Structure and Properties of Nanocrystalline Tetragonal BaTiO3 Prepared by Combustion Solid State Synthesis

Barium titanate (BaTiO3) attracts high scientific and technological attention due to good dielectric and electromechanical properties. Although BaTiO3 is one of the most frequently investigated ferroelectric materials, the need for finding new and/or improved synthesis methods of this material still exists. In this paper, a novel, mild synthesis route for producing tetragonal BaTiO3 from barium nitrate and Ti-oxalate precursor is presented. Morphology of the prepared and subsequently sintered BaTiO3 was determined by SEM. Particle size distribution of the as prepared powder was monitored by the laser diffraction. The phase composition, structure and lattice dynamics were investigated by XRD and Raman spectroscopy. Finally, dielectric parameters were determined in the temperature range from 30 to 180 C, and within a variety of frequencies. Curie temperature was detected at 130 C.


Introduction
Materials based on perovskite structure have found a variety of applications in electronics due to their ferroelectric properties [1,2]. Among them, one of the most investigated is barium titanate (BaTiO 3 ). This ferroelastic and ferroelectric ceramic material has been used for multilayer ceramic capacitors (MLCC), positive temperature coefficient thermistors, and electro-optic devices, due to its high dielectric constant [3,4]. Furthermore, interest in investigating lead-free ferroelectric ceramics is rising because of environmental concern. Here BaTiO 3 finds its opportunity [5].
With an increase of temperature, BaTiO 3 undergoes a few phase transitions: rhombohedral→orthorhombic→tetragonal→cubic [6]. Some of the properties of cubic and tetragonal BaTiO 3 are diametrically different, since cubic BaTiO 3 is paraelectric, while tetragonal BaTiO 3 is ferroelectric.
Till today, various techniques have been used for the preparation of tetragonal BaTiO 3 , sonochemical synthesis, sol-gel, hydrothermal, solvothermal, chemical coprecipitation, mechanochemical, supercritical fluid, etc [7,8]. Solid-state synthesis, followed by calcination at above 1000 °C, presents one of the most cost-effective methods for the preparation of BaTiO 3 at mass scale. However, the reactions at such high temperatures are almost impossible to control and the obtained powders often have large particle size and wide particle size distribution. The literature data indicate that solid-state synthesis from BaCO 3 and TiO 2 was used the most frequently [9][10][11]. The pure BaTiO 3 was dominantly produced by this reaction, but with irregular structure and the presence of large agglomerates. Microstructure could be slightly improved by using mechanical treatment, but problem of agglomeration remains along with increasing microstrain [12]. Contrarily, as the particle size decreases to the nano-level aggregation becomes a significant drawback. It has been shown that reaction for the fabrication of the barium titanate from Ba(NO 3 ) 2 and TiO 2 is promising, because the reaction occurs through liquid-solid phase reaction, which decreases requested temperature [13]. Furthermore, it was demonstrated that the presence of reducing agent in system Ba(NO 3 ) 2 -TiO 2 lowers energy consumption during preparation [2]. On the other hand, the synthesis of BaTiO 3 from oxalates often includes a complicated procedure with dissolving either barium oxalate or titanium oxalate in water or alcohols and subsequent precipitation under low pH, calcinations, etc [14][15][16]. So, the search for an improved method for the fabrication of BaTiO 3 still exists. Generally, when considering techniques to synthesize barium titanate, ideally they should require minimal steps and mild conditions.
In this paper, we sought to reveal a new approach of producing nanocrystalline tetragonal barium titanate powder based on combustion method. The synthesized [Ti 2 O 3 (H 2 O) 2 ](C 2 O 4 )·H 2 O and the commercial barium nitrate was used to develop an alternative low-temperature method for the preparation of crystalline BaTiO 3 powders at low cost. Unlike other titanium compounds, such as lactates, the oxalate complex has optimal content of carbon, enabling fast and easy reaction with barium nitrate at lower temperatures. The comprehensive characterization, including investigation of dielectric properties, of thus prepared powder was performed.

Materials and Experimental Procedures
Synthesis procedure was started by the preparation of Ti precursor. Titanium (IV) oxalate [Ti 2 O 3 (H 2 O) 2 ](C 2 O 4 )·H 2 O was prepared by the procedure described by Boudaren et al. in [17]. Mixture of 1 g (3.49 mmol) of synthesized titanium (IV) oxalate and 0.46 g (1.76 mmol) of commercial p.a. barium nitrate was pulverized in an agate mortar and wetted with methanol to make a homogenous paste. The paste was burned on a Bunsen burner for a 30 min at around 650 °C (measured within the reaction vessel with infrared thermometer). Produced solid was further thermally processed in an electric furnace for 2 h at 700 °C.
XRD patterns of Ti precursor and synthesized sample, were collected using a Rigaku SmartLab automated powder X-ray diffractometer with Cu Kα 1,2 (λ = 1.54059 Å) radiation (U = 40 kV, I = 30 mA) equipped with D/teX Ultra 250 stripped 1D detector in the standard mode. The diffraction angle range was 5-75 °2θ for Ti precursor, while for the synthesized BaTiO 3 it was 10-90 °2θ, with a step of 0.01° at a scan speed of 2 °/min.
Morphology of prepared sample was investigated by SEM. SEM images of powder and sintered sample were taken with a JEOL JSM-6610LV scanning electron microscope. The powder and crushed sintered sample were coated with gold to minimize charging.
Particle size distribution was examined by the laser diffraction on Mastersizer 2000 Malvern Instruments Ltd. The used instrument covers the particle size range of 0.02-2000 μm. For experiment's purpose the powder was dispersed in distilled water. Particle size distribution of obtained dispersion was recorded as it is. Dispersion was subjected to lowintensity ultrasound irradiation (US), at a frequency of 40 kHz and power of 50 W for 5 minutes, and distribution was measured again.
The powder was compacted at 300 MPa using a uniaxial double action pressing process with an 8 mm diameter tool, hydraulic press RING, P-14, VEB THURINGER. Dilatometric curve was obtained by SETSYS Evolution TMA, Setaram Instrumentation, in temperature range from 25 to 1200 o C, with heating rate of 10 o C·min -1 , and dwell time at the highest temperature for 2h. Measurement was performed in air atmosphere.
Raman spectra were collected with a XploRA Raman spectrometer Horiba Jobin Yvon. The system uses a laser at 532 nm, with power of 0.25 MW at the sample. The spectrum was recorded in the range of 100-1400 cm -1 , using the spectrometer equipped with a 2400 lines/mm grating.
Electrical characteristics were measured using Agilent 4284A precision LCR meter in frequency range from 0.2 kHz to 1 MHz. Measurements were performed in temperature interval from 30 to 180 o C, in temperature chamber Heraeus HEP2, which is fully automated. Prior to electrical measurements, silver paste was applied on flat surfaces of specimens.

Results and Discussion
The obtained XRD results were analyzed by the Rietveld method to gain deeper insight into the structural and microstructural parameters. The fundamental parameters approach [18] was implemented in PDXL2 Rigaku software.
The powder XRD patterns for synthesized precursor and BaTiO 3 containing powder are shown in Fig It is important to notice that the X-ray diffraction pattern (Fig. 1b) shows broad peaks indicating the ultrafine nature and small crystallite size of the BaTiO 3 . It should be additionally emphasized that synthesized barium titanate has average crystallite size 36 nm and tetragonal structure, based on data obtained from Rietveld refinement, as presented in Table I. This is even more important having in mind that grain size effects have significant influence on phase transition in BaTiO 3 . It has been shown, that various causes (internal stresses, absence of long-range cooperative interaction, etc.) can lead to transition tetragonal BaTiO 3 into cubic with decreasing crystallite size [20]. The critical size, where the grain size effect occures is in the range 20 to 100 nm, and stronglly depends on preparation method [20]. Among other methods, mechanicall activation can be used for obtaining nanocrystalline BaTiO 3 with tetragonal symetry. But, occurrence of the microstrain higher than 0.5 % and the unstable phonon states diminishes possibilities of use of the mechanical activation for production of the tetragonal, nano-sized BaTiO 3 [21].
The particle size distribution of the synthesized powder (Fig. 2a) shows the polimodal distribution. The small particles (less than 1µm) were associated in agglomerates with average size around 22 µm. Agglomerates bigger than 100 µm were also noticed. This association is the consequence of heating during the synthesis of BaTiO 3 . After short treatment (5 min) in an ultrasound bath (Fig. 2b), distribution was shifted to the left. Mild US irradiation decreases all characteristic values (d(0,1), d(0,5) and d(0,9)), breaks down particles aggregations and slightly reduces width of the distribution. A measured value of the span, which is below 3 (2.931 and 2.837, before and after US, respectively) indicates relatively narrow distribution. 10    The micrograph of the synthesized powder that contains BaTiO 3 is presented in Fig.  3. Two different kinds of particles are noticed. Larger particles are of polygonal shape, approximately 2-3 µm in length, while smaller ones are spherical. The larger correspond to BaCO 3 , as confirmed with XRD results. The spherical particles are ranged from 200 to 500 nm in diameter. They are composed from BaTiO 3 and a smaller amount of TiO 2 particles, which is in good agreement with XRD analysis. Finer grains have a tendency to associate in aggregation. Heat treatment at 700 o C has led to an increase in the particle size. Bigger agglomerates are hard and cannot be crushed in the ultrasound bath, which was shown by analyzing particle size distribution (Fig. 2.). Sintering of pressed sample was performed in high-resolution TMA. Shrinkage curve of the specimen with applied temperature program in the dilatometer is presented in Fig. 4a. With increasing temperature, thermal expansion can be clearly noticed through the nonisothermal stadium. Shrinking occurs slightly before beginning of the isothermal heating regime. The onset of sintering is detected at around 1120 o C. Total detected shrinkage of the sample is about 11 %. The extensive look of the dilatometric curve, in the isothermal regime, indicates that two processes occur simultaneously. It can be noticed that the sintering and the recrystallization take place at the same time. The differential curve, presented in the Fig. 4b, shows more clearly temperatures of processes that are happening during heating. Phase transition from the tetragonal to the cubic phase of BaTiO 3 occurs approximately at 120 o C. This transition is typical for the BaTiO 3 [22][23][24]. The second phase transition is detected in temperature region from 760 to 950 o C. This event originates from the anatase to the rutile phase transformation. It is known that this phase transition undergoes gradually in the temperature range 500-1200 o C, and is strongly related to size and surface effects of the titanium dioxide [25][26][27][28][29]. Besides, phase transition of the BaCO 3 is also possible in this region [29]. It should be mentioned that the BaCO 3 is the most common secondary phase formed during the BaTiO 3 powder preparation by any synthesis route [13]. Phase transition of the α-BaCO 3 into the β-BaCO 3 occurs at 800 o C with an increase in molar volume by 2.7 %. Taking into account the results obtained from XRD, both processes can be responsible for these changes in the region marked with II in the Fig. 4b. The highest shrinkage rate occurs at 1200 o C, at the moment when isothermal sintering starts. Based on available literature data, maximum shrinkage rate, for the un-doped barium titanate, occurs above 1250 o C [30,31]. The presence of the additives can shift the temperature of the maximum shrinkage rate (T MSR ) to the lower [31]. In our case, the shifting of T MSR can be explained by the presence of carbonates. SEM micrograph of the sintered sample is presented in Fig. 5. It is noticeable that the sample is in the medium sintering stage. High open porosity is detected along with randomized closed pores with the irregular shape of edges. Well-sintered areas are also presented, while the largest share of particles retains their individuality. The sintered sample reached 70 % of TD, calculated by measuring mass and dimensions of the sintered specimen. Raman spectrum of the sintered sample was presented in Fig. 6. Appearance of the sharp and narrow picks points out long-range ordered crystal structure. By symmetry, tetragonal BaTiO 3 has 10 Raman active modes. Dominant characteristic of tetragonal structure in the Raman spectra of BaTiO 3 are modes at 306 cm -1 (E(LO + TO), B 1 ) and 716 cm -1 (E(LO), A1(LO)) [32][33][34]. These characteristic modes are detected in Raman spectra of the sintered sample (Fig. 6a). Besides them, other reflections, that originate from the tetragonal phase of barium titanate (146, 260, 513 cm -1 ), were present. The negative intensity peak at 180 cm -1 , marked with red arrow in Fig. 6a, is characteristic of powder samples with sub-micron crystallite size [35]. This is the result of the interference between Raman-scattered radiations from two different modes, as it was proposed by Frey and Payne [19]. The modes of the anatase [36] were also present in the spectra, along with weak modes of intermediary phases BaTi 2 O 5 and Ba 2 TiO 4 [37]. The wide and asymmetric mode in the range of 450 to 670 cm -1 was additionally fitted in order to determine the exact positions of four overleaped modes (Fig. 6b). Deconvolution was performed in the sum of separated peaks, using PeakFit™ software [38].
Synthesis of barium titanate from Ba(NO 3 ) 2 and TiO 2 was followed by the formation of a notable amount of the BaCO 3 [2] . It is known that the reaction between BaCO 3 and titanium dioxide goes through several steps involving BaTi 2 O 5 , Ba 2 TiO 4 , and other barium titanate intermediary compounds. This reaction occurs at the surface of the BaCO 3 grains and the dominant driving force for it is the diffusion process [39]. The existence of intermediate barium titanate phases and anatase in Raman spectra of sintered sample indicates that reaction isn't completed. Modes of the BaCO 3 aren't detected. This is because Raman provides more local images giving information about the thin surface layer of the sample. The BaCO 3 can be located inside of the aggregates covered by one of the intermediate phases, and hence cannot be detected by Raman spectroscopy. The real part of complex dielectric permittivity measured at different temperatures is presenred in Fig. 7a. The main characteristic of these graphics is that ε r decreases at lower frequencies and remains almost constant at higher frequencies. This behavior is common for many dielectric materials [40,41]. Having in mind that the permittivity is a parameter connected with the polarization and that with higher applied frequencies less types of the polarization can be accomplished, reduction in ε r with f is clear [23,42].
Drastic decrease vs. frequency was measured for the dissipation factor (Fig. 7b). The diagram clearly exhibits two regions. The first one can be noticed below 1 kHz where the slop is the steepest, while the second region occurs between 1 kHz and 1 MHz, where the loss tangent remains constant with increasing frequency. The loss tangent takes values from 0.03 to 0.01 for observed temperature range. Temperature dependence of ε r at four selected frequencies is presented in Fig. 8a. Sharp pick at 130 o C occurs for all frequencies. This pick is related with the tetragonal (ferroelectric)-cubic (paraelectric) phase transition within the BaTiO 3 . Obtained Curie temperature is very close to the theoretical value for BaTiO 3 [43][44][45]. It has been shown that the presence of defects and microstrain within the lattice, leads to an increase of free energy inside, so less energy is necessary for phase transition and T c shifts toward lower [46]. Appearance of T c at 130 o C implies a highly ordered crystal structure.
Relatively low dielectric permittivity can be attributed to the low achieved density, the presence of open porosity, inhomogeneous morphology and non-uniform grains distribution. Furthermore, the presence of secondary phases can be responsible for a decrease in dielectric constant [47].
In addition, Curie-Weiss low (Eq. 1) is applied to the results obtained at 500 kHz to calculate the dielectric parameters such as the Curie constant (C) and the Curie-Weiss temperature (T 0 ).
The C and T 0 were determined from the linear extrapolation of reciprocal dependence of the dielectric permittivity versus temperature (Fig. 8b), above Curie temperature. The obtained values are T 0 = 87 o C and C = 3.329·10 4 K. Curie-Weiss constant is reflection of the microstructure. Small grained, dense structure shows higher C values, while porous one mostly displays smaller C. Fine-grained barium titanate ceramics exhibit C with an order of magnitude 10 5 [43]. The Curie-Weiss constant for our sample has value for one order of magnitude smaller, indicating the presence of porous microstructure, which is in good correlation with the SEM micrograph.

Conclusion
In this paper the new approach for the preparation of tetragonal, nanocrystalline BaTiO 3 , was shown. Obtaining tetragonal barium titanate as a major phase at temperature significantly below 1000 o C by using this method was achieved. Sintering temperature was decreased, comparing to other synthesis methods. Raman measurement showed that sintered sample still has small amount of the secondary phases (BaTi 2 O 5 , Ba 2 TiO 4 , TiO 2 ) indicating that reaction hasn't completed. Dielectric parameters confirm significant influence of density on ε r and loss tangent. Low achieved density suggests that the sintering temperature should be increased or dwell time prolonged, toward improving dielectric parameters. Also, ordered crystal structure was demonstrated by Raman and dielectric measurement. It was proven that prepared BaTiO 3 follow Curie-Weiss low and characteristic parameters were determined.
Although obtained material does not possess excellent properties, obtained results and considerable improved experimental conditions -decreased temperature of reaction, adequate precursor, etc. may affect further development in the field, as well as the rational design of new barium titanate with desired properties.