increase the polarizability of the coordinated polyhedra and is another effective strategy for creating polar BBUs.23-28 However, acentric BBUs are not enough for the design of polar materials. To express a macroscopic polarization in bulk materials, these acentric BBUs must be packed in an additive fashion. But that can prove to be quite difficult because the polar anions typically tend to adopt antiparallel arrangements

to increase local electroneutrality, which is energetically favorable according to Pauling’s well-known second rule.29 In order to approach the alignment of polar BBUs, our previous studies have focused on the understanding of the structure-directing properties of oxide-fluoride anions.30-38 We have realized that ETM oxide-fluoride anions could exhibit trans- and cis- structure-directing properties based on the distribution of the ligands, which arise from the inherent difference of oxide and fluoride anions as well as the dπ-pπ metal-oxide orbital interactions.32-33, 36 These structure directing properties can be exploited to qualitatively predict the stable crystallographic orientations of BBUs in oxidefluorides and further to guide designing new materials with specific structure-related properties.32, 34, 38 In addition to these structure-directing properties, early transition metal (ETM) oxide fluoride anions can adopt Λ-shaped bimetallic species. It has been demonstrated that the Λ-shaped BBUs favor polar structures, owing to their predictable crystal packing.39-40 With these strategies, a series of polar oxide-fluorides have been successfully designed.34-42 In this study, we are interested in the polar iodates, which are an important class of visible and mid-IR NLO materials because of their excellent NLO properties, including very strong SHG responses, wide transparent region from near-UV to mid-IR region, and high optical high damage thresholds at the level of GW/cm2.7, 43-50 During the past several years, the reported high-performance iodate NLO materials consist of K(VO)2O2(IO3)3 (3.6 × KTP),43 BaNbO(IO3)5 (14 × KDP),9

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Li2Ti(IO3)6 (500 × ɑ-SiO2),7 and AMoO3(IO3) (A = Rb, Cs) (400 × ɑ-SiO2), etc.45 Notably, although many excellent iodate NLO materials have been successfully synthesized, the strategies for rationally designing polar iodates are still underexploited. Therefore, we pursued the structure-directing properties of ETM oxide-fluoride anions and the Λ-shape42 methodology to develop a new design strategy for polar iodates. Guided by these ideas, we have successfully designed and synthesized two sets of new polymorphic ETM oxide-fluoride iodates, ɑ and β-Ba[VFO2(IO3)2], and ɑ and βBa2[VO2F2(IO3)2]IO3 through introducing transdirecting [VO4F]4- trigonal bipyramid and cis-directing [VO4F2]5octahedron, respectively. ɑ and β-Ba[VFO2(IO3)2] crystallize in the non-polar space groups Pbcn and P212121, respectively, while ɑ and β-Ba2[VO2F2(IO3)2]IO3 crystallize in the polar space groups Pna21 and P21, respectively. These compounds possess excellent NLO properties and are potential visible and mid-IR NLO materials. Herein, we will report their syntheses, crystal structures, phase transitions, the roles of ETM oxide fluoride anions in the design of polar iodates, and their functional properties.

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mol) of HIO3 were used. For β-Ba[VFO2(IO3)2], 0.317 g (1.300 × 10-3 mol) of BaCl2·2H2O, 0.273 g (1.500 × 10-3 mol) of V2O5, and 0.401 g (2.273 × 10-3 mol) of HIO3 were used. For ɑ-Ba2[VO2F2(IO3)2]IO3, 0.603 g (2.456 × 10-3 mol) of BaCl2·2H2O, 0.1 g (0.550 × 10-3 mol) of V2O5, and 0.301 g (1.705 × 10-3 mol) of HIO3 were used. For βBa2[VO2F2(IO3)2]IO3, 0.400 g (1.638 × 10-3 mol) of BaCl2·2H2O, 0.303 g (1.648 × 10-3 mol) of V2O5, and 0.305 g (1.705 × 10-3 mol) of HIO3 were used. These mixtures were filled in the pouches and then 0.3 ml (7 × 10-3 mol) of 48% aqueous HF was also added into each Teflon pouch as fluorine resource and mineralizer. These pouches are further sealed with an impulse sealer and placed into 125 ml Parr autoclaves with 45 ml of deionized water as backfill. These autoclaves were quickly heated to 220 °C, held for 48 h and cooled to the room temperature at 3 °C/h. Millimeter-sized crystals can be obtained with vacuum filtration in air. The yield of ɑ Ba[VFO2(IO3)2] is around 90% based on BaCl2·2H2O. The yields for β-Ba[VFO2(IO3)2] and ɑ and β-Ba2[VO2F2(IO3)2]IO3 are ~70-80% based on BaCl2·2H2O. Crystallographic Determination. Single crystal X-ray diffraction experiments were conducted at 100 K on a BrukerAPEX II CCD diffractometer with monochromatic Mo Kα radiation (λ = 0.71069 Å). The detector distance was set as 60 mm. The data were integrated with SAINT-V7.23A.51 Multiscan techniques were applied for absorption corrections with SADABS. The structures were solved by direct methods with SHELXS crystallographic software package.52 All atoms were refined using full matrix least-squares techniques. The final least-squares refinement is on Fo2 with data having Fo2 ≥ 2σ (Fo2). All the structures were checked by PLATON and no additional symmetry elements were found.53 Their crystallographic data and structure refinement information are given in Table 1. Final atomic coordinates and equivalent isotropic displacement parameters and selected interatomic distances and angles are given in Table S1 and Table S2, respectively.

Experimental Section. Caution. Hydrofluoric acid is toxic and corrosive! It must be handled with extreme caution and the appropriate protective equipment and training. Materials. Barium chloride dihydrate (BaCl2·2H2O, 99.8%Alfa Aesar), Vanadium oxide (V2O5, 99.6%, Alfa Aesar), hydrofluoric acid (HFaq, 48% by weight, Sigma Aldrich), Iodic acid (HIO3, 99.9%, Alfa Aesar) were used as received. Teflon film [fluoro-(ethylenepropylene), FEP] was obtained from American Durafilm. Hydrothermal Syntheses. The crystals of the reported compounds were synthesized by the hydrothermal method. For ɑ-Ba[VFO2(IO3)2], 0.302 g (1.228 × 10-3 mol) of BaCl2·2H2O, 0.112 g (0.615 × 10-3 mol) of V2O5, and 0.613 g (3.485 × 10-3 Table 1. Crystal data and structure refinement for ɑ and β-Ba[VFO2(IO3)2], and ɑ and β-Ba2[VO2F2(IO3)2]IO3. Empirical formula Formula weight Crystal system space group a/Å b/Å c/Å β/O V/ /Å3 Z Dcalcd/g·cm-3 µ(Mo_Kα)/mm-1 Flack factor GOF on F2 R1, wR2 (I > 2σ(I))a R1, wR2 (all data) [a]

ɑBa[VFO2(IO3)2] 589.08 Orthorhombic Pbcn 5.1494(9) 12.644(2) 12.400(2) 90 807.4(2) 4 4.846 13.685 N/A 1.390 0.0123, 0.0295 0.0123, 0.0296

β-Ba[VFO2(IO3)2]

α-Ba2[VO2F2(IO3)2]IO3

β-Ba2[VO2F2(IO3)2]IO3

589.08 Orthorhombic P212121 7.1902(8) 8.1964(8) 13.0420(15) 90 768.61(14) 4 5.091 14.375 0.40(6) 1.062 0.0397, 0.0928 0.0412, 0.0937

920.32 Orthorhombic Pna21 13.578(8) 11.425(7) 7.500(4) 90 1163.5(12) 4 5.254 15.523 0.02(4) 0.983 0.0268, 0.0489 0.0330, 0.0521

920.32 Monoclinic P21 7.38950(10) 7.49940(10) 10.9095(2) 105.4610(10) 582.692(15) 2 5.245 15.497 0.06(2) 1.171 0.0158, 0.0389 0.0158, 0.0390

R1 = Σ||Fo| - |Fc||/Σ|Fo| and wR2 = [Σw(Fo2 – Fc2)2 / Σw Fo4]1/2 for Fo2 > 2σ( Fo2)


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Infrared Spectroscopy. The Fourier transform infrared spectroscopy (FTIR) spectra in the 600-4000 cm-1 range were recorded on a Bruker Tensor 37 FTIR. The UV-vis-NIR diffuse reflectance spectra: The UV-visNIR diffuse reflectance spectra were measured at room temperature with Shimadzu SolidSpec-3700DUV spectrophotometer in the 200 - 2600 nm wavelength range. Thermal analysis: The thermal properties were measured on NETZSCH-Proteus-61 analyzer instrument under flowing nitrogen gas, heated from room temperature to 750 °C at a rate of 10 °C min-1 in a Al2O3 crucible. SHG Measurement. Powder SHG were measured by using the Kurtz-Perry method with Q-switched Nd:YAG lasers at the wavelength of 1064.54 The reported compounds and KDP were ground and sieved into distinct particle size ranges (120 µm). The sieved KDP powder was used as a reference. The intensities of the frequency-doubled output emitted from the sample were measured using a photomultiplier tube. Results and Discussion Syntheses. It is interesting that the different polymorphs were synthesized under similar hydrothermal reaction conditions. The main difference is the amount of starting reagents. In the higher symmetry ɑ Ba[VFO2(IO3)2] and ɑ Ba2[VO2F2(IO3)2]IO3, small amounts of V2O5 (~0.1 g) were used and there was no V2O5 remaining after these reactions. Whereas in the lower symmetry β-Ba[VFO2(IO3)2] and βBa2[VO2F2(IO3)2]IO3, both were synthesized with a larger amounts of V2O5 (~0.3 g) and there was a trace of V2O5 remaining after these reactions. The solubility of V2O5 and its role in these reactions is under further investigation.

Crystal Structures and Phase Transitions for α- and β-Ba[VFO2(IO3)2]. α-Ba[VFO2(IO3)2] and β-Ba[VFO2(IO3)2] have the same stoichiometry, but they possess markedly different structures. α-Ba[VFO2(IO3)2] crystallizes in the centrosymmetric space group Pbcn, while β-Ba[VFO2(IO3)2] crystallizes in NCS and non-polar space group P212121. In order to describe structures conveniently, the phase with high symmetry was labelled the α-phase, and conversely the phase with low symmetry was labelled the β-phase.

Figure 1. The structure of α-Ba[VFO2(IO3)2], (a) the [VFO2(IO3)2]2- poly-anion created by the [VO4F]4- trigonal bipyramid connecting two IO3 units via apical O atoms; (b) the ten-coordinated Ba2+ cations, (c) the [VFO2(IO3)2]2- polyanions are separated by Ba2+ cations to form the structure of αBa[VFO2(IO3)2]. The structure of α-Ba[VFO2(IO3)2] is shown in Figure 1. It is composed of the zero-dimensional (0D) [VFO2(IO3)2]2poly-anions and Ba2+ cations. Firstly, the V5+ cation is

coordinated by four O atoms and one F atom to form a [VO4F]4- trigonal bipyramid. The I5+ cation is coordinated by three O atoms to form the IO3- group. Then, one [VO4F]4trigonal bipyramid connects with two IO3- groups through its two apical O atoms to form the [VFO2(IO3)2]2- poly-anion (Figure 1a). The isolated [VFO2(IO3)2]2- poly-anion clusters are separated by Ba2+ cations to form the structure of αBa[VFO2(IO3)2] (Figure 1b and 1c). In the [VFO2(IO3)2]2poly-anions, both V5+ cations and I5+ cations adopt asymmetric coordination environments. In the [VO4F]4- trigonal bipyramid, the bond lengths of these two apical V-O bonds are 1.910(3) and 1.976(2) Å, respectively. The bond lengths for the two equatorial V-O bonds are both 1.634(2) Å, and the equatorial V-F bond length is 1.910(3) Å. Therefore, the [VO4F]4- unit is an elongated trigonal bipyramid. For I5+ cations, all of the coordinated O atoms fall within the same hemisphere around I5+ cations attributed to the stereochemically active effect of the lone-pair. I-O bond lengths range from 1.792(2) to 1.858(2) Å. The Ba2+ cation is coordinated by two F atoms and eight O atoms with Ba-O bond lengths ranging from 2.741(2) to 2.911(2) Å and the BaF bond length is 2.7379(9) Å. Bond valence calculations resulted in bond valence sums (BVS) of 2.32 for the Ba2+ cation, 4.90 for the I5+ cation, 5.08 for the V5+ cation, 1.18 for the F- anion and for oxygen, range from 1.87-2.27 (Table S1).55-56

Figure 2. The structure of β-Ba[VFO2(IO3)2], (a) the [VFO2(IO3)2]2- poly-anion created by [VO4F]4- square pyramid connecting two IO3 units via apical O atoms; (b) elevencoordinated Ba2+ cation, (c) the [VFO2(IO3)2]2- poly-anions are separated by Ba2+ cation to form the structure of βBa[VFO2(IO3)2]. The structure of β-Ba[VFO2(IO3)2] is shown in Figure 2. Like α-Ba[VFO2(IO3)2], β-Ba[VFO2(IO3)2] contains 0D [VFO2(IO3)2]2- poly-anion clusters, which are composed of [VO4F]4- square pyramids and two IO3- groups that share the two opposite equatorial O atoms of the [VO4F]4- tetragonal pyramid (Figure 2a). The [VFO2(IO3)2]2- clusters are connected by the Ba2+ cations to form the 3D structure of βBa[VFO2(IO3)2] (Figure 2b and 2c). The [VO4F]4- square pyramid is also distorted and contains two short V-O bonds [1.617(6) Å and 1.712(6) Å], two long V-O bonds [1.989(9) Å and 1.990(8) Å], and one V-F bond with the bond length of 1.934(5) Å. The I5+ cations also form into IO3 pyramids and the three O atoms also fall within the same hemisphere around the I5+ cation, indicating that the lone-pair of the I5+ cation is stereo-chemically active. I-O bond lengths range from 1.802(7) to 1.867(8) Å. The Ba2+ cation is coordinated by two F atoms and nine O atoms with Ba-F bond lengths ranging from 2.809(9) to 2.821(8) Å and Ba-O bond lengths in the range from 2.795(8) to 3.118(7) Å. Bond valence calculations resulted in values of 2.28 for the Ba2+ cation, 4.68-4.88 for the



I5+ cations, 4.76 for the V5+ cation, 0.93 for the F- anion and 1.72-2.10 for the O2- anions (Table S1).55-56 The difference between the two structures mainly originates from the different configurations of the [VO4F]4- units. In αBa[VFO2(IO3)2], the [VO4F]4- units adopt the trigonal bipyramid configuration (Figure 1a), while in β-Ba[VFO2(IO3)2], the configuration of [VO4F]4- unit is a square pyramid (Figure 2a). For the five-coordinated ETM cations, the trigonal bipyramid and the square pyramid are two typical configurations and these two configurations can transform one to the other because of the fluxional behavior of the trigonal bipyramid.57-58 The transformation mechanism has been well described in the literature.57-58 Notably, when the [VO4F]4- trigonal bipyramids transform to the [VO4F]4- square pyramids, the steric effect of the equatorial ligands result in elongated equatorial bonds. These bonds reduce the bond valance contribution to V5+ cations. Thus, V5+ cations have more residual positive charges. The residual positive charges allow the V5+ cations and the neighboring O(7) to form the secondary bond with bond length of 2.579 (10) Å (Figure 2c). Hence the [VFO2(IO3)2]∞2chain (Figure 2c). Along this chain, the [VFO2(IO3)2]∞2- BBUs adopt a spiral arrangement that leads to the chiral structure of β-Ba[VFO2(IO3)2].

Crystal Structures and Phase Transitions for α- and β-Ba2[VO2F2(IO3)2]IO3. α-Ba2[VO2F2(IO3)2]IO3 and βBa2[VO2F2(IO3)2]IO3 are also polymorphic. Their space groups, Pna21 and P21, are related by crystallographic groupsubgroup relations, which provide the possibility of a phase transition. Following their symmetry order, they are named as α- and β-phases, respectively.

Figure 3. The BBUs of (a) α-Ba2[VO2F2(IO3)2]IO3 are the [VO2F2(IO3)2]3- poly-anion and (b) isolated IO3- unit; (c and d) Ba cations connects the isolated [VO2F2(IO3)2]3- poly-anions and IO3- units to form the structure of α-Ba2[VO2F2(IO3)2]IO3.

The crystal structure of α-Ba2[VO2F2(IO3)2]IO3 is shown in Figure 3. It is clear that the BBUs are 0D [VO2F2(IO3)2]3- polyanion clusters (Figure 3a) and isolated IO3- units (Figure 3b). In the [VO2F2(IO3)2]3- poly-anion clusters, the V5+ cation forms a [VO4F2]5- octahedron. Four of the vertex sites are occupied by three unique O atoms and one unique F atom, and the other two vertex sites are occupied by disordered O/F at-

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oms (Figure 3a). The I5+ cation is coordinated by three unique O atoms to form the IO3- group. Further, one [VO4F2]5- octahedron connects two IO3- groups through the cis-equatorial O atoms to form the Λ-shaped [VO2F2(IO3)2]3- poly-anion cluster. The Ba2+ cations connect the Λ-shaped [VO2F2(IO3)2]3clusters and isolated IO3- pyramids to complete the structure. (Figure 3c and 3d). Both the V5+ and I5+ cations are coordinated in extremely distorted environments. The [VO4F2]5- octahedron consists of two bridging O atoms and four terminal O or F atoms. Two bridging O atoms, O(4) and O(6), form two relatively long V-O bonds, [2.112(5) Å and 2.156(4) Å]. The terminal V-O bond V-F bonds are 1.628(5) Å and 2.044(4) Å, respectively. The other terminal positions, which are occupied by O/F atoms with a 1:1 occupancy, the V-O/F bond lengths are 1.748(4) Å and 1.786(4) Å. Thus, the [VO4F2]5- octahedron is distorted toward a face (local C3 direction). The magnitude of out-of-center distortion was calculated to be 1.20, indicating that V displays a strong distortion (∆d > 0.80).36 For the IO3 pyramids, all coordinated O atoms fall within the same hemisphere around the I5+ cations and the I-O bond lengths range from 1.808(4) to 1.855(5) Å. In addition, the two unique Ba2+ cations, Ba(1)2+ and Ba(2)2+, are both coordinated by two F atoms and nine O atoms (Figure 3c). The Ba-O bonds range from 2.699(4) to 3.110(5) Å and Ba-F bond lengths range from 2.702(4) to 2.964(4) Å. Bond valence calculations resulted in BVS of 1.87-2.26 for Ba2+ cations, 4.84-4.98 for the I5+ cation, 4.82 for the V5+ cation, 0.61 for the F- anion, and 1.852.20 for O2- anions (Table S1).55-56

Figure 4. The BBUs of (a) β-Ba2[VO2F2(IO3)2]IO3 are the [VO2F2(IO3)2]3- poly-anion and (b) an isolated IO3- unit; (c and d) Ba cations connect the [VO2F2(IO3)2]3- poly-anions and IO3- units to form the structure of β-Ba2[VO2F2(IO3)2]IO3.

The structure of β-Ba2[VO2F2(IO3)2]IO3 is shown in Figure 4. It is clear that β-Ba2[VO2F2(IO3)2]IO3 adopts a very similar structure compared with α-Ba2[VO2F2(IO3)2]IO3. The BBUs of β-Ba2[VO2F2(IO3)2]IO3 are also the [VO2F2(IO3)2]3- poly-anion clusters (Figure 4a) and isolated IO3 units (Figure 4b). The [VO2F2(IO3)2]3- poly-anion cluster in β-Ba2[VO2F2(IO3)2]IO3 is composed of one [VO4F2]5- octahedron and two IO3 groups. The coordination polyhedra of V5+ and I5+ cations in βBa2[VO2F2(IO3)2]IO3 are also extremely distorted. In the [VO4F2]5- octahedra, the bridging O atoms, O(4) and O(6), participate in two relatively long V-O bonds [2.113(4) Å and


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Figure 5. The comparison of α-Ba2[VO2F2(IO3)2]IO3 (a) and β-Ba2[VO2F2(IO3)2]IO3 (b). To better show the difference between the two structures, the Λ-shape of [VO2F2(IO3)2]3- clusters is simplified as VI2 groups. Clearly, in α-Ba2[VO2F2(IO3)2]IO3, the adjacent BBUs adopt anti-parallel arrangement, while in β-Ba2[VO2F2(IO3)2]IO3, the adjacent BBUs simply undergo the translation in the a-c plane.

2.138(4) Å], and the terminal O and F atoms form V-O and VF bonds of 1.621(4) Å and 2.035(3) Å, respectively. Other terminal positions are occupied by O/F atoms with a 1:1 occupancy and V-O/F bond lengths of 1.762(4) Å and 1.764(4) Å. Similar to the previous example, the VO4F2 octahedron is distorted toward a face (local C3 direction) with a strong out-ofcenter distortion (value 1.21). Concerning the IO3 pyramids, all coordinated O atoms fall within the same hemisphere around I5+ cations and I-O bond lengths range from 1.808(4) to 1.855(5) Å. In addition, two unique Ba2+ cations, Ba(1)2+ and Ba(2)2+ , are both coordinated by two F atoms and nine O atoms (Figure 4c), with Ba-O bonds ranging from 2.699(4) to 3.110(5) Å and Ba-F bond lengths ranging from 2.702(4) to 2.964(4) Å. Bond valence calculations resulted in BVS of 1.87-2.24 for the Ba2+ cations, 4.88-4.94 for the I5+ cations, 4.84 for the V5+ cation, 0.60 for the F- anion and 1.91-2.08 for the O2- anions (Table S1).55-56 The main difference between the two structures are the arrangements of the [VO2F2(IO3)2]3- poly-anion clusters and isolated IO3 units. To better show the difference between two structures, the Λ-shape of [VO2F2(IO3)2]3- clusters is simplified as a Λ-shape of VI2 groups (Figure 5). Thus, it is clear that the adjacent [VO2F2(IO3)2]3- poly-anion clusters and the IO3 units in α-Ba2[VO2F2(IO3)2]IO3 both adopt anti-parallel arrangements (Figure 5a), which give two additional glide planes along the a and b axes to α-Ba2[VO2F2(IO3)2]IO3, which crystallizes in the higher symmetry space group, Pna21. In βBa2[VO2F2(IO3)2]IO3, the adjacent [VO2F2(IO3)2]3- polyanion clusters and IO3 units are related by simple translation in a-c plane (Figure 5b), which removes the symmetries in a-c plane and the structure crystallizes in the P21 space group. Notably, the different arrangements between two structures can be attributed to the tiny change of the I5+-coordination. Therefore, the phase transition between α-Ba2[VO2F2(IO3)2]IO3 and βBa2[VO2F2(IO3)2]IO3 has a low energy barrier and the phase transformation can occur in a moderate hydrothermal condition.

The Structure Directing Properties of ETM Oxide Fluoride Anions and Their Assisting Design for Polar Iodates: As described above, the reported compounds contain two distinct V5+-centered oxide fluoride iodate poly-anions, [VFO2(IO3)2]2+ and [VO2F2(IO3)2]3+. The compounds containing the [VFO2(IO3)2]2+ poly-anions are non-polar, while the

compounds containing the [VO2F2(IO3)2]3+ poly-anions are polar. The different polar structures of these compounds can be attributed to the distinct structure directing properties of V5+-centered oxide fluoride anions.

Figure 6. A comparison of the trans- [VFO2(IO3)2]2+ and the cis[VO2F2(IO3)2]3+ poly-anions: (a) In trans- [VFO2(IO3)2]2+, the IO3 units have the opposite orientation. The polarization orientation of these poly-anions mainly originates from [VO4F] and the polarization directions are changeable as the equatorial ligands rotate along the equatorial axis; (b) the Λ-configuration of the cis[VO2F2(IO3)2]3+ poly-anion has a fixed polarization orientation.

In α-Ba[VFO2(IO3)2], the 5-coordinate [VO4F]4- trigonal bipyramid is observed. Owing to the Jahn-Teller distortion, the [VO4F]4- trigonal bipyramid is elongated along the equatorial axis. The BVS contributions for each ligand in [VO4F]4- trigonal bipyramid are shown in Table 2. It is clear that the BVS contributions for the two apical V-O bonds are both 0.63, which is lower than the equatorial V-O bonds, 1.62. In addition, for equatorial V-F bond, although the BVS contribution, 0.59, is also low, it is worth noting that the oxidation state of F- anion is -1. Therefore, for the elongated [VO4F]4- trigonal bipyramid, the apical O atoms possess the most residual negative charge and are the most reactive. Thus, the [VO4F]4- trigonal bipyramid is a trans- director and when the IO3- groups connect with the [VO4F]4- trigonal bipyramid, they preferentially bridge with the apical O atoms to form trans[VFO2(IO3)2]2+ poly-anions. As shown in Figure 6a, two IO3 units in trans-[VFO2(IO3)2]2+ poly-anions possess the opposite orientation and hence the polarization of the [VFO2(IO3)2]2+ poly-anions mainly originates from the difference of bond lengths between equatorial V-O bonds and the equatorial V-F bond (the centered V5+ cation moves to its two equatorial O ligands). However, notably, the polarization directions of the [VFO2(IO3)2]2+ poly-anions are changeable as the equatorial ligands in trans- [VO4F]4- trigonal bipyramid are rotatable



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Table 2. Bond valence sums a for [VO4F]2- in α-Ba[VFO2(IO3)2] and [VO4F2]5- in α-Ba2[VO2F2(IO3)2]IO3. α-Ba[VFO2(IO3)2] Ri, Å Si V-Si [VO4F]2Equatorial bonds V-O(1) 1.634(2) 1.62 0.38 V-O(1) 1.634(2) 1.62 0.38 V-F(1) 1.910(3) 0.59 0.41 Apical bonds V-O(3) 1.976(2) 0.63 1.37 V-O(3) 1.976(2) 0.63 1.37 ΣSV 5.09 α-Ba2[VO2F2(IO3)2]IO3 Ri, Å Si V-Si [VO4F2]5V(1)-O(9) 1.623(6) 1.63 0.37 V(1)-O(3)/F(3) 1.752(6) 1.02 0.48 0.98 (if without O/F disorder) V(1)-O(2)/F(2) 1.798(6) 0.90 0.60 1.11 (if without O/F disorder) V(1)-F(1) 2.025(5) 0.43 0.57 V(1)-O(6) 2.094(6) 0.46 1.54 V(1)-O(4) 2.147(6) 0.43 1.57 ΣSV 4.87

along equatorial axis. Therefore, when they are packed in the structures, their changeable polar orientations would encourage the adjacent BBUs adopt the opposite orientations, just as observed in α-Ba[VFO2(IO3)2] (see Figure 1c). The rotating characteristic of equatorial ligands along the equatorial axis in the trigonal bipyramid can also relate α-Ba[VFO2(IO3)2] and β-Ba[VFO2(IO3)2]. That is, the [VO4F]4- square pyramid in βBa[VFO2(IO3)2] can been seen as a transition state of the rotations.57-58 For α- and β-Ba2[VO2F2(IO3)2]IO3, the six-coordinated [VO4F2]5- octahedron with a C3 distortion is observed. This C3 distorted [VO4F2]5- octahedron is perfect for creating Λ-shaped BBUs with a fixed polarization orientation. Firstly, the C3 distorted [VO4F2]5- octahedron contains three long bonds and three short bonds. The ligands corresponding to three long bonds have more residual negative charges based on the BVS analyses (Table 2), and consequently these ligands are predicted to bond preferentially over the ligands corresponding to the three short bonds. Further, the reactivity of one of the three highly-reactive ligands can be reduced by substituting an O

ligand for F, which leaves two highly reactive cis-ligands to connect two IO3- anions to form the Λ-shaped BBUs (Figure 6b). The reactivity of other ligands can be reduced by partially substituting F- anions for O2- anions, which ensures the stability of these Λ-shaped BBUs. Different from the transdirecting poly-anion, the polar orientation of the Λ-shaped [VO2F2(IO3)2]3- poly-anion is fixed and cannot rotate, owing to a large steric effect (Figure 6b). Therefore, when they are packed in the crystal, their polar orientations can be arranged in an additive or partially additive fashion. Thus, both α- and β-Ba2[VO2F2(IO3)2]IO3 containing the cis-[VO2F2(IO3)2]3+Λshaped poly-anions crystallize in their polar structures. Although phase transitions are also observed for these phases, the polarity of the structures is retained and their polarization orientations are both consistent with the direction of Λ-shaped [VO2F2(IO3)2]3- poly-anions (Figure 5).

Spectroscopic Properties.

Figure 7. The spectroscopic properties of α- and β-Ba[VFO2(IO3)2], and α- and β-Ba2[VO2F2(IO3)2]IO3: (a) the IR spectra shows the absorption peaks of the reported compounds mainly located in the range with λ > 10.5µm; (b) UV-Vis-NIR diffuse reflectance spectra show the band gaps for the reported materials are 2.89 eV, 2.69 eV, 2.59 eV and 2.55 eV, respectively.


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Figure 8. The TG/DTA curves of α-Ba[VFO2(IO3)2] (a), β-Ba[VFO2(IO3)2] (b), α-Ba2[VO2F2(IO3)2]IO3 (c) and β-Ba2[VO2F2(IO3)2]IO3 (d), which show these reported compounds are stable up to ~470 oC.

The IR spectra of the four reported compounds are shown in Figure 7a. Table S3 gives detailed assignments of the absorption peaks.43, 59-61 It is clear that all the reported compounds exhibit the similar IR absorption, in which the absorption peaks in the range from 850 to 958 cm-1 can be assigned as V-O vibrations and the peaks in the range from 697 to 735 cm-1 can be assigned as I-O vibrations. No additional absorption peaks are present in the range from 3300 to 3600 cm-1, indicating that the structure does not contain crystal water or hydroxide radicals. In addition, since the vibration absorptions of both V-O and I-O groups are in the low-frequency region and there is no crystal water or hydroxide radical in the structures, all the reported compounds have wide IR transparent regions, reaching to ~10.5 µm (950 cm-1). These properties indicate that the reported compounds have the potential applications as visible and mid-IR optical and NLO materials.

The analyses of thermal behavior of the four reported compounds are shown in Figure 8. It is clear that all these compounds are stable up to 470 oC. After this temperature, the IO3anions and F- anion in the structures will gradually decompose. For α- and β-Ba[VFO2(IO3)2], their weight loss is 59.36% and 59.59% respectively, which corresponds to the decomposition of IO3 groups and F- anion in the structures with calculated weight loss of 58.52%. The powder XRD pattern of the residuals after TG/DTA is confirmed to be Ba2V2O7 with PDF NO. 39-1432 (Figure S2a), which is consistent with our analysis. For α- and β-Ba2[VO2F2(IO3)2]IO3, more complex decomposition processes are observed. The observed weight loss is 56.58% for α- Ba2[VO2F2(IO3)2]IO3 and 56.02% for β-Ba2[VO2F2(IO3)2]IO3, which again corresponds to the decomposition of IO3 groups and F- anion in the structures with calculated weight loss of 56.79%. The residuals after TG/DTA are confirmed to be Ba2V2O7 and some unknown phases based on powder XRD studies (Figure S2b).

Their UV-vis-NIR diffuse reflectance spectra were measured and shown in Figure 7b. It is clear that all these compounds show little absorption in the range from 500 to 2600 nm, and the band gaps for α-Ba[VFO2(IO3)2], βBa[VFO2(IO3)2], α-Ba2[VO2F2(IO3)2]IO3 and βBa2[VO2F2(IO3)2]IO3 are 2.89, 2.69, 2.59 and 2.55 eV, respectively.

Second-Harmonic Generation (SHG) Properties. The reported compounds, β-Ba[VFO2(IO3)2], αBa2[VO2F2(IO3)2]IO3 and β-Ba2[VO2F2(IO3)2]IO3,crystallize in non-centrosymmetric space groups. Therefore, we measured their SHG responses. The measurement results reveal that βBa[VFO2(IO3)2] displays a relative weak SHG signal, ~1.5 × KDP (KH2PO4), whereas α-Ba2[VO2F2(IO3)2]IO3 and β-

The Analyses for Thermal Behaviors.



Ba2[VO2F2(IO3)2]IO3 show very strong SHG response of ~9 × KDP. All of them are phase-matchable (Figure 9).

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β-Ba2[VO2F2(IO3)2]IO3. The large net dipole moments in their structures favor their large SHG responses. Furthermore, it is also interesting to observe that α- and βBa2[VO2F2(IO3)2]IO3 exhibit the same consistent relationship between their SHG responses and dipole moments with other vanadium oxide iodates, such as K(VO)2O2(IO3)3.43 In K(VO)2O2(IO3)3, the net dipole moments of the unit cell is 129.48 D, which is around four times that in αBa2[VO2F2(IO3)2]IO3, 38.14 D. Interestingly, the SHG response of K(VO)2O2(IO3)3, ~3.6×KTP,43 is also around four times that in ɑ-Ba2[VO2F2(IO3)2]IO3. Note that K(VO)2O2(IO3)3 and ɑ-Ba2[VO2F2(IO3)2]IO3 possess similar cell volumes (1163.5 Å3 for ɑBa2[VO2F2(IO3)2]IO3 and 1162.8 Å3 for K(VO)2O2(IO3)3). These materials demonstrate that the SHG response does relate to the net dipole moments in vanadium oxide/fluoride iodates.

Conclusion. Figure 9. The Phase-matching curves for KH2PO4 (KDP), βα-Ba2[VO2F2(IO3)2]IO3 and βBa[VFO2(IO3)2], Ba2[VO2F2(IO3)2]IO3 (inset: SHG intensity oscilloscope traces). The solid curve is a guide for the eyes, not a fit to the data.

To better understand the relationship of the SHG responses and their crystal structures, the local dipole moments for [VFO2(IO3)2]2- poly-anions, [VO2F2(IO3)2]3- poly-anions and IO3 units, as well as the net dipole moment within a unit cell, were calculated by using a simple bond-valence approach.12, 3132, 62-63 For I5+ cations, the lone pair was given a charge of -2 and localized 1.23 Å from the I5+ cations.64 The dipole moment calculations for ɑ and β-Ba[VFO2(IO3)2] and ɑ and βBa2[VO2F2(IO3)2]IO3 resulted in values of 12.56-14.74 D (D = Debyes) for IO3, 6.04 D for VO4F, and 6.91-10.70 D for VO6 polyhedra. These results are comparable with previously reported vanadium oxide iodates.43 The local dipole moments for BBUs in β-Ba[VFO2(IO3)2] and ɑ and βBa2[VO2F2(IO3)2]IO3 are listed in Table S4. Clearly, in βBa[VFO2(IO3)2], although the trans-[VFO2(IO3)2]2+ poly-anion can display a dipole moment of 8.32 D, the polarizations of the [VFO2(IO3)2]2+ poly-anions completely cancel out in the unit cell, owing to their opposite orientations, resulting in a zero net dipole moment in β-Ba[VFO2(IO3)2]. The counteracting dipole moments and the twining structure may be the main reason for the relative weak SHG response. In α- and βBa2[VO2F2(IO3)2]IO3, the cis-[VO2F2(IO3)2]3+ poly-anions can yield larger dipole moments, 15.05 D in αBa2[VO2F2(IO3)2]IO3 and 14.66 D in β-Ba2[VO2F2(IO3)2]IO3, which are almost two times larger than that of trans[VFO2(IO3)2]2+ poly-anion in β-Ba[VFO2(IO3)2]. In addition, α-Ba2[VO2F2(IO3)2]IO3 and β-Ba2[VO2F2(IO3)2]IO3 contain other isolated I(2)O3- units. The local dipole moments of isolated I(2)O3 units are 14.73 D for α-Ba2[VO2F2(IO3)2]IO3 and 14.69 D for β-Ba2[VO2F2(IO3)2]IO3. As shown in Figure S3, the dipole moments generated by the cis-[VO2F2(IO3)2]3+ polyanions and isolated I(2)O3- units can be constructively added along their polar axis directions, i.e. c axis for αBa2[VO2F2(IO3)2]IO3 and b axis for β-Ba2[VO2F2(IO3)2]IO3. The synergistic effect of cis-[VO2F2(IO3)2]3+ poly-anions and isolated I(2)O3- units makes α-Ba2[VO2F2(IO3)2]IO3 and βBa2[VO2F2(IO3)2]IO3 possess large net dipole moments in their unit cells, 38.14 D for α-Ba2[VO2F2(IO3)2]IO3 and 19.04 D for

By combining the distinct structure-directing properties of ETM oxide-fluoride anions and the Λ-shaped methodology into iodates, two sets of polymorphic ETM oxide-fluoride iodates, ɑ and β-Ba[VFO2(IO3)2], and ɑ and βBa2[VO2F2(IO3)2]IO3 have been successfully synthesized. ɑ and β-Ba[VFO2(IO3)2] contain the trans-[VFO2(IO3)2]2+ polyanions and crystallize in the non-polar space groups Pbcn and P212121, respectively. However, ɑ and β-Ba2[VO2F2(IO3)2]IO3 contain cis-[VO2F2(IO3)2]3+Λ-shaped poly-anions and crystallize in the polar space groups Pna21 and P21, respectively. Structural analyses show that the non-polar structures of ɑ and β-Ba[VFO2(IO3)2] can be attributed to the changeable polar orientations of trans-[VFO2(IO3)2]2+ poly-anions, whereas the polar structures ɑ and β-Ba2[VO2F2(IO3)2]IO3 mainly originate from the Λ-configuration of cis-[VO2F2(IO3)2]3+ poly-anions. It should also be noted that the polar or non-polar behaviors in these structures hold true for both sets of polymorphs, further confirms that the polarizations or non-polarizations of these materials are mainly dominated by the ETM oxide-fluoride anions. Therefore, to combine oxide-fluoride anions with iodates appears to be an effective strategy for discovering polar iodates. The property measurements show that NCS βBa[VFO2(IO3)2], α-Ba2[VO2F2(IO3)2]IO3 and βBa2[VO2F2(IO3)2]IO3 are SHG-active. In particular, the polar ɑ

and β-Ba2[VO2F2(IO3)2]IO3 exhibit excellent NLO properties, including the large SHG responses (~9 × KDP), wide visible and mid-IR transparent region (~0.5-10.5 µm), and high thermal stability (up to 470 oC). Exploration of other ETM oxidefluoride iodates is ongoing.

Associated Content X-ray crystallographic files in CIF format; atomic coordinates, equivalent isotropic displacement parameters, and the bond valence sums of each atom; selected bond distances and angles; experimental and calculated XRD patterns; the dipole moment calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author


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*[email protected] (Kenneth R. Poeppelmeier).

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ACKNOWLEDGMENT

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This work was supported by funding from the National Science Foundation (Awards DMR-1608218). The single crystal X-ray data and FT-IR measurements were acquired at Northwestern University′s Integrated Molecular Structure Education and Research Center (IMSERC) at Northwestern University which is supported by grants from NSF-NSEC, NSF-MRSEC, the KECK Foundation, the State of Illinois, and Northwestern University. This work made use of the J. B. Cohen X-Ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1720139) at the Materials Research Center of Northwestern University.

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For Table of Contents Use Only Title: Assisting the Effective Design of Polar Iodates with Early Transition Metal Oxide Fluoride Anions Authors: Hongwei Yu, Matthew L. Nisbet and Kenneth R. Poeppelmeier *

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Assisting the Effective Design of Polar Iodates with Early Transition

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