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 (P31c) and Na6(W6O19)­(SeO3)2 (C2), 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 d33 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 P31c (No. 159), a = 7.2595(6) Å, b = 7.2595(6) Å, c = 12.4867(13) Å, V = 569.89(9) Å3, Z = 2; Na6(W6O19)­(SeO3)2, monoclinic, space group C2 (No. 5), a = 42.169(8) Å, b = 7.2690(15) Å, c = 6.7494(13) Å, β = 98.48(3)°, V = 2046.2(7) Å3, Z = 4.