Correlation between crystal structure and dielectric response for orthorhombic tungsten bronze ceramics Ba4(Nd1-xSmx)28/3(Ti0.95Zr0.05)18O54

Abstract

Ba4(Nd1-xSmx)28/3(Ti0,95Zr0,05)18O54 ceramics with (x = 0; 0.2; 0.4; 0.8 and 1) were synthesized by solid method at 1250 °C for 10 h. The effects of Nd/Sm ratio on the structure and dielectric behavior were studied by changing the value of x. The study of Ba/RE order with (RE = Nd and Sm) in the solid solution Ba4(Nd1-xSmx)28/3(Ti0,95Zr0,05)18O54 was realized by X-ray diffraction. The crystal structure of these phases belongs to the tungsten bronze type, which is constructed on the basis of the (3x3) Ti/ZrO6 octahedron more than (2x2) (orthorhombic symmetry, space group Pnma, a ≈22.3 Å, b ≈ 7.67 Å, c ≈ 12.1 Å). A structural model has been established, corresponding to an order within the structure. The model of formula [Ba4]A2[Ba2-a(Nd1-xSmx)1.33+b□c]A1'[(Nd1-xSmx)8-d□d]A1(Ti0.95Zr0.05)18O54, corresponds to a model associated to infinite perovskite rows parallels to the Oy axis, constructed on the basis of the octahedron (3x3) Ti/ZrO6. Scanning electron microscopy (SEM) has showed that Ba4(Nd1-xSmx)28/3(Ti0,95Zr0,05)18O54 ceramics have a typical columnar grain, which indicates that the phase structure of tungsten bronze exists in the x range and all samples show a dense microstructure. The average grain size ranges from 1.179 to 0.912 μm. The dielectric properties were studied by complex impedance spectroscopy in the temperature range from 30 °C to 800 °C where an anomaly was observed in a few compositions characterized by a maximum of the dielectric permittivity that shifts with increasing frequency at higher temperatures. The presence of a strong dispersion over a wide range of temperatures, is probably related to cationic disorder within the Ba4(Nd1-xSmx)28/3(Ti0,95Zr0,05)18O54 structure.