Chang H Y, Sivakumar T, Ok K M, Halasyamani P Shiv
Department of Chemistry, University of Houston, 136 Fleming Building, Houston, Texas 77204-5003, USA.
Inorg Chem. 2008 Oct 6;47(19):8511-7. doi: 10.1021/ic800573k.
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 KNbW 2O 9, RbNbW 2O 9, and KTaW 2O 9 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 MO 6 (M = Nb (5+)/W (6+) or Ta (5+)/W (6+)) octahedral framework, with K (+) or Rb (+) occupying the "hexagonal" tunnels. The d (0) transition metals, Nb (5+), Ta (5+), and W (6+), 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-SiO 2. 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 muC K (-1) m (-2). The reported materials are also ferroelectric, as demonstrated by hysteresis loops (polarization vs electric field). Spontaneous polarization values, P s, ranging from 2.1 to 8.4 muC 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: KNbW 2O 9, orthorhombic, space group Cmm2 (No. 35), a = 21.9554(2) A, b = 12.60725(15) A, c = 3.87748(3) A, V = 1073.273(13) A (3), and Z = 6; RbNbW 2O 9, orthorhombic, space group Cmm2 (No. 35), a = 22.00985(12) A, b = 12.66916(7) A, c = 3.8989(2) A, V = 1086.182(10) A (3), and Z = 6; KTaW 2O 9, orthorhombic, space group Cmm2 (No. 35), a = 22.0025(2) A, b = 12.68532(14) A, c = 3.84456(4) A, V = 1073.05(2) A (3), and Z = 6.
报道了三种极性非中心对称（NCS）六方钨青铜型氧化物的合成、晶体结构、二次谐波产生（SHG）、压电、热释电和铁电性能。通过标准固态技术合成了材料KNbW₂O₉、RbNbW₂O₉和KTaW₂O₉，并通过实验室粉末X射线衍射对其结构进行了表征。这些化合物是同构的，结晶于极性NCS空间群Cmm2。材料呈现出角共享的MO₆（M = Nb(⁵⁺)/W(⁶⁺)或Ta(⁵⁺)/W(⁶⁺)）八面体框架，其中K⁺或Rb⁺占据“六方”隧道。d(0)过渡金属Nb(⁵⁺)、Ta(⁵⁺)和W(⁶⁺)由于二阶 Jahn-Teller 效应而偏离其氧化物八面体的中心。使用1064 nm辐射进行的SHG测量显示倍频效率在180至220×α-SiO₂之间。逆压电测量得到的d₃₃值在10至41 pm V⁻¹之间。在50℃时的总热释电系数p在-6.5至-34.5 μC K⁻¹ m⁻²之间。如滞后回线（极化与电场）所示，所报道的材料也是铁电的。测量得到的自发极化值Ps在2.1至8.4 μC cm⁻²之间。SHG效率、压电响应、热释电系数和铁电极化的大小强烈依赖于d(0)过渡金属的偏离中心畸变。讨论并探索了结构-性能关系。晶体数据：KNbW₂O₉，正交晶系，空间群Cmm2（编号35），a = 21.9554(2) Å，b = 12.60725(15) Å，c = 3.87748(3) Å，V = 1073.273(13) ų，Z = 6；RbNbW₂O₉，正交晶系，空间群Cmm2（编号35），a = 22.
