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Polar Hexagonal Tungsten Oxide (HTO) Materials: (1) Synthesis, Characterization, Functional Properties, and Structure−Property Relationships in A2(MoO3)3(SeO3) (A = Rb+ and Tl+) and (2) Classification, Structural Distortions, and Second-Harmonic Generating Properties of Known Polar HTOs

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posted on 25.05.2010, 00:00 authored by Hong Young Chang, Sun Woo Kim, P. Shiv Halasyamani
Two new polar noncentrosymmetric (NCS) oxides, Rb2(MoO3)3(SeO3) and Tl2(MoO3)3(SeO3), have been synthesized and characterized. The materials exhibit the layered hexagonal tungsten oxide (HTO) structural topologyClass 2, i.e., the MoO6 layers are “capped” on one side by the SeO3 polyhedra. The Rb+ and Tl+ cations are found between the layers. The Mo6+ and Se4+ cations are present in asymmetric coordination environments that are attributable to second-order Jahn−Teller (SOJT) effects. In addition to structural characterization, the materials were characterized by second-harmonic generation (SHG), piezoelectric, and polarization measurements. SHG measurements using 1064-nm radiation revealed doubling efficiencies of 300 and 400 × α-SiO2 for the Rb- and Tl-phases, respectively. Piezoelectric experiments revealed d33 values of ∼9−13 pm/V. Polarization measurements indicate that the materials are not ferroelectric, i.e., the polarization is not “switchable”. The materials are pyroelectric, with total pyroelectric coefficients (p), at 45 °C, of −1.1 μC m−2 K−1 for Rb2(MoO3)3(SeO3) and −2.1 μC m−2 K−1 for Tl2(MoO3)3(SeO3). Thermogravimetry, differential scanning calorimetry, UV−vis, and infrared spectroscopy measurements were also performed. We also examine all of the known polar HTO-type materials to gain a better understanding of the functional properties and structure−property relationships. Crystal data for Rb2(MoO3)3(SeO3): hexagonal, space group P63 (No. 173), a = b = 7.2992(14) Å, c = 11.978(5) Å, V = 552.7(3) Å3, and Z = 2; crystal data for Tl2(MoO3)3(SeO3): trigonal, space group P31c (No. 159), a = b = 7.2867(10) Å, c = 11.794(3) Å, V = 542.33(18) Å3, and Z = 2.