Alignment of Lone Pairs in a New Polar Material: Synthesis, Characterization, and Functional Properties of Li 2 Ti(IO 3 ) 6 Hong-Young Chang, † Sang-Hwan Kim, † P. Shiv Halasyamani,* ,† and Kang Min Ok* ,‡ Department of Chemistry, UniVersity of Houston, 136 Fleming Building, Houston, Texas 77204-5003, and Department of Chemistry, Chung-Ang UniVersity, 221 Heukseok-dong, Dongjak-gu, Seoul 155-756, Republic of Korea Received October 30, 2008; E-mail: psh@uh.edu; kmok@cau.ac.kr The synthesis of new materials with advanced functional properties, e.g., piezoelectricity, battery applications, multiferroic behavior, etc. is of current and broad interest. 1 This is particularly true with polar materials, i.e., compounds exhibiting a macroscopic dipole moment. Polar materials are of great interest because of two technologically important properties: pyroelectricity and ferro- electricity. 2,3 In molecular compounds, such as NH 3 , HCl, and H 2 O, the concept of polarity is straightforward. For a solid-state material to be considered polar, the compound must crystallize in one of ten crystal classes: 1, 2, 3, 4, 6, m, mm2, 3m,4mm, or 6mm. 4 Clearly, polarity and polar materials are important, yet the question of how to synthesize a new polar material remains. To address this question, we have synthesized several new polar oxides that contain cations susceptible to second-order Jahn-Teller (SOJT) effects: 5 octahedrally coordinated d 0 transition metals and lone-pair cations. 6 Because of SOJT effects, both groups of cations are in asymmetric coordination environments. With the d 0 cations, a displacement of the metal toward a corner, edge, or face of the oxide octahedron occurs, 7 whereas with the lone-pair cations, a nonbonded electron pair is observed. 8 For both types of cation, the local coordination is changed from nonpolar centrosymmetric to polar noncentrosym- metric. In fact, the lone-pair cation may be considered as predis- torted, 9 since the cations are almost always found in asymmetric polar coordination environments. We have focused our attention on d 0 transition metal iodates. 10 We suggest that the local polar environments observed in the d 0 transition metals and the lone- pair cation I 5+ are retained in the solid state, resulting in a macroscopically polar material. In this communication, we describe the synthesis, experimental and computational characterization, and functional properties of a new polar material, Li 2 Ti(IO 3 ) 6 . A novel and unique feature of this material that has profound implications for the functional properties is that the lone pairs on the iodate groups are aligned. Li 2 Ti(IO 3 ) 6 was synthesized by combining Li 2 CO 3 , TiO 2 , HIO 3 , and water in an autoclave at 230 °C for 4 days. 11 Li 2 Ti(IO 3 ) 6 crystallizes in the polar noncentrosymmetric space group P6 3 (No. 173). The structure consists of a TiO 6 octahedron that is linked to six IO 3 polyhedra. It should be noted that the Ti 4+ cation is disordered over two sites with 50% occupancy on each site. These groups of polyhedra are separated by the Li + cations. In connectivity terms, the structure can be written as {[TiO 6/2 ] 2- · 6[IO 1/2 O 2/1 ] 0 } 2- , with charge balance maintained by the two Li + cations. Effectively, the structure may be considered as “zero-dimensional”, with the large anionic polyhedra separated by Li + cations (see Figure 1). The Ti-O and I-O bond distances range from 2.028(7) to 2.053(7) Å and from 1.791(5) to 1.871(6) Å, respectively. Bond valence calculations 12 resulted in values of 0.92, 4.18, and 4.94 for Li + , Ti 4+ , and I 5+ , respectively. Both the Ti 4+ and I 5+ cations are in asymmetric coordination environments as a result of SOJT effects. The Ti 4+ cation is slightly distorted toward a face of its octahedron (a C 3 -type distortion), resulting in three “short” [2.028(7) Å] and three “long” [2.053(7) Å] Ti-O bonds. Each I 5+ is bonded to three oxygen atoms, and because of its lone pair, a trigonal pyramidal coordination environment is observed. All of the lone pairs on the I 5+ cations are aligned in a parallel manner (see Figure 2). This alignment of the lone pairs creates a macroscopic dipole moment, resulting in a polar material. The IO 3 polyhedra strongly influence the structure as well as the functional properties of Li 2 Ti(IO 3 ) 6 . As Figure 1 shows, the TiO 6 octahedron is surrounded by six IO 3 groups. As we noted earlier, 9 when octahedrally coordinated d 0 cations are linked to lone- pair polyhedra, the SOJT distortion associated with the d 0 cation is in a direction away from the oxide ligand that bridges the two metals. In Li 2 Ti(IO 3 ) 6 , the Ti 4+ cation is completely surrounded by six IO 3 groups, effectively “trapping” the Ti 4+ cation in the center of its oxide octahedron. Because of this trapping, a very weak distortion (3.8 × 10 -4 Å 2 ) is observed, 13 which is substantially smaller than the average for Ti 4+ (∼0.056 Å 2 ). 9 Li 2 Ti(IO 3 ) 6 is thermally stable up to ∼400 °C. Above this temperature, the material decomposes to Li 2 TiO 3 .The electronic structure of Li 2 Ti(IO 3 ) 6 provides insight into its structure-property relationships. 14 The band structure (see Figure S8 in the Supporting Information) reveals an energy gap of ∼1.6 eV at the Fermi level, which is less than the measured value of 3.0 eV. It has been shown that these types of calculations underestimate the band gap. 15 The top of the valence band (O 2p) and the bottom of the conduction † University of Houston. ‡ Chung-Ang University. Figure 1. Ball-and-stick diagram of Li 2 Ti(IO 3 ) 6 in the ab plane. It should be noted that the groups of metal oxide polyhedra are separated by Li + cations. Published on Web 02/04/2009 10.1021/ja808469a CCC: $40.75  2009 American Chemical Society 2426 9 J. AM. CHEM. SOC. 2009, 131, 2426–2427