Polar hexagonal tungsten bronze-type oxides: KNbW2O9, RbNbW2O9, and KTaW2O9

Abstract

The synthesis, crystal structures, second-harmonic generation (SHG), piezoelectric, pyroelectric, and ferroelectric properties of three polar noncentrosymmetric (NCS) hexagonal tungsten bronze-type oxides are reported. The materials KNbW2O9, RbNbW2O9, and KTaW2O9 were synthesized by standard solid-state techniques and structurally characterized by laboratory powder X-ray diffraction. The compounds are isostructural, crystallizing in the polar NCS space group Cmm2. The materials exhibit a corner-shared MO6 (M = Nb5+/W6+ or Ta5+/W6+) octahedral framework, with K+ or Rb+ occupying the "hexagonal" tunnels. The d(0) transition metals, Nb5+, Ta5+, and W6+, are displaced from the center of their oxide octahedra attributable to second-order Jahn-Teller effects. SHG measurements using 1064 nm radiation revealed frequency-doubling efficiencies ranging from 180 to 220 x alpha-SiO2, Converse piezoelectric measurements resulted in d(33) values ranging from 10 to 41 pm V-1. The total pyroelectric coefficient, p, at 50 degrees C ranged from -6.5 to -34.5 mu C K-1 m(-2). The reported materials are also ferroelectric, as demonstrated by hysteresis loops (polarization vs electric field). Spontaneous polarization values, P, ranging from 2.1 to 8.4 mu C cm(-2) were measured. The magnitudes of the SHG efficiency, piezoelectric response, pyroelectric coefficient, and ferroelectric polarization are strongly dependent on the out-of-center distortion of the d(0) transition metals. Structure-property relationships are discussed and explored. Crystal data: KNbW2O9, orthorhombic, space group Cmm2 (No. 35), a = 21.9554(2) angstrom, b = 12.60725(15) angstrom, c = 3.87748(3) angstrom, V = 1073.273(13) angstrom(3), and Z = 6; RbNbW2O9, orthorhombic, space group Cmm2 (No. 35), a = 22.00985(12) angstrom, b = 12.66916(7) angstrom, c = 3.8989(2) angstrom, V = 1086.182(10) angstrom(3), and Z = 6; KTaW2O9, orthorhombic, space group Cmm2 (No. 35), a = 22.0025(2) angstrom, b = 12.68532(14) angstrom, c = 3.84456(4) angstrom, V = 1073.05(2) angstrom(3) and Z = 6.