The Lone‐Pair Cation I⁵⁺ in a Hexagonal Tungsten Oxide‐Like Framework: Synthesis, Structure, and Second‐Harmonic Generating Properties of Cs₂I₄O₁₁

Abstract

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. [20-24] The IO 5 and IO 3 polyhedra share corners to form the layered structure. In connectivity terms, the layer can be described as a [3(2À anion, with charge balance attained by two Cs + cations. One of the most novel aspects of the structure is the twodimensional IO 5 layer (Figure 1). The HTO-like layer consists of six-membered rings containing corner-sharing IO 5 polyhedra in alternating orientation as one proceeds around the ring. Cs 2 I 4 O 11 is the first example of a lone-pair cation in a HTO-like topology. In the rings themselves, three lone pairs point inward and three outward. Thus, the lone pairs and oxide anions alternate around the ring. The lone pairs have two important structural consequences. First, the local dipole moment on each IO 5 polyhedron is in the direction of the lone pair. If we sum all of the dipole moments with respect to the IO 5 group, the resultant moment is zero. In other words, there is complete cancellation of the dipole moments with respect to the IO 5 polyhedra, that is, the IO 5 layer is pseudocentrosymmetric. This type of pseudocentrosymmetric layer was observed previously in Rb 2 TeW 3 O 12 and Cs 2 TeW 3 O 12 .[17] The cancellation is relevant to the SHG efficiency. Second, the Cs + cations are not coplanar with the IO 5 ring, attributable to the lone pairs. As shown in Figure 2, the Cs + cations sit above and below the rings. The environments of the Cs + cations influence any possible ion-exchange capabilities of the material.[25] The IO 5 layer is capped on one side by an asymmetric three-coordinate I 5+ cation, that is, an IO 3 group (see Figure 3). Alignment of the IO 3 polyhedra in the [001] direction results in an NCS and polar structure. The lone pairs associated with the IO 3 polyhedra also point along the [001]

Rb2Na(NO3)3: A Congruently Melting UV-NLO Crystal with a Very Strong Second-Harmonic Generation Response

“…Recently, materials containing carbonates have been widely studied, and a series of relevant materials exhibiting excellent NLO characteristics have been uncovered [26][27][28][29][30][31][32][33]. Systematic examinations of [NO 3 ]´-containing materials with proper NLO properties, however, have rarely been achieved in contrast to the successful investigations of carbonates and borates [34][35][36][37]. In fact, there is no known nitrate compound that can be used as a practical NLO material up to now.…”

“…The unique advantage of the [BO 3 ] 3´g roup has motivated many chemists' interests further in finding new NLO materials with other functional groups, such as nitrates and carbonates. Specifically, [NO 3 ]á nd [CO 3 ] 2´g roups possess the similar π-conjugated molecular orbitals in trigonal planar structures. Recently, materials containing carbonates have been widely studied, and a series of relevant materials exhibiting excellent NLO characteristics have been uncovered [26][27][28][29][30][31][32][33].…”

“…In fact, there is no known nitrate compound that can be used as a practical NLO material up to now. On the basis of Li's calculations, the microscopic second-order susceptibility, β (2) , of the [NO 3 ]´group is expected to be larger than that of the [BO 3 ] 3´o r [CO 3 ] 2´g roup [38]. Therefore, we have chosen the [NO 3 ]á nionic group as a basic building unit to develop novel UV-NLO crystals with high NLO coefficients.…”

“…On the basis of Li's calculations, the microscopic second-order susceptibility, β (2) , of the [NO 3 ]´group is expected to be larger than that of the [BO 3 ] 3´o r [CO 3 ] 2´g roup [38]. Therefore, we have chosen the [NO 3 ]á nionic group as a basic building unit to develop novel UV-NLO crystals with high NLO coefficients. In order to obtain NLO materials exhibiting strong second-harmonic generation (SHG) effects, a number of methods have been discussed.…”

“…Powder SHG measurements indicate that Rb 2 Na(NO 3 ) 3 has a strong SHG efficiency of five times that of KH 2 PO 4 (KDP) and is type I phase-matchable. The calculated average nonlinear optical (NLO) susceptibility of Rb 2 Na(NO 3 ) 3 turns out to be the largest value among the NLO materials composed of only [NO 3 ]á nion. In addition, Rb 2 Na(NO 3 ) 3 exhibits a wide transparency region ranging from UV to near IR, which suggests that the compound is a promising NLO material.…”