Noncentrosymmetric (NCS) oxides exhibit a variety of technologically important properties including ferroelectricity, piezoelectricity, pyroelectricity, and second-order nonlinear optical behavior. [1][2][3] The rational design of new NCS materials, however, remains a challenge, although a number of strategies have been suggested. [4][5][6][7][8][9] We have focused on synthesizing materials containing cations with lone pairs (e.g., Sb 3+ , Se 4+ , Te 4+ , etc.) in order to increase the incidence of NCS in any new compound. [10,11] A structural topology that is often observed in NCS is the hexagonal tungsten oxide (HTO) framework. This structure has been reported for a variety of cations, including V 5+ , Mo 6+ , W 6+ , and Sb 5+ . [12][13][14][15][16][17][18] The framework consists of corner-sharing MO6 = 2 octahedra that are linked to form an array of three-and six-membered rings. To date, structures have been observed in which other cations, for example, Se 4+ , Te 4+ , P 5+ , and Sb 3+ , bridge, cap one side, or cap both sides of the HTO layer. In instances of onesided capping, an NCS polar material is observed, often with highly efficient second-harmonic generating (SHG) properties, that is, SHG > 400 SiO 2 . [13,17] In the reported materials, the HTO framework has been restricted to octahedrally coordinated d 0 transition metals and the d 10 Sb 5+ cation. We describe herein Cs 2 I 4 O 11 , the first material with a layered HTO-like framework that contains a lone-pair cation, I 5+ . The material also has I 5+ cations capping the layer on one side, which renders the structure NCS. Hence, SHG measurements are also presented.
Crystals of Cs 2 I 4 O 11 were grown hydrothermally by combining Cs 2 CO 3 , Nb 2 O 5 , HIO 3 , and H 2 O in a Teflon-lined autoclave at 220 8C for 4 d. The product consisted of large (maximum dimension 3 mm), colorless, faceted, hexagonal crystals.
Cs 2 I 4 O 11 exhibits a layered HTO-like framework consisting of asymmetric IO 5 and IO 3 polyhedra. [19] Both the fiveand three-coordinate I 5+ cations are in asymmetric coordina-[*] Dr.