Chang Hong-Young, Kim Sang-Hwan, Ok Kang Min, Halasyamani P Shiv
Department of Chemistry, University of Houston, 136 Fleming Building, Houston, Texas 77204-5003, USA.
J Am Chem Soc. 2009 May 20;131(19):6865-73. doi: 10.1021/ja9015099.
We have synthesized a series of new alkali-metal or Tl(+) titanium iodates, A(2)Ti(IO(3))(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 TiO(6) octahedra, each of which is bonded to six IO(3) polyhedra. These polyhedral groups are separated by the A(+) cations. Our calculations on Na(2)Ti(IO(3))(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: Li(2)Ti(IO(3))(6): hexagonal, space group P6(3) (No. 173), a = b = 9.3834(11) A, c = 5.1183(6) A, Z = 1. Na(2)Ti(IO(3))(6): hexagonal, space group P6(3) (No. 173), a = b = 9.649(3) A, c = 5.198(3) A, Z = 1. K(2)Ti(IO(3))(6): trigonal, space group R3 (No. 148), a = b = 11.2703(6) A, c = 11.3514(11) A, Z = 3. Rb(2)Ti(IO(3))(6): trigonal, space group R3 (No. 148), a = b = 11.3757(16) A, c = 11.426(3) A, Z = 3. Cs(2)Ti(IO(3))(6): trigonal, space group R3 (No. 148), a = b = 11.6726(5) A, c = 11.6399(10) A, Z = 3. Tl(2)Ti(IO(3))(6): trigonal, space group R3 (No. 148), a = b = 11.4167(6) A, c = 11.3953(11) A, Z = 3.
我们合成了一系列新的碱金属或Tl(+)碘酸钛化合物，A₂Ti(IO₃)₆（A = Li、Na、K、Rb、Cs、Tl）。有趣的是，Li相和Na相是非中心对称（NCS）且具有极性的，而K、Rb、Cs和Tl的类似物是中心对称（CS）且无极性的。我们能够利用阳离子尺寸论据、配位要求和键价概念来解释从NCS极性到CS无极性的转变。这六种材料在拓扑结构上相似，由TiO₆八面体组成，每个八面体都与六个IO₃多面体相连。这些多面体基团由A⁺阳离子隔开。我们对Na₂Ti(IO₃)₆的计算表明，极化反转在能量上非常不利，使得该材料具有极性但不是铁电体。对于所有材料，都报道了其合成、结构表征、电子结构分析、红外光谱、紫外可见光谱和热重测量以及离子交换反应。对于极性材料，进行了二次谐波产生、压电性和极化测量。晶体数据：Li₂Ti(IO₃)₆：六方晶系，空间群P6₃（编号173），a = b = 9.3834(11) Å，c = 5.1183(6) Å，Z = 1。Na₂Ti(IO₃)₆：六方晶系，空间群P6₃（编号173），a = b = 9.649(3) Å，c = 5.198(3) Å，Z = 1。K₂Ti(IO₃)₆：三方晶系，空间群R3（编号148），a = b = 11.2703(6) Å，c = 11.3514(11) Å，Z = 3。Rb₂Ti(IO₃)₆：三方晶系，空间群R3（编号148），a = b = 11.3757(16) Å，c = 11.426(3) Å，Z =
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