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 [2]. The C site (smallest interstice) is generally empty.
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) [12], Ba5−xSrxDyTi3V7O30 (x = 0 - 5) [13], Sr5EuCr3Nb7O30 [14], Ba4SrRTi3V7O30 (R = Sm, Dy) [15]. These compounds show many interesting properties like diffuse phase transition with wide variation of transition temperature. One such compound which has drawn our attention is Ba5HoTi3V7O30 (BHTV) which has already been examined elsewhere [16] and some of its properties have been reported. To know more about this compound and to explore its application, we have summarized the structural, impedance and modulus properties of the BHTV compound in this paper.
2. Experimental
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.
3. Results and Discussion3.1. Structural Study
The room temperature XRD patterns (Figure 1) of the calcined powder of BHTVshow the formation of singlephase compound with orthorhombic crystal structure. The reflection peaks of the patterns were indexed in tetragonal or orthorhombic crystal system using computer software “POWDMULT” [17] (Since TB compounds have generally tetragonal or orthorhombic crystal structure). The position of the strongest peak in this compound is around 30˚. An orthorhombic unit cell was selected on the basis of the best agreement between observed and calculated interplanar spacing d (i.e.,–) for this compound. The unit cell parameters of this compound are a = 13.5479 Å, b = 8.3346 Å and c = 5.6697 Å. The crystallite size (Dhkl) of the powder sample was estimated from the broadening of the peaks (), using Debye-Scherrer’s equation [18];, where λ = 1.5405 Å and θhkl = Bragg angle. Broadening due to mechanical strain, instrumental error and other factors was ignored during calculation. The average crystallite size of BHTV was found to be 16.61 nm.
The inset of Figure 1 shows the scanning electron micrograph of the BHTV pellet at room temperature. It is found that the grains are of platelet like morphology and size are non-uniform and densely distributed throughout the sample. A certain degree of porosity persists which may be due to the low sintering temperature. The average grain size of the compound was found as ~3 μm. The
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 [13].
3.2. Electrical Analysis
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). Figure 2 shows the Nyquist plots (symbol ■) through experiment and fitted data (symbol □) for 573 K, 623 K, 673 K and 723 K temperature by Equation (1) below:
where and Bode plot (Z experimental ■ and calculated □ vs log frequency) is shown in
inset of Figures 2(a)-(d). The complex impedance plots (Figure 2) for all mentioned temperature comprise of two semicircular arcs (due to bulk at high frequency and grain boundary at low frequency) with center below the real axis suggesting the departure from ideal Debye behavior. This departure is due to the presence of constant phase element (CPE) in RC system. CPE is used in a model to compensate non-homogeneity in the system. For a rough or porous surface, α value is between 0.9 and 1 which can cause a double-layer capacitance to appear as a constant phase element. If α equals to 1 than the equation is identical to that of a capacitor and if α is 0 then Z is equals to resemble to ideal resistor. The CPE can also yield to an inductance, if α = –1. For ideal Debye-like response, the equivalent circuit comprises of a parallel combination of a resistor and capacitor with single relaxation time. Here, in Figure 2(e) a single semicircular arc can be modeled to an equivalent circuit of parallel combination of a resistance, capacitance along with a constant phase element [19]. The values of the temperature dependent electrical parameters corresponding to the equivalent circuit (shown in Figure 2(e)) are given in Table 1.
Figure 3(a) shows the variation of imaginary part of impedance (Z″) with frequency at different temperatures. The Z″-frequency patterns exhibits some important features such as; 1) the value of (i.e., peak value) shifts towards higher frequency on increasing temperature; 2) appearance of a peak at a particular frequency (known as relaxation frequency, fmax); 3) the peak value of Z″ decreases as the temperature increases and 4) the peaks in the plot could be observed upto a temperature of 673K. Beyond this temperature peaks could not be observed because it shifts towards higher frequencies which overshoots the limits in which our measurements have been carried out. These features may be considered due to occurrence of the temperature-dependent relaxation phenomena in the materials [20].
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doi:10.1007/s00339-005-3330-1
–
) for this compound. The unit cell parameters of this compound are a = 13.5479 Å, b = 8.3346 Å and c = 5.6697 Å. The crystallite size (Dhkl) of the powder sample was estimated from the broadening of the peaks (
), using Debye-Scherrer’s equation [
, where λ = 1.5405 Å and θhkl = Bragg angle. Broadening due to mechanical strain, instrumental error and other factors was ignored during calculation. The average crystallite size of BHTV was found to be 16.61 nm.
and Bode plot (Z experimental ■ and calculated □ vs log frequency) is shown in
resemble to ideal resistor. The CPE can also yield to an inductance, if α = –1. For ideal Debye-like response, the equivalent circuit comprises of a parallel combination of a resistor and capacitor with single relaxation time. Here, in 
(i.e., peak value) shifts towards higher frequency on increasing temperature; 2) appearance of a peak at a particular frequency (known as relaxation frequency, fmax); 3) the peak value of Z″ decreases as the temperature increases and 4) the peaks in the plot could be observed upto a temperature of 673K. Beyond this temperature peaks could not be observed because it shifts towards higher frequencies which overshoots the limits in which our measurements have been carried out. These features may be considered due to occurrence of the temperature-dependent relaxation phenomena in the materials [