A₃V₅O₁₄ (A = K⁺, Rb⁺, or Tl⁺), New Polar Oxides with a Tetragonal Tungsten Bronze Related Structural Topology: Synthesis, Structure, and Functional Properties

Three polar noncentrosymmetric (NCS) oxide materials, A3V5O14 (A = K+, Rb+, or Tl+), have been synthesized by hydrothermal and conventional solid state techniques. Their crystal structures and functional properties (second-harmonic generation, piezoelectricity, and polarization) have been determined. The iso-structural materials exhibit a layered structural topology that consists of corner-sharing VO4 tetrahedra and VO5 square pyramids. The layers stack parallel to the c-axis direction and are separated by the K+, Rb+, or Tl+ cations. Powder second-harmonic generation (SHG) measurements using 1064 nm radiation indicate the materials exhibit moderate SHG efficiencies of ∼100 × α-SiO2. Additional SHG measurements, that is, particle size versus SHG efficiency, indicate the materials are type-I phase-matchable. Converse piezoelectric measurements for K3V5O14, Rb3V5O14, and Tl3V5O14 revealed d33 values of 28, 22, and 26 pm/V, respectively. Pyroelectric measurements, that is, temperature-dependent polarization measurements, resulted in pyroelectric coefficients of −2.2, −2.9, and −2.8 μC/m2·K at 65 °C, for K3V5O14, Rb3V5O14, and Tl3V5O14 respectively. Frequency-dependent polarization measurements confirmed that all of the materials are nonferroelectric, consistent with our first principle density functional theory (DFT) electronic structure calculations. Infrared, UV−vis, thermogravimetric, and differential scanning calorimetry measurements were also performed. Crystal data: K3V5O14, trigonal, space group P31m (No. 157), a = 8.6970(16) Å, c = 4.9434(19) Å, V = 323.81(15), and Z = 1; Rb3V5O14, trigonal, space group P31m (No. 157), a = 8.7092(5) Å, c = 5.2772(7) Å, V = 346.65(5), and Z = 1; Tl3V5O14, trigonal, space group P31m (No. 157), a = 8.7397(8) Å, c = 5.0846(10) Å, V = 336.34(8), and Z = 1.