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

Two new polar noncentrosymmetric (NCS) oxides, Rb 2 (MoO 3 ) 3 (SeO 3 ) and Tl 2 (MoO 3 ) 3 (SeO 3 ), have been synthesized and characterized. The materials exhibit the layered hexagonal tungsten oxide (HTO) structural topology—Class 2, i.e., the MoO 6 layers are “capped” on one side by the SeO 3 polyhedra. The Rb + and Tl + cations are found between the layers. The Mo 6+ and Se 4+ 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-SiO 2 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 Rb 2 (MoO 3 ) 3 (SeO 3 ) and −2.1 μC m −2 K −1 for Tl 2 (MoO 3 ) 3 (SeO 3 ). 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 Rb 2 (MoO 3 ) 3 (SeO 3 ): hexagonal, space group P 6 3 (No. 173), a = b = 7.2992(14) Å, c = 11.978(5) Å, V = 552.7(3) Å 3, and Z = 2; crystal data for Tl 2 (MoO 3 ) 3 (SeO 3 ): trigonal, space group P 31 c (No. 159), a = b = 7.2867(10) Å, c = 11.794(3) Å, V = 542.33(18) Å 3, and Z = 2.