1. Introduction

ABO₃ Perovskite type materials are of considerable technological importance, particularly with respect to their physical properties such as ferro, pyro-and piezo-electricity, dielectric susceptibility, linear & non linear electro-optic properties etc. The change in physical properties is particularly large when the external conditions, such as temperature, pressure, electric-field, composition etc. are altered. Such effects occur in connection with the simultaneous presence of phase transition in the system, where the atomic structure of the perovskite changes either discontinuously or continuously into another form. The dielectric properties of ABO₃ perovskite structure K_1-xNa_x NbO₃ for some of the compositions has been extensively studied at high temperature[1] and it shows a number of ferroelectric phases with high spontaneous polarization. The solid solution of ferroelectric KNbO3 and antiferroelectric NaNbO3 exhibits good piezoelectric properties[2,3]. Potassium Sodium Niobate ceramic (K_1-xNa_xNbO₃) with perovskite structures are widely used for transducer applications with broad ranges of technologically important dielectric, piezoelectric, ferroelectric and electro-optic properties. The structure of PSN system at room temperature is basically of the orthorhombic type. K_1-xNa_xNbO₃ (x=0) is a particularly promising ferroelectric with its combination of relatively low dielectric constant ( with extremely high electro-optic coefficients (r₄₂=380pm/V, r₃₃=64pm/V). This makes KNbO₃ vary attractive for opto-electronic devices, including high-speed electro-optic switches, modulators, and frequency doublers[4-7]. Dielectric measurements on this material were first reported by Matthias and Remeika [8] and then by Shirane et. al.[9] and observed well defined ferroelectric hystersis loops from room temperature to 400⁰C, for compositions form about 10 to 100% (mol) of K in K_1-xNa_xNbO₃. Dielectric properties were reported by Narayan Murty et. al. [10]. Cross [11] has predicted, theoretically, the phase-diagram of KNbO₃-NaNbO₃ mixture from phenomenological arguments. The mixing of NaNbO₃ in KNbO₃ ceramics play an important role on the ferroelectric Curie temperature, dielectric constant, grain size as well as the planer mechanical coupling coefficient (k_p) value [12-13], because NaNbO₃ is anti-ferroelectric at room temperature, when mixed with small amount of KNbO₃, becomes ferroelectric, which creates interest in the present investigation to investigate this system with a varying composition and temperature range. In this paper, we report the results of investigation on dielectric constant, tangent loss and electrical conductivity of the ceramic system K_1-xNa_xNbO₃ ( x=0.4, 0.2 & 0) prepared by solid state reaction method and sintering process, in the temperature range 45⁰C-245⁰C, and at 1 M Hz frequency.

2. Preparation

The starting material was dried at 200⁰C for one hour to remove absorbed moisture. Different compositions of K_1-xNa_xNbO₃ for (x=0.4, 0.2 & 0) were prepared by weighing the sodium carbonate, potassium carbonate and niobium penta-oxide (starting materials) in proper stoichiometric proportions. The mixture was calcined in the platinum crucible, in air, at 950⁰C for 2h, for carbonate removal. After cooling, in dry air, the calcined mixtures were weighed to ensure complete carbonate removal.

The pre-sintered mixture was ground and pressed into pellets of 10mm diameter. All the pellets were placed on a platinum crucible and sintered, in air, at 1050⁰C for 26 h. The sintered pellets were electroded using air-drying silver paste for dielectric measurements.

3. Characterization

X-ray powder studies were performed at room temperature with a SEIFERT 3000P X-ray diffractometer using filtered Cu K₁ radiation of 1.540598⁰ A wavelength, in which, Ni, is used as filter. The instrument is well calibrated with silicon standard samples and the lines obtained are matching with the standard lines. At room temperature all PSN ceramic samples exhibit the orthorhombic symmetry. The sub cell parameters were obtained using the auto-X computer software and were compatible with those obtained earlier for ceramics [10,12-17]. The results are summarized in following table.

Table A. Lattice parameters of K1-XNaXNbO3 for different compositions at room temperature composition Lattice parameters oA a b c KNbO3 4.027 4.057 3.958 K0.8Na0.2NbO3 4.002 4.040 3.946 K0.6Na0.4NbO3 3.998 4.011 3.919

4. Measurements

The variation of dielectric constant, loss tangent and electrical conductivity at 1MHz frequency, in the temperature range 45⁰C-245⁰C have been studied and plotted in Fig. 1, 2 & 3 respectively. These measurements were performed on HP Impedance Analyzer and FLUKE RCL meter, PM 6306.

5. Result and Discussions

We have measured dielectric constant, loss tangent of K_0.6Na_0.4NbO₃, K_0.8Na_0.2NbO₃ and KNbO₃ ceramic pellets using HP Impedance Analyzer and Fluke RCL meter PM 6304, at the temperatures from 45⁰C to 245⁰C.The temperature dependence of dielectric constant, loss tangent and conductivity at 1MHz have been shown in Figs 1, 2 & 3, and tables-1,2 & 3, respectively.

From these figures (Fig.1-3) and tables (Tables 1-3), it is observed that the mixed system of K_1-xNa_xNbO₃ has a transition from orthorhombic to tetragonal at about 220⁰C. It is 225⁰C for KNbO₃, 215⁰C for K_0.8Na_0.2NbO₃ and 205⁰C for K_0.6Na_0.4NbO₃, which shows that transition temperature from orthorhombic to tetragonal shifts towards lower temperature as we increases the quantity of anti- ferroelctric material sodium niobate in the mixed system of K_1-XNa_XNbO₃, which is in agreement with previous observations[1,18]. Lattice parameters of K_1-XNa_XNbO₃ for different compositions at room temperature have been shown in table-A, which shows a decrease in lattice parameters as we add antiferroelectric material NaNbO₃ in mixed system of K_1-xNa_xNbO₃ ceramic.

Figture 1. Variation of dielectric constant with temperature for K1-xNaxNbO3 at 1 MHz

Table 1. Variation of dielectric constant with temperature for K1-xNaxNbO3, at 1 MHz frequency Temperature 0C KNbO3 K0.8Na0.2NbO3 K0.6Na0.4NbO3 45 76.17 70.57 83.680 55 76.17 069.88 81.910 65 76.52 071.41 83.270 75 76.88 076.81 88.670 85 77.22 083.57 95.320 95 77.39 087.35 99.690 105 77.71 092.11 105.20 115 78.58 096.23 110.40 125 79.18 097.81 112.06 135 81.10 102.61 115.49 145 82.74 105.50 120.27 155 83.95 107.75 124.01 165 85.14 110.37 127.45 175 86.13 115.11 130.77 185 87.35 120.25 134.93 195 88.54 135.34 148.03 205 89.39 149.11 141.86 215 89.73 144.07 139.39 225 98.58 141.05 138.67 235 97.17 139.48 143.46 245 96.60 140.48 147.98

Figture 2. Variation of tangent loss with temperature for K1-xNaxNbO3 system at 1 MHz

Table 2. Variation of loss tangentfor K1-xNaxNbO3, with temperature, at 1 MHz frequency Temperature 0C KNbO3 K0.8Na0.2NbO3 K0.6Na0.4NbO3 45 0.033 0.064 0.082 55 0.034 0.051 0.066 65 0.035 0.049 0.065 75 0.035 0.060 0.074 85 0.035 0.068 0.080 95 0.035 0.060 0.073 105 0.036 0.054 0.065 115 0.038 0.049 0.057 125 0.043 0.047 0.054 135 0.046 0.048 0.055 145 0.040 0.050 0.055 155 0.032 0.050 0.055 165 0.032 0.050 0.053 175 0.038 0.050 0.051 185 0.037 0.055 0.056 195 0.037 0.066 0.067 205 0.038 0.082 0.086 215 0.038 0.102 0.130 225 0.040 0.110 0.148 235 0.044 0.114 0.171 245 0.045 0.117 0.195

Figture 3. Variation of electrical conductivity with temperature for K1-xNaxNbO3 system at 1 MHz

Table 4. Variation of electrical conductivity of K1-xNaxNbO3 with temperature, at 1 MHz frequency Temperature 0C KNbO3(X 10-06) K0.8Na0.2NbO3(X10-6) K0.6Na0.4NbO3(X10-6) 45 141 250 372 55 145 198 299 65 147 195 298 75 150 255 364 85 151 265 422 95 152 290 405 105 155 278 380 115 164 264 349 125 188 257 340 135 205 276 350 145 184 290 366 155 182 301 380 165 181 310 378 175 181 322 369 185 181 365 417 195 183 498 528 205 187 677 707

6. Conclusions

Our results shows that the mixing of Na is effective in promoting the densification of the ceramics and can be well sintered and exhibit a dense, pure perovskite structure and this is attributed to the fact of the presence of oxygen vacancies. For most KNN-based solid solutions, the piezoelectric properties are enhanced but with a reduced T_c [19]. Therefore as 20% mixing of Na, transition temperature is 215^oC whereas 40% mixing of Na, transition temperature reduces to 205^oC, which shows that transition temperature from orthorhombic to tetragonal shifts towards lower temperature as we increase the quantity of anti - ferroelectric material NaNbO₃ in the mixed system, K_1-XNa_XNbO₃.

ACKNOWLEDGEMENTS

The authors are thankful to IIT Delhi, NPL Delhi & IIT Roorkee for library facilities and Material Science Research Centre. IIT Madras, for providing laboratory facilities for sample preparation and characterization.