Toward the Rational Design of Novel Noncentrosymmetric Materials

Nov 14, 2016 - He attended Sogang University, Korea, earning B.S. (Cum Laude) and M.S. degrees in Chemistry under the direction of Professor Chong Shi...
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Toward the Rational Design of Novel Noncentrosymmetric Materials: Factors Influencing the Framework Structures Kang Min Ok* Department of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea CONSPECTUS: Solid-state materials with extended structures have revealed many interesting structure-related characteristics. Among many, materials crystallizing in noncentrosymmetric (NCS) space groups have attracted massive attention attributable to a variety of superb functional properties such as ferroelectricity, pyroelectricity, piezoelectricity, and nonlinear optical (NLO) properties. In fact, the characteristics are pivotal to many industrial applications such as laser systems, optical communications, photolithography, energy harvesting, detectors, and memories. Thus, for the past several decades, a great deal of synthetic effort has been vigorously made to realize these technologically important properties by improving the occurrence of macroscopic NCS space groups. A bright approach to increase the incidence of NCS structures was combining local asymmetric units during the initial synthesis process. Although a significant improvement has been achieved in obtaining new NCS materials using this strategy, the majority of solid-state materials still crystallize in centrosymmetric (CS) structures as the locally unsymmetrical units are easily lined up in an antiparallel manner. Therefore, discovering an effective method to control the framework structure and the macroscopic symmetry is an imminent ongoing challenge. In order to more effectively control the overall symmetry of solid-state compounds, it is critical to understand how the backbone and the subsequent centricity are affected during the crystallization. In this Account, several factors influencing the framework structure and centricity of solid-state materials are described in order to more systematically discover novel NCS materials. Recent studies on crystalline solid-state materials suggest three factors affecting the local coordination environment as well as the overall symmetry of the framework structure: (1) size variations of the various template cations, (2) a variable backbone arrangement occurring from the hydrogen-bonding interactions, and (3) the presence of framework flexibility. With regard to the first factor, the impact of size of the various metal cations and coordination numbers on the alignment of other adjacent polyhedra, linkers, and lone pairs determining the framework geometries of mixed metal oxides is analyzed. The second factor considers the regulation of crystallographic centricity determined by the availability of hydrogen-bonding interactions between anionic frameworks containing local asymmetric polyhedra and organic cations. Finally, the third factor explores the framework architecture and the space group symmetry influenced by the flexibility of polyhedra revealing variable coordination numbers. The centricity and framework of new solid-state materials might be controlled by using a variety of synthetically controllable asymmetric units such as organic structure-directing cations and linkers with different sizes and functional groups.

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INTRODUCTION Functional materials crystallizing in noncentrosymmetric (NCS) space groups, namely, crystalline materials lacking a center of symmetry, have been drawing enormous recognition attributable to the enchanting symmetry-dependent properties such as second-harmonic generation (SHG), piezoelectricity, pyroelectricity, and ferroelectricity.1−6 The characteristics are extremely important in many applications such as lasers, optical communications, solid-state batteries, sensing devices, thermal detectors, memories, and electronics. From the continuous efforts of many synthetic chemists to discover novel NCS materials, several representative NCS chromophores have been proposed: (1) cations susceptible to second-order Jahn−Teller (SOJT) distortions, i.e., high-valent d0 transition metal cations in an octahedral coordination environment (Ti4+, V5+, Nb5+, Mo6+, W6+, etc.) and cations possessing stereochemically active lone pairs (Pb2+, Sb3+, Bi3+, Se4+, Te4+, I5+, etc.),7−14 (2) d10 © XXXX American Chemical Society

transition metal cations revealing highly polar displacement in the center of the coordination environment (Zn2+, Cd2+, etc.),15−19 and (3) anions with trigonal-planar geometry exhibiting asymmetric π-conjugated molecular orbitals (NO3−, CO32−, BO33−, etc.).20−30 Approaches combining the local asymmetric units during the syntheses have substantially improved the possibility to obtain macroscopic NCS structures. In addition, secondary distortions such as lattice stresses arising from interactions between the constituent polyhedra are possible.12,31 However, the majority of crystalline materials still tend to crystallize in centrosymmetric (CS) space groups because the presence of local asymmetric groups is not a sufficient condition for NCS structures. In other words, the unsymmetrical units can easily align in an antiparallel manner, Received: September 7, 2016

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Figure 1. (a, b) ORTEP drawings (with 50% probability ellipsoids) for (a) NCS polar A2M(IO3)6 (A = Li, Na; M = Ti, Sn) and (b) CS nonpolar A2M(IO3)6 (A = K, Rb, Cs; M = Ti, Sn). (c) While the lone pairs on the iodates are aligned in a parallel manner around the MO6 octahedra to create the octahedral coordination mode around each of the Li+ and Na+ cations, (d) the larger alkali metal cations contacting with oxide ligands on nine different IO3 polyhedra and an MO6 octahedron have IO3 dipole moments that point in equal and opposite directions, causing the materials to crystallize in the CS nonpolar space group.

templating cations in the same group of the periodic table during the synthesis. In general, alkali and alkaline-earth metal cations and even various organic cations are employed as templating cations. Interestingly, stoichiometrically similar materials often reveal different centricities attributable to the structure-directing effects of the template cations. A2M(IO3)6 (A = Li, Na, K, Rb, Cs; M = Ti, Sn). A series of stoichiometrically identical alkali metal iodates, A2M(IO3)6 (A = Li, Na, K, Rb, Cs; M = Ti, Sn) share a common molecular structure composed of an MO6 octahedron connected to six IO3 polyhedra (Figure 1a,b).34−36 As shown in Figure 1a,b, while the materials containing the smaller cations (Li+ and Na+) reveal NCS polar structures, those with the larger ones (K+, Rb+, and Cs+) exhibit CS nonpolar structures. The NCS polar iodates, Li2Ti(IO3)6, Na2Ti(IO3)6, Li2Sn(IO3)6, and Na2Sn(IO3)6, exhibit very strong SHG efficiencies of 500, 400, 400, and 400 times that of α-SiO2, respectively, and all of them are phase-matchable (type I) because of the parallel alignment of the dipole moments in the IO3 polyhedra. In iodates with the smaller Li+ and Na+ cations, the alkali metal cations are in a pseudo-octahedral coordination environment. Thus, the IO3 polyhedra should be aligned in a parallel manner around the MO6 octahedra in order to create the octahedral coordination mode around each of the Li+ and Na+ cations, which results in macroscopic NCS polar structures (Figure 1c). However, the

resulting in the crystallization of CS structures. In addition, the aforementioned methodology is based on experience rather than a firm theoretical background. Although a few in situ diffraction studies have been performed to directly monitor the crystallization of NCS materials such as BaTiO 3 and KTiOPO4,32,33 detailed formation mechanisms for many new NCS materials have not been explored yet because of the lack of facilities and information on the preparation conditions for individual compounds. In this Account, a few important effects controlling the framework structure and macroscopic centricity will be suggested in order to increase the attainability of overall NCS structures more systematically after close structural examinations of recently reported solid-state materials with asymmetric units. The factors include cation size effects influencing the alignment of other constituent polyhedra, hydrogen-bonding interactions between organic templates and ligands in the frameworks, and framework flexibility of asymmetric polyhedra generated from rich coordination moieties.

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EFFECT OF CATION SIZE ON THE CENTRICITY AND THE FRAMEWORK STRUCTURE

Cation Size Effect Influencing the Macroscopic Centricity

Many stoichiometrically equivalent solid-state materials are obtained by adding the respective sources of different B

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Figure 2. Ball-and-stick models of (a) SrMo2O5(SeO3)2 representing a CS pseudo-two-dimensional structure and (b) BaMo2O5(SeO3)2 revealing an NCS three-dimensional framework (blue, Mo; green, Se; red, O; yellow, Sr or Ba). (c) The MoO6 octahedra and SeO3 polyhedra are coordinated in an antiparallel manner to maintain the eight-coordnate square-antiprismatic environment around each Sr2+ cation in CS SrMo2O5(SeO3)2. (d) The Mo2O11 groups completely encompass the larger Ba2+ cation; thus, the two SeO3 groups are connected in a parallel manner to complete the 10coordinate environment around the Ba2+ cation in NCS BaMo2O5(SeO3)2.

ligands on four MoO6 octahedra and four SeO3 polyhedra in an eight-coordinate square-antiprismatic environment. To maintain the crowded eight-coordinate square-antiprismatic environment around the Sr2+ or Pb2+ cation, the MoO6 octahedra and SeO3 polyhedra are coordinated in an antiparallel manner. Any unfavorable repulsion between the polyhedra can be minimized by adopting an inversion center at the Sr2+ or Pb2+ cation, which renders the material CS nonpolar (Figure 2c). The larger Ba2+ cation in BaMo2O5(SeO3)2, however, is in a 10-coordinate environment and interacts with oxide ligands on eight MoO6 octahedra and two SeO3 polyhedra. Here, four pairs of cornersharing MoO6 octahedra (i.e., Mo2O11 groups) completely surround the Ba2+ cation. Therefore, the only way to maintain the 10-coordinate environment in the Ba2+ cation with two more SeO3 polyhedra is to link Se4+ cations to O(4) and O(6) [the O(4)−O(6) contact is 2.541(9) Å], which spontaneously results in a parallel alignment of lone pairs and a NCS polar structure (Figure 2d). All of the other O−O distances observed around the Ba2+ cation in BaMo2O5(SeO3)2 are too long for the connection of SeO3 groups. YVQ2O8 (Q = Se, Te). The stoichiometrically equivalent quaternary vanadium selenite and tellurite, YVSe2O8 and YVTe2O8, respectively, synthesized via hydrothermal and solid-state reactions revealed another type of interesting cation size effect influencing the macroscopic centricity.39 Both of the materials exhibit three-dimensional frameworks that are

larger alkali metal cations contact with oxide ligands on nine different IO3 polyhedra and an MO6 octahedron, where three of the oxide ligands are linked to both M4+ and I5+ cations (Figure 1d). Thus, the IO3 groups rotate with respect to the M4+ cation for the larger A+ cations to maintain the coordination number of 9. The resulting IO3 dipole moments pointing in equal and opposite directions cause the materials to crystallize in CS nonpolar space groups. AMo2O5(SeO3)2 (A = Sr, Pb, Ba). Molybdenum selenites with different templating cations, AMo2O5(SeO3)2 (A = Sr, Pb, Ba), were synthesized through solid-state reactions.37 Whereas the isostructural materials containing cations with relatively smaller ionic radii (Sr 2+ , 1.26 Å; Pb 2+ , 1.29 Å), 38 SrMo2O5(SeO3)2 and PbMo2O5(SeO3)2, exhibit a CS pseudotwo-dimensional structure with distorted MoO6 octahedra linked by asymmetric SeO3 polyhedra, BaMo2O5(SeO3)2 with a larger cation (Ba2+, 1.42 Å) shows an NCS three-dimensional framework (Figure 2a,b). The NCS polar selenite BaMo2O5(SeO3)2 reveals a moderate SHG response of ca. 80 times that of α-SiO2 and is phase-matchable (type I). Although the moments arising from the C2 out-of-center distorted MoO6 octahedra and SeO3 polyhedra point in opposite directions, a larger net moment originating from the alignment of SeO3 polyhedra is responsible for the observed SHG efficiency of BaMo 2 O 5 (SeO 3 ) 2 . The smaller templating cation in SrMo2O5(SeO3)2 or PbMo2O5(SeO3)2 interacts with oxide C

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Figure 3. Ball-and-stick representations showing the three-dimensional frameworks composed of VO6 octahedra, YO8 polyhedra, and SeO3 or TeO3 linkers for (a) NCS YVSe2O8 and (b) CS YVTe2O8 in the ac planes (blue, V; green, Se or Te; yellow, Y; red, O). While the smaller SeO3 linkers can reside in a limited space without any repulsion in NCS YVSe2O8, the larger TeO3 linkers in CS YVTe2O8 align in opposite directions in a confined space to avoid unfavorable lone pair−lone pair repulsions.

determining the centricity of the material. The SeO3 linkers with the smaller cation, Se4+, can reside in a limited space without any repulsion and stabilize the NCS structure (Figure 3a). However, Te4+ with the larger average ionic radius and longer cation−lone pair length requires more space around YO8 polyhedra in YVTe2O8. To avoid the unfavorable lone pair−lone pair repulsions, the larger TeO3 linkers in YVTe2O8 align in opposite directions in a confined space, leading the material to crystallize in a CS space group (Figure 3b). ZnMoSb2O7 and CdMoSb4O10. The two quaternary molybdenum(VI) antimony(III) oxides ZnMoSb2O7 and CdMoSb4O10, consisting of highly polarizable cations such as d10 (Zn2+ or Cd2+), d0 (Mo6+), and lone-pair (Sb3+) cations,

composed of layers of corner-shared VO6 octahedra, layers of edge-shared YO8 polyhedra, and SeO3 or TeO3 groups (Figure 3). The SeO3 and TeO3 polyhedra in YVSe2O8 and YVTe2O8, respectively, serve as both intra- and interlayer linkers. The selenite, YVSe2O8, which crystallizes in the NCS polar space group Abm2, shows an SHG efficiency ca. 10 times that of αSiO2 and is non-phase-matchable (type I). The observed weaker SHG response is attributed to a considerable cancellation of polarizations between the dipole moments arising from the VO6 octahedra with the C4 out-of-center distortion and the asymmetric SeO3 groups that are aligned in opposite directions. In this case, the size of the lone-pair-cation linker, i.e., SeO3 or TeO3, is very important in influencing and D

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Figure 4. Ball-and-stick models revealing (a) the three-dimensional framework obtained by the connection of MoO4 tetrahedra, SbO4 polyhedra, and ZnO4 tetrahedra in NCS ZnMoSb2O7 and (b) the unidimensional chains composed of CdO6 octahedra, SbO3 polyhedra, and MoO4 tetrahedra in CS CdMoSb4O10 (cyan, Zn; orange, Cd; blue, Mo; green, Sb; red, O). To maintain the framework in a limited space, the coordinated polyhedra around the small Zn2+ cation in NCS ZnMoSb2O7 tend to be lined up, whereas the larger Cd2+ cation in CS CdMoSb4O10 can accommodate more polyhedra in the coordination environment, in which the interacting groups are lined up in an antiparallel manner.

were prepared through hydrothermal and solid-state reactions.40 Whereas ZnMoSb2O7 reveals a three-dimensional framework structure with ZnO4, MoO4, and SbO4 polyhedra in the NCS polar space group P21, CdMoSb4O10 exhibits a unidimensional tubule structure with CdO6, MoO4, and SbO3 polyhedra in the CS space group P21/m (Figure 4). Powder SHG measurements indicated that polar ZnMoSb2O7 exhibits an SHG efficiency about 10 times that of α-SiO2, similar to that of ZnO, and is non-phase-matchable (type I). Detailed structural analysis of the various polyhedra of polarizable cations suggests that a net polarization arising from the distorted MoO4 tetrahedra is mainly responsible for the observed SHG signal of ZnMoSb2O7. Although ZnMoSb2O7 and CdMoSb4O10 are not stoichiometrically similar, the ionic radius of the d10 cation plays a role in determining the centricity of the material here. Specifically, while the smaller d10 transition metal cation, Zn2+, tends to have a low coordination number of 4, the larger cation, Cd2+, has a high coordination number of 6 by interacting with six oxide ligands in the polyhedra of Mo6+ and Sb3+. Then the coordinated polyhedra around the smaller Zn2+ cation can be aligned to uphold the backbone in a confined space, and a polar NCS structure is obtained. The larger Cd2+ cation, however, can interact with more polyhedra around its coordination environment, where the interacting polyhedra are lined up in an antiparallel manner for more effective packing, which results in a CS structure. A similar cation size effect affecting the macroscopic centricity has been observed in a series of borate materials as well.41,42 Detailed SHG properties for the NCS materials influenced by the cation size effect are summarized in Table 1.

Table 1. Detailed SHG Properties of NCS Materials Influenced by the Cation Size Effect compound 34,35

Li2Ti(IO3)6 Na2Ti(IO3)635 Li2Sn(IO3)636 Na2Sn(IO3)636 BaMo2O5(SeO3)237 YVSe2O839 ZnMoSb2O740

space group

powder SHG efficiency @ 1064 nma

phase matching (type I)b

P63 (No. 173) P63 (No. 173) P63 (No. 173) P63 (No. 173) Cmc21 (No. 36) Abm2 (No. 39) P21 (No. 4)

500 400 400 400 80 10 10

PM PM PM PM PM NPM NPM

Relative to α-SiO2. bPM = phase-matchable; NPM = non-phasematchable. a

alkali metal indium selenites, AIn(SeO3)2 (A = Na, K, Rb, Cs), was synthesized via standard solid-state and hydrothermal reactions. A common structural motif, a network of cornershared InO6 octahedra and SeO3 trigonal pyramids, is observed in the family (Figure 5).43 However, while AIn(SeO3)2 (A = Na, K, Rb) have a three-dimensional framework structure, CsIn(SeO3)2 exhibits a two-dimensional structure. As shown in Figure 5, similar channel structures constructed by corner sharing of InO6 octahedra and SeO3 polyhedra are observed in the family. The alkali metal cations reside in the respective cavities formed by the 12-membered rings (12-MRs) and/or interlayer spaces. One can notice that the size of the alkali metal cations significantly influences the mode of bonding of the SeO3 group to the InO6 octahedra. The ionic radii for eightcoordinate Na+, K+, and Rb+ and for 12-coordinate Cs+ are 1.18, 1.51, 1.61, and 1.88 Å, respectively.38 In the selenites with the relatively smaller Na+ and K+ cations, NaIn(SeO3)2 and KIn(SeO3)2, the lone pairs on the SeO3 groups point inward within the larger 12-MR channels, in which sufficient space is available for the lone pairs (Figure 5a). In RbIn(SeO3)2,

Cation Size Effect Affecting the Framework Structure

The size of the templating cations influences not just the centricity but also the framework structure of solid-state materials by changing the bonding nature of the constituent polyhedra. A family of stoichiometrically identical quaternary E

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Accounts of Chemical Research [N(CH3)4]ZnCl3

An organically templated zinc chloride, [N(CH3)4]ZnCl3, exhibits a unidimensional crystal structure consisting of corner-shared chains of ZnCl4 tetrahedra that are separated by [N(CH3)4]+ cations (Figure 6).51 As shown in Figure 6, all of the ZnCl4 tetrahedra in the chains are approximately aligned in the [001] direction, resulting in a polar structure. More close structural analysis suggests that the organic template cations, [N(CH3)4]+, reside in the crystal lattice and form hydrogen bonds with the ZnCl4 tetrahedra. Hydrogen bonds specifically occur from C(1), C(2), C(3), and C(6) in tetramethylammonium cations to the chlorine atoms in the ZnCl4 tetrahedra [C(1)−H···Cl(3), 3.538(9) Å; C(2)−H···Cl(3), 3.963(9) Å; C(2)−H···Cl(4), 3.852(3) Å; C(3)−H···Cl(3), 3.871(6) Å; C(6)−H···Cl(3), 3.562(7) Å; C(6)−H···Cl(5), 3.772(7) Å] (Figure 6). Bond valence sum calculations on the chains in [N(CH3)4]ZnCl3 also confirm the effect of hydrogen-bonding interactions. Specifically, while bonds to terminal chlorines affected by hydrogen-bonding interactions reveal valences of 0.57−0.63, those to bridging chlorines exhibit rather weaker values of 0.41−0.42. The polar structure arising from the asymmetric alignment of ZnCl4 tetrahedra in the chains of [N(CH3)4]ZnCl3 exhibits an SHG efficiency 15 times that of αSiO2 and is non-phase-matchable (type I). The SHG efficiency of [N(CH3)4]ZnCl3 is on the same order as that of ZnO, which is composed of aligned ZnO4 tetrahedra.52

Figure 5. Ball-and-stick models of (a) AIn(SeO3)2 (A = Na, K), (b) RbIn(SeO3)2, and (c) CsIn(SeO3)2 (blue, In; green, Se; red, O; yellow; Na, K, Rb, or Cs). A similar structural motif constructed by corner sharing of InO6 octahedra and SeO3 polyhedra is observed in this family. The lone pairs on the SeO3 groups point inward within the larger 12-MR channels in NaIn(SeO3)2 and KIn(SeO3)2. In RbIn(SeO3)2, the lone pairs on the SeO3 polyhedra point in opposite directions because of the slightly larger size of the Rb+ cation. In CsIn(SeO3)2, the larger Cs+ cation forces the Se4+ to bond to three oxygen atoms only within one layer, generating a layered structure.

A2TiF6 (A = [N(CH3)4], [C(NH2)3])

Two titanium fluorides templated by different organic cations, [N(CH3)4]TiF6 and [C(NH2)3]TiF6, were prepared through either a solvothermal reaction or a simple mixing method (Figure 7).53 Although the two titanium fluorides share a common structural motif, i.e., zero-dimensional TiF6 octahedra and organic cations, their centricities are completely different. Whereas [N(CH3)4]TiF6 crystallizes in the CS space group R3̅, [C(NH2)3]TiF6 crystallizes in the polar NCS space group Cm. Powder SHG measurements using 1064 nm radiation indicate that NCS [C(NH2)3]TiF6 has an SHG efficiency 25 times that of α-SiO2 attributable to the aligned C3 out-of-center distortion of TiF6 octahedra and is non-phase-matchable (type I). Detailed structural analysis suggests that the asymmetric outof-center displacement and the subsequent alignment of TiF6 octahedra to define a polar structure of [C(NH2)3]TiF6 are due to hydrogen-bonding interactions between the H−N atoms in the organic cation [N(CH2)3]+ and the F atoms in the TiF6 octahedra (Figure 7b). Specifically, hydrogen-bonding interactions are clearly observed between the fluorine atoms in TiF6 and nitrogen atoms in the [N(CH2)3]+ cation [F(1)···N(4), 2.912(5) Å; F(2)···N(1), 2.893(5) Å; F(3)···N(1), 2.947(6) Å; F(3)···N(2), 2.814(5) Å; F(4)···N(3), 2.949(5) Å; F(4)···N(4), 2.892(7) Å]. In fact, the hydrogen-bonding interactions result in the formation of a pseudo-two-dimensional framework for [C(NH2)3]TiF6. However, neither hydrogen-bonding interactions nor out-of-center distortions of TiF6 octahedra are observed in CS [N(CH3)4]TiF6. A similar effect of hydrogen bonding on the asymmetric polar structure has been observed in the polar layered uranium phosphate fluorides Cs(UO2)F(HPO4)·0.5H2O and Rb(UO2)F(HPO4).54 Detailed SHG properties of the NCS materials that are affected by hydrogen-bonding interactions are summarized in Table 2.

however, the lone pairs on the SeO3 polyhedra point in opposite directions compared with those in NaIn(SeO3)2 and KIn(SeO3)2 because of the slightly larger size of the Rb+ cation. Because the larger Rb+ cations reside there, it would be difficult for the lone pairs on the SeO3 groups to find any space within the 12-MR channels. Instead, the lone pairs on the SeO3 groups in RbIn(SeO3)2 point inward in the smaller 4-MR channels (Figure 5b). In the selenite CsIn(SeO3)2 with the largest cation, Cs+, the large interlayer spacing forces each Se4+ cation to bond to three oxygen atoms only within one layer. Although the closest interlayer O−O contact distance is 4.79 Å, the nearest Se−O contact on an adjacent layer is at a distance of 2.63 Å. Thus, the Cs+ cation should reside in the interlayer space and generate a layered structure of CsIn(SeO3)2 (Figure 5c). Similar cation size effects on framework structures have been observed in various stoichiometrically equivalent alkali metal selenites and tellurites.44−50

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EFFECT OF HYDROGEN-BONDING INTERACTIONS Another important factor regulating the crystallographic centricity of solid-state materials is the hydrogen-bonding interactions. In general, both organic templating cations and anionic ligands are involved, where the availability of hydrogenbonding interactions among the constituent elements is critical for the effect. F

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Figure 6. Ball-and-stick and polyhedral representations of [N(CH3)4]ZnCl3 in (a) the bc plane and (b) the ac plane (cyan, Zn; orange, Cl, purple, N; yellow, C; white, H). The hydrogen-bonding interactions (red dashed lines) between the hydrogens in the [N(CH3)4]+ groups and the chlorines in the ZnCl4 tetrahedra have the ZnCl4 tetrahedra aligned along the [001] direction.

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EFFECT OF FRAMEWORK FLEXIBILITY Most of the solid-state materials introduced here so far exhibit different structures and dimensionalities attributed to the various constituent building units such as distorted MO6 octahedra and QOn (Q = Sb, Se, Te; n = 3−5) polyhedra with lone pairs. For example, the variable coordination environments of Te4+ with oxide ligands can exhibit a variety of structural motifs such as trigonal pyramid, seesaw, and square pyramid.14,55−63 Introducing other cations with flexibility such as p-block elements can generate further structural diversity. The presence of framework flexibility can often influence the backbone architecture as well as the space group symmetry.

In2Zn(SeO3)4 is in a six-coordinate octahedral environment, whereas the Ga3+ cation in Ga2Zn(TeO3)4 is in a fourcoordinate tetrahedral geometry. The observed different coordination environments are consistent with the ionic radii of the respective cations In3+ (0.80 Å) and Ga3+ (0.47 Å).38 Thus, the larger cation, In3+, possesses a great deal of flexibility within the framework structure. In fact, In3+ cation can maintain the InO6 octahedral coordination moiety through distortions with its flexibility. Thus, the SeO3 groups are linked in an antiparallel manner to minimize any unfavorable repulsion of lone pairs, which renders the material a two-dimensional CS structure (Figure 8a). On the other hand, the smaller cation, Ga3+, in Ga2Zn(TeO3)4, forms rather rigid GaO4 tetrahedra. The rigid backbone originating from the regular GaO4 tetrahedra spontaneously directs the alignment of lone pairs in TeO3 polyhedra, resulting in an NCS structure (Figure 8b). Similar structural differentiation arising from backbone flexibility has been observed in other selenites M2(Se2O5)3 (M = Al, Ga, and In).65

In2Zn(SeO3)4 and Ga2Zn(TeO3)4

The stoichiometrically similar quaternary mixed metal selenite and tellurite compounds In2Zn(SeO3)4 and Ga2Zn(TeO3)4, respectively, were synthesized by solid-state reactions.64 While CS In2Zn(SeO3)4 shows a layered structure composed of distorted InO 6 octahedra, ZnO 6 octahedra, and SeO 3 polyhedra, NCS Ga2Zn(TeO3)4 exhibits a three-dimensional framework with GaO4 or ZnO4 tetrahedra and TeO3 trigonal pyramids (Figure 8). Ga2Zn(TeO3)4 exhibits a moderate SHG efficiency (10 times that of α-SiO2) and is non-phase-matchable (type I). A net moment observed along the [1̅1̅1̅] direction attributable to the alignment of asymmetric TeO3 groups is responsible for the SHG of Ga2Zn(TeO3)4. The In3+ cation in

InVTe2O8 and InVSe2O8

The degree of framework flexibility for solid-state materials is also affected by the lone-pair cations. Two stoichiometrically equivalent quaternary tellurite and selenite compounds, InVTe2O8 and InVSe2O8, respectively, were synthesized by solid-state reactions.66 Whereas CS InVTe2O8 shows a twoG

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Figure 7. Ball-and-stick representations of (a) CS [N(CH3)4]2TiF6 in the ac plane and (b) NCS polar [C(NH2)3]2TiF6 in the ab plane (blue, Ti; green, F; purple, N; yellow, C). Hydrogen atoms have been omitted for clarity. Hydrogen-bonding interactions (red dashed lines) between the F and H−N groups in [C(NH2)3]TiF6 are responsible for the asymmetric out-of-center displacement and the subsequent alignment of TiF6 octahedra.

Table 2. Detailed SHG Properties of NCS Materials Affected by Hydrogen-Bonding Interactions

a

compound

space group

powder SHG efficiency @ 1064 nma

phase-matching (type I)b

[N(CH3)4]ZnCl351 [C(NH2)3]TiF653 Cs(UO2)F(HPO4)·0.5H2O54 Rb(UO2)F(HPO4)54

Pmc21 (No. 26) Cm (No. 8) Pca21 (No. 29) Cmc21 (No. 36)

15 25 0.5 0.5

NPM NPM − −

Relative to α-SiO2. bNPM = non-phase-matchable.

monitored in extended structures. In InVTe2O8, Te4+ cations encompass corner-shared InO6 zigzag chains because of its flexibility, forming a layered structure. The symmetric layered structure is possible only because the Te−O bond distance ranges from 1.856(9) Å to 2.506(9) Å. In other words, the flexibility generated from the variable coordination modes of Te4+ cations allow the lone pairs to point in opposite directions, resulting in a CS layered structure (Figure 9a). The smaller Se4+ cation, however, can show only the SeO3 trigonal-pyramidal

dimensional layered structure that is composed of InO6 octahedra, VO4 tetrahedra, and TeO4 polyhedra, NCS InVSe2O8 exhibits a three-dimensional framework structure with distorted InO6 octahedra, VO5 square pyramids, and SeO3 trigonal pyramids (Figure 9). InVSe2O8 has an SHG efficiency 30 times that of α-SiO2 attributable to a net moment along the [1̅00] direction and is non-phase-matchable (type I). The Te4+ cation can exhibit variable coordination modes with oxide ligands; thus, TeO3, TeO4, and TeO5 polyhedra are often H

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Figure 8. Ball-and-stick and polyhedral representations of (a) CS In2Zn(SeO3)4 and (b) NCS Ga2Zn(TeO3)4 (blue, In; green, Se or Te; cyan, Zn or Ga; red, O). As a result of the flexibility of the InO6 octahedra, the SeO3 groups in In2Zn(SeO3)4 are linked in an antiparallel manner. The smaller Ga3+ cation in Ga2Zn(TeO3)4 forms rather rigid GaO4 tetrahedra and directs the alignment of the lone pairs in the TeO3 polyhedra.

Figure 9. Ball-and-stick models of (a) CS InVTe2O8 and (b) NCS InVSeO8 in the ac plane (blue, In; cyan, V; green, Te or Se; red, O). The flexible TeO4 groups in InVTe2O8 allow the lone pairs to point in opposite directions, resulting in a CS layered structure. The rigid SeO3 groups in InVSe2O8 link the InO6 chains and the VO5 chains to form a three-dimensional framework in which all of the VO5 square pyramids are aligned, generating a polar NCS structure.

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CONCLUSIONS A few important factors affecting the framework structures and overall centricities of solid-state materials have been examined in this Account. We have discussed how the size of templating cations influences the macroscopic centricity and backbone geometry by changing the connection modes and the alignments of other constituent polyhedra. The cations

geometry with oxide ligands. Thus, the rigid SeO3 groups in InVSe2O8 link the InO6 chains and the VO5 chains to form a three-dimensional framework in which all of the VO5 square pyramids are aligned to generate a polar NCS structure (Figure 9b). The SHG properties of the NCS materials influenced by framework flexibility are listed in Table 3. I

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Accounts of Chemical Research Table 3. Detailed SHG Properties of NCS Materials Influenced by Framework Flexibility

a

compound

space group

powder SHG efficiency @ 1064 nma

Ga2Zn(TeO3)464 In2(Se2O5)365 InVSe2O866

I43̅ d (No. 220) Pc (No. 7) Pm (No. 6)

10 10 30

he serves the Korean Chemical Society (KCS) as a director for general affairs.

phasematching (type I)b NPM NPM NPM

ACKNOWLEDGMENTS

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REFERENCES

This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Grants 2014M3A9B8023478 and 2016R1A2A2A05005298).

Relative to α-SiO2. bNPM = non-phase-matchable.

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included alkali metal, alkaline-earth metal, transition metal, and even lone-pair cation linkers. Hydrogen-bonding interactions between organic templating cations and anionic ligands are another important factor regulating the crystallographic centricity. The arrangement of asymmetric units in the framework is varied depending on the availability of the hydrogen-bonding interactions. Framework flexibility achieved by the presence of p-block elements and/or cations with variable coordination environments also helps to determine the framework architecture and the space group symmetry. Considering the key factors described in this Account, the centricity and framework of novel solid-state materials may be tuned by introducing various organic structure-directing cations with different sizes and functional groups during the syntheses. Also, systematic explorations of metal−organic hybrid materials containing a variety of different asymmetric organic linkers may help to discover novel NCS materials. We will continue to examine the influence of the suggested factors on the macroscopic centricities for synthetically challenging materials containing elements with an extremely high melting point and/ or sparing solubility.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kang Min Ok: 0000-0002-7195-9089 Notes

The author declares no competing financial interest. Most of the NCS materials introduced in this Account are available from the Noncentrosymmetric Materials Bank (http://ncsmb.knrrc.or.kr). Biography Kang Min Ok was born in Busan, Korea, in 1970. He attended Sogang University, Korea, earning B.S. (Cum Laude) and M.S. degrees in Chemistry under the direction of Professor Chong Shik Chin, followed by a Ph.D. degree from the University of Houston working for Professor P. Shiv Halasyamani on discovering new second-harmonicgenerating materials. He began postdoctoral research at Houston on the new noncentrosymmetric materials with Professor Halasyamani. His second postdoctoral research was conducted in the group of Professor Dermot O’Hare at the University of Oxford, where he investigated the kinetics of solid-state reactions using in situ diffraction. Since joining the faculty of Chung-Ang University (CAU) in Korea, he has performed research on discovering novel solid-state materials with asymmetric coordination environments and functional coordination polymers. In 2014 he was highlighted as one of 13 emerging investigators in an ACS select virtual issue on solid-state chemistry. He has held the title of CAU Distinguished Scholar since 2014. Currently, J

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(39) Kim, Y. H.; Lee, D. W.; Ok, K. M. Noncentrosymmetric YVSe2O8 and Centrosymmetric YVTe2O8: Macroscopic Centricities Influenced by the Size of Lone Pair Cation Linkers. Inorg. Chem. 2014, 53, 1250−1256. (40) Jo, H.; Ok, K. M. Polar Noncentrosymmetric ZnMoSb2O7 and Nonpolar Centrosymmetric CdMoSb4O10: d10 Transition Metal Size Effect Influencing the Stoichiometry and the Centricity. Inorg. Chem. 2016, 55, 6286−6293. (41) Han, S.; Wang, Y.; Pan, S.; Dong, X.; Wu, H.; Han, J.; Yang, Y.; Yu, H.; Bai, C. Noncentrosymmetric versus Centrosymmetric: Influence of the Na+ Substitution on Structural Transition and Second-Harmonic Generation Property. Cryst. Growth Des. 2014, 14, 1794−1801. (42) Bai, C.; Yu, H.; Han, S.; Pan, S.; Zhang, B.; Wang, Y.; Wu, H.; Yang, Z. Effect of Halogen (Cl, Br) on the Symmetry of Flexible Perovskite-Related Framework. Inorg. Chem. 2014, 53, 11213−11220. (43) Lee, D. W.; Kim, S. B.; Ok, K. M. Cation Size Effect on the Framework Structures in a Series of New Alkali-Metal Indium Selenites, AIn(SeO3)2 (A = Na, K, Rb, and Cs). Inorg. Chem. 2012, 51, 8530−8537. (44) Lee, D. W.; Ok, K. M. New Alkali-Metal Gallium Selenites, AGa(SeO3)2 (A = Li, Na, K, and Cs): Effect of Cation Size on the Framework Structures and Macroscopic Centricities. Inorg. Chem. 2013, 52, 5176−5184. (45) Bae, S.-w.; Kim, C.-Y.; Lee, D. W.; Ok, K. M. Cooperative Effects of Cation Size and Variable Coordination Modes of Te4+ on the Frameworks of New Alkali Metal Indium Tellurites, NaIn(TeO3)2, KIn(TeO3)2, RbInTe3O8, and CsInTe3O8. Inorg. Chem. 2014, 53, 11328−11334. (46) Bang, S.-e.; Lee, D. W.; Ok, K. M. Variable Framework Structures and Centricities in Alkali Metal Yttrium Selenites, AY(SeO3)2 (A = Na, K, Rb, and Cs). Inorg. Chem. 2014, 53, 4756− 4762. (47) 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. (48) Song, S. Y.; Ok, K. M. Modulation of Framework and Centricity: Cation Size Effect in New Quaternary Selenites, ASc(SeO3)2 (A = Na, K, Rb, and Cs). Inorg. Chem. 2015, 54, 5032−5038. (49) Kim, Y.; Kim, B.; Ok, K. M. New niobium selenites, ANbO(SeO3)2 (A = Na, K, Rb, and Cs): Effect of alkali metal cation size on the dimensionality and intraoctahedral distortion. J. Alloys Compd. 2016, 662, 381−387. (50) Song, S. Y.; Ok, K. M. LiSc(SeO3)2·xH2O (0 ≤ x ≤ 1): New Selenites Revealing Water Molecule-Driven Extremely High Temperature Single-Crystal-to-Single-Crystal Transformations. Cryst. Growth Des. 2016, 16, 3076−3080. (51) Choi, M.-H.; Kim, S.-H.; Chang, H.-Y.; Halasyamani, P. S.; Ok, K. M. New Noncentrosymmetric Material - [N(CH3)4]ZnCl3: Polar Chains of Aligned ZnCl4 Tetrahedra. Inorg. Chem. 2009, 48, 8376− 8382. (52) Kurtz, S. K.; Perry, T. T. A Powder Technique for the Evaluation of Nonlinear Optical Materials. J. Appl. Phys. 1968, 39, 3798−3813. (53) Kim, E.-a.; Lee, D. W.; Ok, K. M. Centrosymmetric [N(CH3)4]2TiF6 vs. noncentrosymmetric polar [C(NH2)3]2TiF6: A hydrogen-bonding effect on the out-of-center distortion of TiF6 octahedra. J. Solid State Chem. 2012, 195, 149−154. (54) Ok, K. M.; Baek, J.; Halasyamani, P. S.; O’Hare, D. New Layered Uranium Phosphate Fluorides: Syntheses, Structures, Characterizations, and Ion-Exchange Properties of A(UO2)F(HPO4)· xH2O (A = Cs+, Rb+, K+; x = 0−1). Inorg. Chem. 2006, 45, 10207− 10214. (55) Kim, M. K.; Jo, V.; Lee, D. W.; Ok, K. M. Anionic templating in a new layered bismuth tellurium oxychloride, Bi3Te4O10Cl5. Dalton Trans. 2010, 39, 6037−6042. K

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Accounts of Chemical Research (56) 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. (57) Kim, Y. H.; Lee, D. W.; Ok, K. M. α-ScVSe2O8, β-ScVSe2O8, and ScVTe2O8: New Quaternary Mixed Metal Oxides Composed of Only Second-Order Jahn-Teller Distortive Cations. Inorg. Chem. 2013, 52, 11450−11456. (58) Lee, D. W.; Ok, K. M. Na1.4InTe3.6O9.4: New Variant of a Hexagonal Tungsten Oxide (HTO)-Like Layered Framework Containing Both a Main-Group Cation, In3+, and a Lone-Pair Cation, Te4+. Inorg. Chem. 2013, 52, 6236−6238. (59) Kim, Y. H.; Lee, D. W.; Ok, K. M. Strong Second Harmonic Generation (SHG) Originating from Combined Second-Order JahnTeller (SOJT) Distortive Cations in a New Noncentrosymmetric Tellurite, InNb(TeO4)2. Inorg. Chem. 2014, 53, 5240−5245. (60) Lee, D. W.; Ok, K. M. New Polymorphs of Ternary Sodium Tellurium Oxides: Hydrothermal Synthesis, Structure Determination, and Characterization of β-Na2Te4O9 and Na2Te2O6·1.5H2O. Inorg. Chem. 2014, 53, 10642−10648. (61) Song, S. Y.; Lee, D. W.; Ok, K. M. Rich Structural Chemistry in Scandium Selenium/Tellurium Oxides: Mixed-Valent Selenite-Selenates, Sc2(SeO3)2(SeO4) and Sc2(TeO3) (SeO3) (SeO4), and Ternary Tellurite, Sc2(TeO3)3. Inorg. Chem. 2014, 53, 7040−7046. (62) Kim, Y. H.; Jeon, B.-Y.; You, T.-S.; Ok, K. M. New quaternary oxides with both families of second-order Jahn-Teller (SOJT) distortive cations: Solid-state synthesis, structure determination, and characterization of YNbTe2O8 and YNbSe2O8. J. Alloys Compd. 2015, 637, 155−161. (63) Bae, S.-w.; Yoo, J.; Lee, S.; Choi, K. Y.; Ok, K. M. Synthesis and Characterization of Three New Layered Vanadium Tellurites, MVTe2O8 (M = Al, Ga, and Mn): Three-Dimensional (3-D) Antiferromagnetic Behavior of MnVTe2O8 with a Zigzag S = 2 Spin Chain. Inorg. Chem. 2016, 55, 1347−1353. (64) Lee, D. W.; Bak, D.-b.; Kim, S. B.; Kim, J.; Ok, K. M. Effect of the Framework Flexibility on the Centricities in Centrosymmetric In2Zn(SeO3)4 and Noncentrosymmetric Ga2Zn(TeO3)4. Inorg. Chem. 2012, 51, 7844−7850. (65) Ok, K. M.; Halasyamani, P. S. New Selenites: Syntheses, Structures, and Characterization of Centrosymmetric Al2(Se2O5)3 and Ga2(Se2O5)3 and Non-centrosymmetric In2(Se2O5)3. Chem. Mater. 2002, 14, 2360−2364. (66) Lee, D. W.; Oh, S.-J.; Halasyamani, P. S.; Ok, K. M. New Quaternary Tellurite and Selenite: Synthesis, Structure, and Characterization of Centrosymmetric InVTe2O8 and Noncentrosymmetric InVSe2O8. Inorg. Chem. 2011, 50, 4473−4480.

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