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, TlMVO 5 (M = Se 4+ or Te 4+ ), have been synthesized by hydrothermal techniques using Tl 2 CO 3, SeO 2 (TeO 2 ), and V 2 O 5 as reagents. The structures were determined by single-crystal X-ray diffraction (TlSeVO 5, orthorhombic, space group Pna 2 1 (No. 33) with a = 7.1639(15) Å, b = 8.6630(19) Å, c = 7.8946(17) Å, V = 489.95(18) Å 3, and Z = 4; TlTeVO 5, orthorhombic, space group Pna 2 1 (No. 33) with a = 7.319(3) Å, b = 8.749(4) Å, c = 7.868(3) Å, V = 503.8(4) Å 3, Z = 4). The materials exhibit NCS and polar three-dimensional structures consisting of chains of corner shared VO 6 octahedra connected by SeO 3 (TeO 4 ) and TlO 8 polyhedra. The V 5+, Se 4+ (Te 4+ ), and Tl + cations are in asymmetric coordination environments attributable to second-order Jahn−Teller (SOJT) effects. The V 5+ cations undergo intra-octahedral distortions toward an edge (local C 2 direction), whereas the Se 4+ (Te 4+ ) 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-SiO 2 for both TlSeVO 5 and TlTeVO 5 . 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/m 2 ·K, for TlSeVO 5 and TlTeVO 5, respectively. Infrared, Raman, UV−vis diffuse reflectance spectroscopy, and thermal analyses are also presented.