Synthesis, Structure, and Characterization of Two New Polar Sodium Tungsten Selenites: Na₂(WO₃)₃(SeO₃)·2H₂O and Na₆(W₆O₁₉)(SeO₃)₂

Two new quaternary sodium tungsten selenites, Na₂(WO₃)₃(SeO₃)·2H₂O (<i>P</i>3₁<i>c</i>) and Na₆(W₆O₁₉)­(SeO₃)₂ (<i>C</i>2), have been synthesized and characterized. The former exhibits a hexagonal tungsten oxide layered structure, whereas the latter has a one-dimensional “ribbon” structure. The layers and “ribbons” consist of distorted WO₆ and asymmetric SeO₃ polyhedra. The layers in Na₂(WO₃)₃(SeO₃)·2H₂O and the “ribbons” in Na₆(W₆O₁₉)­(SeO₃)₂ are separated by Na⁺ cations. Powder second-harmonic-generation (SHG) measurements on Na₂(WO₃)₃(SeO₃)·2H₂O and Na₆(W₆O₁₉)­(SeO₃)₂ using 1064 nm radiation reveal SHG efficiencies of approximately 450× and 20× α-SiO₂, respectively. Particle size versus SHG efficiency measurements indicate that the materials are type 1 non-phase-matchable. Converse piezoelectric measurements result in <i>d</i>₃₃ values of approximately 23 and 12 pm/V, whereas pyroelectric measurements reveal coefficients of −0.41 and −1.10 μC/m²·K at 60 °C for Na₂(WO₃)₃(SeO₃)·2H₂O and Na₆(W₆O₁₉)­(SeO₃)₂, respectively. Frequency-dependent polarization measurements confirm that the materials are nonferroelectric; i.e., the macroscopic polarization is not reversible, or “switchable”. IR and UV–vis spectroscopy, thermogravimetric and differential thermal analysis measurements, and electron localization function calculations were also done for the materials. Crystal data: Na₂(WO₃)₃(SeO₃)·2H₂O, trigonal, space group <i>P</i>3₁<i>c</i> (No. 159), <i>a</i> = 7.2595(6) Å, <i>b</i> = 7.2595(6) Å, <i>c</i> = 12.4867(13) Å, <i>V</i> = 569.89(9) ų, <i>Z</i> = 2; Na₆(W₆O₁₉)­(SeO₃)₂, monoclinic, space group <i>C</i>2 (No. 5), <i>a</i> = 42.169(8) Å, <i>b</i> = 7.2690(15) Å, <i>c</i> = 6.7494(13) Å, β = 98.48(3)°, <i>V</i> = 2046.2(7) ų, <i>Z</i> = 4