1. Introduction

The relaxor ferroelectrics with the perovskite ABO₃ structure are characterized by smeared maximum at the temperature dependence of dielectric constant ε and its frequency dispersion in the region of phase transition[1]. Optimization of physical properties of these materials can be made by simple or complicated doping of the A or B positions of the perovskite structure[2-4]. The BaTiO₃–based ferroelectrics systems attract permanent interest as Pb-free ecologically clean materials[5,6].

An isovalent doping of B sites of the perovskite structure is usually used for tailoring the Curie temperature T_C and lowering the temperatures of the tetragonal-orthorhombic and orthorhombic-rhombohedral phase transitions. For example, doping by Zr⁴⁺ and Sn⁴⁺ ions results in linear decrease of T_C and increase of the temperatures of phase transitions[7-9]. Isovalent doping by ions of similar radius does not influence the charge states and no electrical fields appear. Isovalent doping of B-sites by ions with large (~20-30%) difference of ion radius results in appearance of random elastic fields and relaxor properties[9]. Heterovalent doping of B-sites substantially hinders the ordering processes in the perovskite structure and superstructures may appear.

Heterovalent doping of A-sites in the BaTiO₃ structure may result in both deterioration of ferroelectric order and appearance of relaxor behavior[10], and substantial variation of the temperatures of ferroelectric phase transitions. Substitution of Ba²⁺ by Bi³⁺ practically does not influence T_C till doping level of 10%, however abnormal behavior of dielectric constant is observed at bismuth ion concentration of 2%. Such behavior is characteristic of the ferroelectric phase and characterized by substantial frequency dispersion [10].

Simultaneous doping of both A- and B-sites in the BaTiO₃ structure can be characterized by complicated mechanism of charge compensation and also results in substantial variation of ferroelectric properties: shift of the phase transition temperatures, relaxor behavior at small and low concentrations of doping ions, abnormal behavior of dielectric constant and other physical characteristics. Recent investigations of the xBiScO₃-(1-х)BaTiO₃ solid solutions[11,12] showed that they are characterized by high values of the piezoelectric constants and electromechanical coupling factor due to peculiarities of their phase diagram and existence of morphotropic phase boundary. The relaxor behavior is determined by disordering of heterovalent ions in the structurally equivalent sites of crystal lattice.

At room temperature the BiScO₃ compound has the monoclinic type of lattice symmetry and the question about existence of structural phase transitions still remains open[13]. At present BiScO₃ is actively used as doping compound in various ferroelectric systems: BiScO₃–PbTiO₃ [14], (K_0.5Bi_0.5)TiO₃–BiScO₃–PbTiO₃[15], BiScO₃-(K_0.475 Na_0.475Li_0.05)(Nb_0.95Sb_0.05)O₃[16].

According to[7,8] the xBiScO₃-(1-х)BaTiO₃ solid solutions with perovskite structure are formed for the concentration region 0≤x≤0.4. For 0≤x≤0.05 such solid solutions have tetragonal structure (the P4mm space group) with the degree of tetragonality linearly decreasing from 1.0103 (undoped BaTiO₃) to 1.103 (x=0.5), such behavior is explained by cumulative effect from simultaneous c lattice parameter decrease and a lattice parameter increase. Note that in[8] calculations were made under assumption of equivalent of substitution of Ba and Ti by Bi and Sc ions, respectively, with equal values of the thermal disorder parameters. Coexistence of two phases – tetragonal (space group P4mm) and trigonal (space group R3m) - is observed for 0.05≤x≤0.20. The existence of phase with perovskite-like structure (space group Pm3m) was observed for х>0.2. It was shown that addition of BiScO₃ may influence the ferroelectric properties of the xBiScO₃-(1-х)BaTiO₃ solid solutions [10]: for 0.02≤x≤0.10 substantial variations of the ferroelectric loop shape and remnant polarization were observed. However no data were obtained for lower doping concentrations.

The principal problem of synthesis of Bi-containing oxide compounds using Bi₂O₃ as one of the starting components is its relatively low melting temperature 825℃[17]. Depending on the preparation method the influence of Bi volatility is minimized in different ways. When the samples are prepared by the solid state reactions method, long milling with subsequent anneal at relatively low temperature of 900℃ for the short period of time (2 h.)[18] is used. When the complex oxide compounds are prepared using molten salt synthesis with calcinations in sealed alumna crucibles at 1100℃ for 2 h with subsequent thorough washing with hot deionized water[19]. When the sol-gel method is used for synthesis, the samples are sintered at various temperatures from 1100 to 1150℃, for 1 h in air[20]. When the solid state reactions method is used at higher temperatures (1050-1320℃) and for longer durations (till 20 hours) the samples are prepared in a closed crucible with excess of Bi₂O₃ since more than two-thirds of its excess quantity can be evaporated during sintering[10]. The disadvantage of addition of excess quantities of Bi₂O₃ during sintering is in possible Bi incorporation into BaTiO₃. Such Bi doping of BaTiO₃ induces a decrease in T_c as was shown in[21].

In this work the xBiScO₃-(1-х)BaTiO₃ solid solutions with 0.00 ≤ x ≤0.03 were prepared by solid state reactions method by heat treatment of stoichiometric mixtures of BaTiO_3, Bi₂O₃, and Sc₂O₃ in the temperature interval from 500 till 850℃. The structural, dielectric, and ferroelectric properties of thus obtained bulk ceramics were studied.

2. Experimental

Powder samples of xBiScO₃-(1-х)BaTiO₃, where x = 0, 0.010, 0.015, 0.020, 0.025, 0.030, were prepared by conventional solid state reactions method from reagent grade BaCO₃, TiO₂, Bi₂O₃, and Sc₂O₃, taken in stoichiometric proportions. The undoped BaTiO₃ was synthesized by gradual increase of the sintering temperature from 800℃ to 1100℃ with intermediate grindings. The xBiScO₃-(1-х) BaTiO₃ samples were obtained by addition to BaTiO₃ the stoichiometric amounts of Bi₂O₃ and Sc₂O₃ mixtures. These mixtures were first mixed in ethanol with the approximately equal quantities of barium titanate, then gradually the rest amount of barium titanate was added till the homogeneous mixture of the desired stoichiometry have been obtained. The mixtures were annealed with increase of the sintering temperature from 600℃ to 850℃ with intermediate grindings. Higher annealing temperatures were deliberately avoided due to volatility of Bi₂O₃. Total duration of sintering at the temperature of 850℃ was 46 hours. All anneals were carried out in air.

X-ray diffraction analysis was performed at room temperature for structural analysis and lattice parameters determination using the Shimadzu XRD-7000 diffractometer with monochromatic CuKα radiation (λ=0,15418 нм) for the 2θ interval from 20˚ to 80˚ and scan speed 0.9˚/min. Phase identification was carried out using the JCPDSD-ICDD cards No.00-005-0626, and No.01-075-3569. Structural parameters were calculated using the FullProf Suite (version 2.00) software. In order to validate correctness of utilization of X-ray diffraction analysis for structural characterization of the xBiScO₃-(1-х)BaTiO₃ solid solutions with small doping levels (in our case x was less than 0.030) special calibration measurements have been made on the unannealed mixtures of BaTiO₃ with the corresponding quantities of Bi₂O₃ and Sc₂O₃. The surface topography of the samples was studied using the JEOL JSM-6390LA scanning electron microscope with X-ray dispersive microanalysis. The size distribution of powder particles was measured using the Horiba partica LA-950V2 laser analyzer. For electrical measurements the powders were cold isostatically pressed at 15 MPa and disk-shaped bulk samples with typical dimensions of in diameter and 1÷3 mm in thickness were annealed at 850℃ for 6 h. in air. The density of bulk samples was determined by weighing in liquid. For electrical measurements on bulk pellet samples the sintered pellets were polished to achieve parallel, smooth surfaces, and electrodes were made by sintering of the Ag-containing paste at the temperature of 850℃ for 10 min in air. Dielectric measurements were performed from 30 to 190℃ in custom designed furnace with Actacom AM-3001 LCR meter. Ferroelectric hysteresis loops were measured at room temperature under the action of triangle ac field with frequency 1 Hz and amplitude up to 33 kV/cm. Pulses of the switching electric field were generated by DAC card PCI-6251 with LabVIEW-based software and amplified by high voltage amplifier TREK 20/2℃. The switching current data were recorded by measuring the signal on the serious resistor of 20 kΩ. The switching charge was obtained by digital integration of the current data.

3. Results and Discussion

Figure 1 shows the X-ray diffraction patterns recorded at room temperature. Their analysis permitted to conclude that the solid solutions are formed and all samples have the tetragonal symmetry (the space group P4mm), which correspond to the known data[14,15].

Figure 1. X-ray diffraction patterns of the xBiScO3-(1-х)BaTiO3 samples

The crystal lattice parameters are given in Table accordance with the Vegard law[16], linear variation of lattice parameters is observed for the xBiScO₃-(1-х)BaTiO₃ system with x increasing from zero to 0.03. The question about sensitivity of X-ray analysis and its applicability for correct determination of structural parameters at small doping levels was clarified by precise measurements on the initial mixtures of BaTiO₃ with the corresponding quantities of Bi₂O₃ and Sc₂O₃. The results obtained for the sample with x = 0.01 just after mixing and after annealing are presented in Fig.2(a) and Fig.2(b) respectively.

Table 1. Crystal Structure Parameters for the xBiScO3-(1-x)BaTiO3 Samples xBiScO3-(1-x)BaTiO3 Space group Symmetry Lattice costant, (Å) Volume, Å 3 Bragg R-factor a c x=0.000 P4mm Tetragonal 3.99400 4.03116 64.3052 1.43 x=0.010 3.99690 4.03480 64.4567 1.68 x=0.015 3.99810 4.03580 64.5115 2.80 x=0.020 3.99989 4.03720 64.5917 2.85 x=0.025 4.00079 4.03800 64.6335 2.72 x=0.030 4.00244 4.03871 64.6982 1.52

In the diffraction pattern of Fig.2(a) the peaks from BaTiO₃, Bi₂O₃, Sc₂O₃ are clearly seen. In Fig.2(b) the peaks from Bi₂O₃ and Sc₂O₃ are absent while the peaks corresponding only to the perovskite phase are clearly seen. These data confirm the sufficient sensitivity of our X-ray experimental setup for correct determination of the structural parameters of the xBiScO₃-(1-х)BaTiO₃ system with x below 0.03.

Figure 3 shows the SEM images of the 0.03 BiScO₃- 0.97BaTiO₃ sample surface. Analysis of the backscattered image (Fig.3a) permits to conclude that the sample consists of grains with homogeneous phase compo sition. The secondary electron image (Fig.3b) permits to evaluate the mor- phology of the samples: after thermal treatment the powders with rounded shapes formed bigger agglomerates. Such sample surface morphology is typical of the solid state reactions synthesis method.

Figure 2. X-ray diffraction patterns of the xBiScO3-(1-х)BaTiO3, x=0.01, just after mixing (a) and after annealing (b) of BaTiO3 with the corrsponding quantities of Bi2O3 and Sc2O3

Figure 3. SEM images of the 0.03BiScO3-0.97BaTiO3 sample surface. Backscattered image (a); secondary electron image (b)

Figure 4 shows the results of EDX analysis of the 0.03BiScO₃-0.97BaTiO₃ sample with distributions of individual elements. Spatial coincidence of these distributions, in particular of Bi, gives additional evidence for phase homogeneity of the samples and substantial reduction of influence of Bi volatility by proper choice of heat treatment temperature. Homogeneous distribution of elements over the sample surface was additionally confirmed by recording and comparing the full EDX spectra in more that 20 points over the image.

Figure 4. SEM-image of the 0.03BiScO3-0.97BaTiO3 sample surface (a)with the results of EDX analysis: spatial distribution of the Ba element (b); spatial distribution of the Ti element (c); spatial distribution of the Sc element (d); spatial distribution of the Bi element (e)

Temperature dependences of dielectric constant ε recorded at frequencies of 10 Hz and 100 kHz are shown in Fig.5. The positions of maxima T_max of ε(T) dependences and the maximum values of dielectric constant ε_max almost linearly increase with the content of BiScO₃ and corresponding lattice parameters. Such behavior can be attributed to the increasing lattice disorder with x and probable formation of nanoclusters. Almost no frequency dispersion is observed. The values of ε_max are lower than those observed, for example, in[10]. This fact is the result of combined influence of two factors: lower sintering temperature and average grain size dependence of ε. As was shown in[22] lowering the sintering temperature from 1200℃ to 900℃ results in almost 9 times decrease of ε in the vicinity of T_max, while very sharp dependence of ε on an average grain size d_av is observed[23]. According to[23] the maximum of ε(d_av) dependence corresponds to d_av ≈ 1 μm with ≈2.75 times drop for d_av increasing from 1 to 3 μm.

Figure 5. Temperature dependences of the dielectric constant for the xBiScO3-(1-х)BaTiO3 samples recorded at frequencies of 100 Hz (a) and 100 kHz(b)

The inverse of the dielectric constant at 100 kHz as a function of temperature was investigated to confirm whe- ther or not the behavior of ε(T) dependences above T_max can be described by the Curie–Weiss law[1]. The 100 kHz data were used to minimize possible space charge contribution to the dielectric constant. It was observed that the Curie– Weiss law is obeyed for all compositions under investigation. The Curie constant C_CW varied linearly with x from C_CW = 4.8·104℃ for x=0.00 to C_CW= 1.4·105℃ for x≤0.03. Such values are comparable with other ferroelectric materials with displacive phase transitions[1].The observed agreement with the Curie–Weiss law in this system is characteristic of a proper ferroelectric. Since both dielectric dispersion and deviation from the Curie–Weiss law are characteristic of relaxor ferroelectric materials, our results suggest that at small doping levels the xBiScO₃-(1-х) BaTiO₃ solid solution behaves like a normal ferroelectric.

Figure 6 shows the ferroelectric loops for the xBiScO₃- (1-х)BaTiO₃ 0.00≤x≤0.03 system.

Figure 6. Ferroelectric hysteresis loops for the xBiScO3-(1-х)BaTiO3, x=0÷0.03 samples

The principal characteristics of polarization loops - saturation polarization Ps, remnant polarization Pr, and coercive field Hc, are given in Table 2.

Table 2. Physical characteristics of the xBiScO3-(1-x)BaTiO3 samples. εmax - maximum value of the dielectric constant; Tmax - temperature of the maximum of the ε(T) curve; ρrel - relative density (with respect to theoretical X-ray density); dav - average grain size; Ps - saturation polarization; Pr - remnant polarization; Hc - coercive field Composition, x εmax Tmax ρrel% dav,μm Ps, μC/сm2 Pr, μC/сm2 Hc, kV/cm 100 Hz 100 kHz 100 Hz 100 kHz 0.000 498.41 463.67 124.8 126.1 88.0 1.87 1.32 0.80 6.20 0.010 1449.47 1350.74 124.9 126.6 88.9 1.64 4.87 1.80 2.64 0.015 1569.87 1506.21 125.6 127.4 89.7 1.77 4.10 2.05 5.51 0.020 1793.60 1636.89 126.4 128.3 79.8 1.94 2.81 0.97 2.89 0.025 1929.88 1861.87 126.6 128.9 92.1 1.93 4.09 1.86 4.23 0.030 2886.50 2727.40 128.6 128.8 88.2 1.97 2.48 0.97 5.51

Nonmonotonic dependence on x is observed. Analysis of these data must be made with an account for the concentration dependence of relative density ρrel. For all compositions its values are in the range from 88.0 till 92.1%, except for the sample with x=0.020. Such small density resulted in lower values of all parameters of ferroelectric loop. Maximum values of each parameter are observed for different compositions: saturation polarization is maximum for x = 0.010, remnant polarization is maximum for x = 0.015, while the coercive field is maximum for the same composition. With an account for the fact that the maximum value of squareness ratio Pr/Ps = 0.5 is observed for x= 0.015, one can conclude that this composition is characterized by optimum combination of ferroelectric properties.

4. Conclusions

The structural, dielectric, and ferroelectric properties of bulk ceramic xBiScO₃-(1-х)BaTiO₃ solid solutions with 0.00≤ x ≤ 0.03 were investigated. The samples were prepared by solid state reactions method at maximum heat treatment temperature of 850℃. Even at this low temperature it is possible to obtain stoichiometric solid solutions with tetragonal symmetry. Homogeneous distribution of principal elements, including Bi, over the sample surface was observed. Though the values of dielectric constant linearly increased with the doping level, optimum combination ferro- electric properties was found for x = 0.015.

ACKNOWLEDGEMENTS

This work was supported by the Russian Foundation for basic Research (grant No. 08-02-99062_r_ofi), Program of the fundamental research of the Presidium of RAS No.21, Project of joint research of Ural and Siberian Branches of RAS No.09-S-2-1016, Program of the Department of physical sciences of RAS “Spin phenomena in solid-state structures and spintronics”.