Two new noncentrosymmetric polar oxides:: Synthesis, characterization, second-harmonic generating, and pyroelectric measurements on TlSeVO5 and TlTeVO5

Two new noncentrosymmetric (NCS) polar quaternary oxides, TlMVO5 (M = Se4+ or Te4+), have been synthesized by hydrothermal techniques using Tl2CO3, SeO2 (TeO2), and V2O5 as reagents. The structures were determined by single-crystal X-ray diffraction (TlSeVO5, orthorhombic, space group Pna2(1) (No. 33) with alpha = 7.1639(15) angstrom, b = 8.6630(19) angstrom, c = 7.8946(17) angstrom, V = 489.95(18) angstrom(3), and Z = 4;TlTeVO5, orthorhombic, space group Pna2(1) (No. 33) with a = 7.319(3) angstrom, b = 8.749(4) angstrom, c = 7.868(3) angstrom, V = 503.8(4) angstrom(3), Z = 4). The materials exhibit NCS and polar three-dimensional structures consisting of chains of corner shared VO6 octahedra connected by SeO3 (TeO4) and TlO8 polyhedra. The V5+, Se4+ (Te4+), and Tl+ cations are in asymmetric coordination environments attributable to second-order Jahn-Teller (SOJT) effects. The V5+ cations undergo intra-octahedral distortions toward an edge (local C-2 direction), whereas the Se4+ (Te4+) and TI+ cations are in distorted coordination environments attributable to their]one-pair. As the materials are NCS and polar, second-harmonic generating (SHG), ferroelectric, and pyroelectric measurements were performed. The SHG measurements, using 1064 nm radiation, revealed doubling efficiencies of similar to 40 x alpha-SiO2 for both TlSeVO5 and TlTeVO5. Ferroelectric measurements indicated the materials are not "switchable"; that is, the local dipole moment cannot be reversed in the presence of an external electric field. Pyroelectric measurements revealed a total pyroelectric coefficient, p, of -2.9 and - 1.9 mu C/m(2)center dot K, for TlSeVO5 and TlTeVO5, respectively. Infrared, Raman, UV-vis diffuse reflectance spectroscopy, and thermal analyses are also presented.