Phase Transition in Tl2TeO3: Influence and Origin of the Thallium Lone Pair Distortion

Rich Non-centrosymmetry in a Na-U-Te Oxo-System Achieved under Extreme Conditions

Two new sodium uranyl tellurites and two new sodium uranyl tellurates have been synthesized from high-temperature/high-pressure conditions and structurally characterized. We demonstrated that crystalline phases, forming in a Na-U-Te system under extreme conditions, appear to favorably have non-centrosymmetric structures. Three out of four novel uranyl tellurium compounds, Na[(UO2)Te(IV)2O5(OH)], Na2[(UO2)(Te(VI)2O8)], and Na2[(UO2)Te(VI)O5], crystallize in non-centrosymmetric space groups. The crystal structure of Na[(UO2)Te(IV)2O5(OH)] is based on two-dimensional [UO2Te2O5(OH)](-) corrugated sheets, which are charge balanced by guest Na(+) cations. The structure of Na2[(UO2)Te(VI)2O8] is constructed from [(UO2)2Te2O8](2+) anionic layers composed of UO7 pentagonal bipyramids and TeO6 octahedra. Na2[(UO2)(Te(VI)O5)] is a new type of three-dimensional anionic open framework built from the interconnection of UO7 pentagonal bipyramids and TeO6 octahedra with different types of interlacing channels within the U-Te anionic framework. Na[(UO2)Te(IV)6O13(OH)], as the only centrosymmetric compound isolated in the Na-U-Te family, is crystallized in space group Pa3̅, and its structure is highly related to that of cliffordite (UO2(Te3O7)), which is composed from UO8 hexagonal bipyramids and TeO5 square pyramids. The vibrational modes associated with U-O, Te(IV)-O, and Te(VI)-O bonds are discussed, and the Raman spectra of the four compounds are characterized for signature bands.

Macroscopic polarity control with alkali metal cation size and coordination environment in a series of tin iodates

The IO₃ groups in Na₂Sn(IO₃)₆ are aligned in a parallel manner attributed to the six-coordinate Na⁺ cation, whereas the IO₃ polyhedra in Cs₂Sn(IO₃)₆ rotate with respect to Sn⁴⁺ due to the nine-coordinate Cs⁺ cation.

Polar or nonpolar? A+ cation polarity control in A2Ti(IO3)6 (A = Li, Na, K, Rb, Cs, Tl)

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.