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

• Published in: Chemistry of materials 2010-05, Vol.22 (10), p.3241-3250
• Main Authors: Chang, Hong Young, Kim, Sun Woo, Halasyamani, P. Shiv
• Format: Article
• Language: English
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm100476m

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 d 33 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.