New Polar Oxides: Synthesis, Characterization, Calculations, and Structure-Property Relationships in RbSe2V3O12 and TlSe2V3O12

Two new polar noncentrosymmetric oxides, RbSe2V3O12 and TlSe2V3O12, have been synthesized and characterized. The oxides are isostructural, as Tl+ exhibits an inert rather than a stereoactive lone pair. The reported materials were structurally characterized by single-crystal X-ray diffraction. The materials exhibit a two-dimensional hexagonal tungsten oxide (HTO) type topology with layers of corner-shared VO6 octahedra. The layers are capped, above and below, by SeO3 polyhedra. The Rb+ and Tl+ cations are found between the layers. The V5+ and Se4+ cations are in asymmetric coordination environments attributable to second-order Jahn-Teller (SOJT) effects. In addition to structural characterization, second-harmonic generation (SHG), piezoelectric, and polarization measurements were performed. SHG measurements using 1064 nm radiation revealed doubling efficiencies ranging from similar to 40-50 x alpha-SiO2. Piezoelectric experiments revealed d(33) values of similar to 6-12 pm V-1. Polarization measurements indicate the materials are not ferroelectric, i.e., the polarization is not "switchable". The materials are pyroelectric, with a total pyroelectric coefficient, p, at 45 degrees C, of -4.4 and -2.6 mu C m(-2) K-1 for RbSe2V3O12 and TlSe2V3O12, respectively. Thermogravimetric measurements, UV-vis, and infrared spectroscopy were also performed, as were electronic structure calculations. Crystal data: RbSe2V3O12, hexagonal, space group P6(3) (No. 173), a = b = 7.1364(4) angstrom, c = 11.4687(13) angstrom, V = 505.83(7) angstrom(3), and Z = 2; TlSe2V3O12, hexagonal, space group P6(3) (No. 173), a = b = 7.1248(3) angstrom, c = 11.4287(11) angstrom, V = 502.43(6) angstrom(3), and Z = 2.