1. Introduction

The perovskite niobate (ANbO₃) constitutes an interesting structural family, which undergoes several sequences and mechanisms of structural phase transitions. Silver lithium niobate (Ag_1-xLi_xNbO₃) system shows interesting properties[1, 2]. The constituents of this system are silver niobate (AgNbO₃) and lithium niobate (LiNbO₃). Silver niobate is known to exhibit several sequences and mechanisms of structural phase transitions as a function of the temperature[3-5]. Silver niobate AgNbO₃ undergoes a sequence of phase transitions[6-8] observed at the following temperatures:

(i) At 67℃, orthorhombic M₁[FE] → orthorhombic M_2[AFE];

(ii) At 267℃, orthorhombic M₂[AFE] → orthorhombic M_3[AFE];

(iii) At 353℃, orthorhombic M₃[AFE] → orthorhombic O_1[PE];

(iv) At 361℃, orthorhombic O_1[PE] → orthorhombic O_2[PE];

(v) At 387℃, orthorhombic O₂[PE] → tetragonal T[PE];

(vi) At 567℃, tetragonal T[PE] → cubic C[PE].

where M₁, M₂ & M₃ denote the phases with orthorhombic symmetry in rhombic orientation, while O₁ & O₂ are the phases with orthorhombic symmetry in parallel orientation. The parallel orientation means the orthorhombic axes are parallel to the pseudo cubic directions, whereas in the case of rhombic orientation the orthorhombic a- and b-axes are parallel to diagonals contained inside the faces of the same pseudo cubic axes[6]. It was established that M₁ phase exhibit, weak ferroelectric[FE] properties, phase M₂ & M₃ are antiferroelectric[AFE] and phases O₁, O₂, T and C are paraelectric[PE][9]. The temperature evolution of the lattice parameters and the sequence of phase transitions are similar to those observed in sodium niobate (NaNbO₃)[10, 11].

Lithium niobate, LiNbO₃ undergoes the phase transition at 1210 ˚C, from the trigonal[Ferroelectric] to the trigonal[Paraelectric]. Bhatt and Semwal[12] have reported dielectric properties of LiNbO₃. The non- linear acoustical properties of LiNbO₃ have led to the observation of a new physical effect. An acoustical tone burst stores energy with in the crystal that is re emitted at a later time of order of 70 μs. This effect can be characterized as an ‘acoustic memory’. This phenomenon is dependent on frequency and temperature[13]. X-ray investigations of Ag_1-xLi_xNbO₃ solid solution ceramics (0 ≤ x ≤ 0.15) showed that small Li substitution causes a change of symmetry[14]. At room temperature, a phase boundary between orthorhombic and rhombohedral symmetry is observed for x = 0.05; this phase boundary is indicated also by dielectric properties. The Li- substitution, leads to a gradual rise and shift of diffuse ε' (T) maximum which is observed for (AN) as 227℃. Instead of this diffuse maximum, a sharp one at 197 ℃ is already observed for Ag_0.94Li_0.06NbO₃[14].

2. Material and Methods

The final physical and electromechanical properties of ferroelectric material components greatly depend upon the preparation method used. For studies of fundamental properties of materials, large homogeneous single crystals are usually desirable to minimize the effects of surfaces and imperfections. However, single crystals are very expensive and difficult to grow, whereas ceramics have the advantage of being a great deal easier to prepare compare to their single crystal.

2.1. Sample Preparation

The starting materials used for preparing silver lithium niobate Ag_1-xLi_xNbO₃ system were silver oxide (Ag₂O), purity 97% (Qualigens Fine Chemicals); lithium carbonate (Li₂CO₃), purity 99% (Qualigens Fine Chemicals) and niobium pentaoxide (Nb₂O₅), purity 99.9% (Loba Chemie). Similar to preparation of silver niobate[7], silver sodium niobate[15,16], holmium doped lithium niobate[17] and silver potassium niobate[18,19], the samples of silver lithium niobate were prepared by solid-state reaction method. The starting materials were initially dried at 200ºC for 2h to remove the absorbed moisture and then quantities of the reagent required to prepare silver lithium niobate were weighed in stoichiometric proportion. All the samples were prepared according to following reaction:

(1-x) Ag₂O + x Li₂CO₃ + Nb₂O₅ → 2 Ag_1-xLi_xNbO₃ + x CO₂

Each composition was manually dry ground into fine powder for two and half hours and then wet mixed using reagent methyl alcohol and pestle for two hrs. The mixture was calcined in a silica crucible, in air, at 650 ºC for 2h, at 850 ºC for 2h and at 950 ºC for 1 ½ h.

The pre-sintered mixture was ground again for 2h and pressed into pellets of 11 mm diameter applying the pressure of 3 tons. All pellets of a composition were placed on a silica crucible and sintered in air at 1000 ºC for 14h. All the samples were taken out from furnace when they cooled to room temperature. The sintered pellets of all compositions were gold polished for characterization (SEM) and electroded in metal-insulator-metal (MIM) configuration using air-drying silver paste for dielectric measurements. Prepared samples were characterized using XRD and SEM.

2.2. Characterization

X-ray diffraction (XRD) patterns of all the prepared samples at room temperature have been obtained on a D-8 ANCE X-ray diffractometer (Bruker) using Cu-K_α radiation of 1.540598Å wavelength. The instrument was well calibrated with the silicon standard sample. Peak indexing was done by using JCPDS data. From the observed diffraction pattern, lattice spacing d was determined which was used to determine the lattice parameters. Surface topography of the samples was studied by scanning electron micrographs (SEM) using LEO-440 scanning electron microscope.

3. Results and Discussion

The prepared ceramic samples of Ag_1-xLi_xNbO₃ for x = 0, 0.3, 0.5 and 0.7 were analyzed for phase and crystallinity by powder X-ray diffraction technique. X-ray diffraction pattern for all the samples were recorded at room temperature. The diffraction data were collected in the 2θ ranges of 10–70º with a scan step of 0.02º. X-ray diffraction patterns of obtained for all prepared samples is shown in Figure. 1. From the observed diffraction patterns the lattice spacing was determined which was used to determining the unitcell parameters. The unit-cell parameters were determined using the ‘WinPLOTR’ computer software (2005 version), which includes CRYSFIRE and FULLPROF software. From X-ray patterns, it was found that at room temperature all the compositions are in orthorhombic phase. Lattice parameters (Table 1) also reveal the structures of present systems.

Figure 1. Powder X-ray diffraction pattern of Ag1-xLixNbO3 for different x values.

Table 1. Lattice parameters and average grain sizes of Ag1-xLixNbO3 for different x values at room temperature S. no. Compositions Lattice Parameters Grain Size (SEM) (µm) a (Å) b (Å) c (Å) 1. AgNbO3 5.5972 5.5423 3.9100 5.127 2. Ag0.7 Li 0.3 NbO3 8.8653 7.0365 5.3847 3.873 3. Ag0.5 Li 0.5 NbO3 8.7997 8.3403 7.2954 3.711 4. Ag0.3 Li 0.7 NbO3 7.4974 6.6379 5.1534 2.573

Surface morphology and grain sizes of the prepared samples were studied by SEM technique. Scanning Electron Micrographs for Ag_1-xLi_xNbO₃ (x = 0, 0.3, 0.5 and 0.7) are shown in Figs 2(a) - (d) respectively. The grains of different sizes are randomly distributed with large inter-granular porosity. The grains of different sizes with orthorhombic shape grow in the prepared samples. Smaller grains (~ 1 to 2 µm) occupy the space between the bigger grains (~ 4 to 8 µm), and thus reducing the porosity. Variation of average grain size with different composition of Ag_1-xLi_xNbO₃ is shown in Table 1. Average grain size of AgNbO₃ is found 5.127 µm, on doping of Li grain size decreased to 3.873 µm, on further increasing the doping percentage of Li, grain size further decreased. The decrease in grain sizes is due to the smaller size of lithium.

4. Conclusions

In the present study, formation and characterization of lithium doped silver niobate perovskite system, Ag_1-xLi_xNbO₃ (x = 0, 0.3, 0.5, 0.7) have been investigated. Ceramic samples were prepared by solid state reaction and sintering method. The sintered samples were investigated for phase and crystallinity by powder X-ray diffraction. From X-ray diffraction patterns and SEM studies, it has been observed that at room temperature all the compositions are in orthorhombic phase. It was also found that average grain size of Ag_1-xLi_xNbO₃ decrease on increasing the value x, i.e., doping of Li. As Li is smaller than Ag, it causes a decrease in grain size on increasing concentration of lithium.

Figure 2. Scanning Electron Micrographs of AgNbO3 (a), Ag0.7Li0.3NbO3 (b), Ag0.5Li0.5NbO3 (c) and Ag0.3Li0.7NbO3 (d) samples.

ACKNOWLEDGEMENTS

Authors express hearty thanks to Dr. Rajendra Dobhal, Director, USERC Dehradun, Uttarakhand, Dr. B.S. Semwal, Department of Physics, H.N.B. Garhwal University Srinagar, Uttarakhand, India and Dr. R.P. Pant, National Physical Laboratory, New Delhi, India for their help, support, constructive criticism & encouragement.