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Narrative Progress Report

A narrative progress report is presented that describes research results from ACS-PRF. During the grant period, we published 14 papers in peer-reviewed journals, with two additional papers in press, as well as a book chapter. Highlights of selected published research will be discussed. Other research results that are not presented, but acknowledge this grant, can be found in the list of publications uploaded to the ACS-PRF web-site.

A₂Ti(IO₃)₆ (A = Li, Na, K, Rb, Cs, or Tl): We synthesized a series of new alkali-metal or Tl⁺ titanium iodates, A₂Ti(IO₃)₆ (J. Am. Chem. Soc., 131, 2426-2427, 2009; J. Am. Chem. Soc., 131, 6865-6873, 2009). All six materials exhibit a ‘zero-dimensional' crystal structure consisting of a TiO₆ octahedron linked to six IO₃ polyhedra. Between these polyhedra are the alkali-metal or thallium cations. One of the most interesting features about the series of materials is the change in macroscopic polarity, from polar non-centrosymmetric to non-polar centrosymmetric, as we progress from the smaller, Li⁺ and Na⁺ to the larger, K⁺, Rb⁺, Cs⁺, and Tl⁺, cations (see Figure 1). For the reported materials, the local polarity is attributable solely to the IO₃ polyhedra, since the polarization associated with the TiO₆ octahedron is negligible, Li and Na phases, or zero, K, Rb, Cs, and Tl phases. We demonstrated that it is the ionic size, coordination preferences, and bond valence requirements of the smaller cations, i.e., Li⁺ and Na⁺, that result in the non-centrosymmetric polar environment. The synthesis and characterization of these materials was done by a graduate student, Hong-Young Chang, who is currently a post-doctoral associate at UT-Austin.

KNbW₂O₉, RbNbW₂O₉, and KTaW₂O₉: We also synthesized three new polar hexagonal tungsten bronze-type oxides, KNbW₂O₉, RbNbW₂O₉, and KTaW₂O₉. The publication appeared as part of the Forum on Functional Inorganic Materials (Inorg. Chem., 47, 8511-8517, 2008), for which the PI was a co-editor. The reported materials, are iso-structural and consist of MO₆ (M = Nb⁵⁺ / W⁶⁺) octahedra that form a three-dimensional topology. It should be noted that the M⁵⁺ and W⁶⁺ cations are statistically disordered 0.33 : 0.67 on all the metal sites. The MO₆ octahedra are corner-shared, with their connectivity resulting in ‘hexagonal channels' where the alkali-metals are observed (see Figure 2). Attributable to second-order Jahn-Teller (SOJT) effects, the four unique octahedrally coordinated d⁰ cations, M1, M2, M3, and M4, are asymmetrically bonded to six oxygen atoms. There is a marked decrease in the magnitude of the distortion between KNbW₂O₉ and KTaW₂O₉. By examining local and net cationic distortions in the materials, we were able to rationalize the observed physical properties. For example, the SHG efficiency for KNbW₂O₉ compared with KTaW₂O₉ is 220 vs 180 x a-SiO₂ respectively, and spontaneous polarization, P_s, values are 8.4 and 4.8 mC/cm² respectively. Between KNbW₂O₉ and RbNbW₂O₉ the magnitude of the physical properties are similar, although the P_s for RbNbW₂O₉ is surprisingly small, 2.1 mC/cm². It is likely that the relatively small piezoelectric, pyroelectric, and ferroelectric responses are attributable to the cancellation of many of the local polarizations.

Broader Impacts

With support from the ACS-PRF I was able to partially support two graduate students, Hong-Young Chang and Jeongho Yeon, as well as partially support two post-doctoral associates, Drs. Sang-Hwan Kim and Weiguo Zhang. Hong-Young finished his Ph.D. in my research group in August 2009 and is currently a post-doctoral associate in the Department of Chemical Engineering at UT-Austin. Also, with ACS funding I was able to expand on this area of research in the direction of single crystal growth. In fact, Dr. Zhang has recently been able to grow large, centimeter-size, single crystal of a novel piezoelectric oxide.