Two New Noncentrosymmetric Polar Oxides: Synthesis, Characterization, Second-Harmonic Generating, and Pyroelectric Measurements on TlSeVO₅ and TlTeVO₅

Abstract

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 Pna21 (No. 33) with a = 7.1639(15) Å, b = 8.6630(19) Å, c = 7.8946(17) Å, V = 489.95(18) Å3, and Z = 4; TlTeVO5, orthorhombic, space group Pna21 (No. 33) with a = 7.319(3) Å, b = 8.749(4) Å, c = 7.868(3) Å, V = 503.8(4) Å3, 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 C2 direction), whereas the Se4+ (Te4+) and Tl+ cations are in distorted coordination environments attributable to their lone-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 ∼40 × α-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 μC/m2·K, for TlSeVO5 and TlTeVO5, respectively. Infrared, Raman, UV−vis diffuse reflectance spectroscopy, and thermal analyses are also presented.