New Vanadium Selenites: Centrosymmetric Ca₂(VO₂)₂(SeO₃)₃(H₂O)₂, Sr₂(VO₂)₂(SeO₃)₃, and Ba(V₂O₅)(SeO₃), and Noncentrosymmetric and Polar A₄(VO₂)₂(SeO₃)₄(Se₂O₅) (A = Sr²⁺ or Pb²⁺)

Five new vanadium selenites, Ca₂(VO₂)₂(SeO₃)₃(H₂O)₂, Sr₂(VO₂)₂(SeO₃)₃, Ba­(V₂O₅)­(SeO₃), Sr₄(VO₂)₂(SeO₃)₄(Se₂O₅), and Pb₄(VO₂)₂(SeO₃)₄(Se₂O₅), have been synthesized and characterized. Their crystal structures were determined by single crystal X-ray diffraction. The compounds exhibit one- or two-dimensional structures consisting of corner- and edge-shared VO₄, VO₅, VO₆, and SeO₃ polyhedra. Of the reported materials, A₄(VO₂)₂(SeO₃)₄(Se₂O₅) (A = Sr²⁺ or Pb²⁺) are noncentrosymmetric (NCS) and polar. Powder second-harmonic generation (SHG) measurements revealed SHG efficiencies of approximately 130 and 150 × α–SiO₂ for Sr₄(VO₂)₂(SeO₃)₄(Se₂O₅) and Pb₄(VO₂)₂(SeO₃)₄(Se₂O₅), respectively. Piezoelectric charge constants of 43 and 53 pm/V, and pyroelectric coefficients of −27 and −42 μC/m²·K at 70 °C were obtained for Sr₄(VO₂)₂(SeO₃)₄(Se₂O₅) and Pb₄(VO₂)₂(SeO₃)₄(Se₂O₅), respectively. Frequency dependent polarization measurements confirmed that the materials are not ferroelectric, that is, the observed polarization cannot be reversed. In addition, the lone-pair on the Se⁴⁺ cation may be considered as stereo-active consistent with calculations. For all of the reported materials, infrared, UV–vis, thermogravimetric, and differential thermal analysis measurements were performed. Crystal data: Ca₂(VO₂)₂(SeO₃)₃(H₂O)₂, orthorhombic, space group Pnma (No. 62), a = 7.827(4) Å, b = 16.764(5) Å, c = 9.679(5) Å, V = 1270.1(9) ų, and Z = 4; Sr₂(VO₂)₂(SeO₃)₃, monoclinic, space group P2₁/c (No. 12), a = 14.739(13) Å, b = 9.788(8) Å, c = 8.440(7) Å, β = 96.881(11)°, V = 1208.8(18) ų, and Z = 4; Ba­(V₂O₅)­(SeO₃), orthorhombic, space group Pnma (No. 62), a = 13.9287(7) Å, b = 5.3787(3) Å, c = 8.9853(5) Å, V = 673.16(6) ų, and Z = 4; Sr₄(VO₂)₂(SeO₃)₄(Se₂O₅), orthorhombic, space group Fdd2 (No. 43), a = 25.161(3) Å, b = 12.1579(15) Å, c = 12.8592(16) Å, V = 3933.7(8) ų, and Z = 8; Pb₄(VO₂)₂(SeO₃)₄(Se₂O₅), orthorhombic, space group Fdd2 (No. 43), a = 25.029(2) Å, b = 12.2147(10) Å, c = 13.0154(10) Å, V = 3979.1(6) ų, and Z = 8.