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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+)

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posted on 2016-02-22, 10:28 authored by Jeongho Yeon, Sang-Hwan Kim, Sau Doan Nguyen, Hana Lee, P. Shiv Halasyamani
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) Å3, 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) Å3, 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) Å3, 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) Å3, 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) Å3, and Z = 8.

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