Polycrystalline sample of Ba5HoTi3V7O30 was prepared using solid-state reaction technique. X-ray structural analysis indicated a single-phase formation with orthorhombic structure. Microstructural study by SEM showed non-uniform distribution of grains over the surface of the sample. Impedance and modulus spectroscopy studies were carried out, as functions of frequency (42 Hz - 5 MHz) and temperature (RT-773K). The Nyquist plots clearly showed the presence of both bulk and grain boundary effect in the compound. Electrical phenomena in the material can appropriately be modeled in terms of an equivalent circuit with R, C and CPE in parallel. The fitting procedure used here allows us to determine the value of R and C with good precision. Here R2 and R3 correspond to the resistance contributed from the grain boundary and bulk, respectively. C1 and C2 correspond to the capacitance contributed from the grain boundary and bulk, respectively. The real part of electrical modulus shows that the material is highly capacitive. The asymmetric peak of the imaginary part of electric modulus M″, predicts a non Debye type relaxation. The activation energy of the compound (calculated both from impedance and modulus spectrum) is same, and hence the relaxation process may be attributed to the same type of charge carriers.
Ferroelectric compounds of tungsten bronze (TB) structural family covers a large number of ferroelectric materials which are found to be useful for applications in various electronic devices such as transducers, actuators, capacitors, and ferroelectric random access memory [1,2]. They have also some associated properties like pyroelectric, piezoelectric, and nonlinear optical properties [3-5]. The TB structure consists of a skeleton framework of distorted BO6 octahedral sharing corners in such a way that a variety of cations can be substituted at three different types of interstices (A, B, and C) of a general formula (A1)2(A2)4(C4)(B1)2(B2)8O30. Thus it provides scope for improving the ferroelectric properties in search of new and better materials for device applications .where A-type (mono or divalent) cations can be accommodated at different types of interstitial (A1, A2) sites. Trior pentavalent atoms are substituted at octahedral sites B1 and B2 [
Detailed literature study reveals that a lot of work has been carried out on ferroelectric niobates, vanadates and tantalates having TB structure [6-9] such as, Ba5RTi3–x- ZrxNb7O30 (x = 0, 1, 2, 3) [10,11], Ba5RTi3Nb10O30 (R = Dy and Sm) [
Preparation of the material. A polycrystalline sample of BHTV was fabricated using solid state reaction technique. Powders of BaCO3 (Loba chemie, 99%), Ho2O3 (Merck, 99.5%+), TiO2 (Loba chemie, 99.5%) and V2O5 (Loba chemie, 99%) in stoichiometric proportion were weighed and thoroughly ground in an agate mortar to maintain proper stoichiometry and homogeneity in mixture, and then calcined at 1023 K for 12 hrs. The calcined powder was pressed into cylindrical pellets 12 - 13 mm in diameter and 1 - 2 mm thick under the pressure of 7 tons using a hydraulic press. Polyvinyl butyral (PVB) was used as a binder to reduce the brittleness of the pellets, which was burnt out during sintering. The pellets were then sintered in an air atmosphere at 1073 K for 12 hrs, and then polished with fine emery paper to make their faces flat and parallel. The pellets were finally coated with conductive silver paint and dried at 423 K for 2 hour before carrying out impedance measurements.
Characterization of the material. X-ray diffraction (XRD) studies of the materials were carried out at room temperature in the Bragg angle range 20˚ - 80˚ by Rigaku X-ray diffractometer (model: Miniflex). Surface morphology of the compound was studied by scanning electron microscopy (JEOL JSM-5800F). The impedance measurements were carried out at an input signal level of 300 mV in the temperature range of RT-773 K using a computer-controlled impedance analyzer (HIOKI LCR Hi TESTER, Model: 3532-50) in the frequency range of 42 Hz - 5 MHz along with a laboratory made sample holder and a heating arrangement.
The room temperature XRD patterns (
The inset of
shape, size and distribution of grains in the microstructure suggest that the sample has polycrystalline nature. Similar microstructures are observed with that of some other materials of this family [
Complex impedance analysis technique is an important and powerful tool to investigate the electrical properties of materials over a wide range of frequency (42 Hz - 5 MHz) and temperature (573 K - 773 K).
where and Bode plot (Z experimental ■ and calculated □ vs log frequency) is shown in
inset of Figures 2(a)-(d). The complex impedance plots (