Polar or Nonpolar? A+ Cation Polarity Control in A2Ti(IO3)6(A = Li, Na, K, Rb, Cs, Tl)

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Abstract
We have synthesized a series of new alkali-metal or Tl+ titanium iodates, A2Ti(IO3)6 (A = Li, Na, K, Rb, Cs, Tl). Interestingly the Li and Na phases are noncentrosymmetric (NCS) and polar, whereas the K, Rb, Cs, and Tl analogues are centrosymmetric (CS) and nonpolar. We are able to explain the change from NCS polar to CS nonpolar using cation-size arguments, coordination requirements, and bond valence concepts. The six materials are topologically similar, consisting of TiO6 octahedra, each of which is bonded to six IO3 polyhedra. These polyhedral groups are separated by the A+ cations. Our calculations on Na2Ti(IO3)6 indicate that polarization reversal is energetically very unfavorable, rendering the material polar but not ferroelectric. For all of the materials, synthesis, structural characterization, electronic structure analysis, infrared spectra, UV−vis and thermogravimetric measurements, and ion-exchange reactions are reported. For the polar materials, second-harmonic generation, piezoelectricity, and polarization measurements were performed. Crystal data: Li2Ti(IO3)6: hexagonal, space group P63 (No. 173), a = b = 9.3834(11) Å, c = 5.1183(6) Å, Z = 1. Na2Ti(IO3)6: hexagonal, space group P63 (No. 173), a = b = 9.649(3) Å, c = 5.198(3) Å, Z = 1. K2Ti(IO3)6: trigonal, space group R3̅ (No. 148), a = b = 11.2703(6) Å, c = 11.3514(11) Å, Z = 3. Rb2Ti(IO3)6: trigonal, space group R3̅ (No. 148), a = b = 11.3757(16) Å, c = 11.426(3) Å, Z = 3. Cs2Ti(IO3)6: trigonal, space group R3̅ (No. 148), a = b = 11.6726(5) Å, c = 11.6399(10) Å, Z = 3. Tl2Ti(IO3)6: trigonal, space group R3̅ (No. 148), a = b = 11.4167(6) Å, c = 11.3953(11) Å, Z = 3.

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