Structural, Ferroelectric, Pyroelectric, and Dielectric Study of Bi 0.5 Na 0.5 TiO 3 Ceramics synthesized with Precursors Obtained by Sol-Gel Method doped with Lanthanum

In this work, the ferroelectric, pyroelectric and dielectric properties of La-doped bismuth sodium titanate ceramics (Bi 0.5 Na 0.5 ) 1−x La x TiO 3 (BNLT) using amounts from 0 to 6.7 at.% La were studied. The precursor powders used to make dense BNLT ceramics were obtained by the sol-gel method using the acetic acid route. All samples were calcined at 700°C for 1hr and sintered at 1150°C for 30 min in an encapsulated crucible to avoid Na and Bi volatilization, obtaining relative densities equal or higher than 94%. The obtained X-ray diffraction patterns typical of the perovskite structure, confirm the incorporation of lanthanum into the lattice, evolving from rhombohedral phase to a mixture of rhombohedral and cubic structure for higher concentrations. Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC) results indicate that the crystallization of precursors powders of BNT takes place between 500 and 700°C. Additionally, the Scanning Electron Microscopy (SEM) micrographs reveal a decrement of grain size from 4.5 to 0.5 μm with increasing La content. The ferroelectric hysteresis curves show that the best ferroelectric properties were obtained for BNT 1.3% La, where the obtained values of remnant polarization and coercive field were P r =29 μC/cm 2 and E c =39 kV/cm respectively. Also, at this concentration, the pyroelectric response shows the higher value, 4 times the pyroelectric signal of the pure BNT.


Introduction
Nowadays, lead-based electro-ceramics as PZT are the most used due they have excellent piezoelectric (d33=590-610 pC/N) and dielectric (r=1700-1790) properties [1,2]. However, considering the high toxicity of lead and its volatility at high temperatures, required for the synthesis of lead-based ceramics, there is a trend towards the development of lead-free ferroelectric systems [3]. Since its discovery by Smolenskii et al., in 1960 [4], the bismuth sodium titanate Na0.5Bi0.5TiO3 (BNT) perovskite system has emerged as a good candidate to replace lead-based systems, due to its high remnant polarization (Pr=38µC/cm 2 ) and Curie temperature (Tc=320°C) [5,6]. However, it is well known that the BNT is considered as a hard ferroelectric, due to its high coercive field values(Ec~73 kV/cm) [7], also presents relatively high leakage currents, hindering the polarization process. Being the latest, limiting characteristics for the use of this system in several applications [3,8].
The modification of the BNT system by ion-doping is another method to improve its piezoelectric and ferroelectric properties [13]. Particularly, the addition of La 3+ in certain concentrations has led to an improvement on pyroelectric and dielectric properties into systems such as PZT (maximum energy density improvement from 799.5 J1 -1 per cycle for 5/65/35 PLZT to 1014 J1 -1 per cycle for 7/65/35 PLZT) [14], BT (from r=900 for undoped BT to r=1450 for 8 at.% La) [15] and BNT (from r=350 for undoped BNT to r=928 for 5 at% La) [16].
It should be noted that the starting powders, used during the synthesis process, have a strong influence on the final properties of the ferroelectric systems. The precursor powders can be obtained from different ways: raw powders (oxides and carbonates), by the co-precipitation method, and sol-gel method, to mention just a few of them. Particularly the precursor powders derived from the sol-gel method have many advantages like a high compositional homogeneity [17] and great dispersibility [18]. Additionally, nanosized powders obtained by the sol-gel method decrease significantly temperatures and times of processing stages of calcination and sintering [19].
In a previous work [20] we explored the BNT system doped with La in a concentration range of 0 to 6% at. However, it was necessary to use an excess of 100% of the Na source in the initial precursors to be able to compensate the volatilization of Na in the calcination and sintering stages, to obtain samples with appropriate ferroelectric characteristics of the doped BNT, this because in the calcination and sintering process did not use the encapsulation stage.
In this work, we added this encapsulation stage in the synthesis process, which it is possible to obtain samples with high densification and good ferroelectric properties, without the need to add excess of Na or Bi in the initial precursors.
The aim of the present work is the study of the effect of Lanthanum doping in low range concentrations to avoid the formation of secondary phases, range from 0 to 6.7 % at. on structural, ferroelectric, pyroelectric, and dielectric behavior of a modified BNT system, obtained using precursor powders derived by the sol-gel method.

Materials and methods
(Bi 0.5 Na 0.5 ) 1−x La x TiO 3 (0, 0.003, 0.007, 0.01, 0.013, 0.033, and 0.067) powders were prepared by a sol-gel process. The precursor solution was prepared by dissolution of an were ground to obtain fine powders with homogenous size after that were uniaxially pressed at 275 MPa for 5 min to obtain pellets with 10.14 mm of diameter and 0.5 mm of thickness followed by sintering at 1150°C for 30 min using encapsulation to minimize loss of volatile elements such as bismuth and sodium.
Thermal behavior and decomposition of metalorganic compounds were obtained by thermogravimetric analysis (TGA) and differential thermal analysis (Mettler Toledo which is related to loss of remnant water, acetone, and carbon dioxide [24]. Finally, even though during the BNT crystallization it must not show weight losses, the TG curve shows a mass loss of 1.2%, which is associated with the volatilization of residuary CO2.

Microstructural Analyses
The density of the sintered samples, with different atomic concentrations of La (0, 0.003, 0.007, 0.01, 0.013, 0.033, and 0.067) was determined by the Archimedes method. The results presented in Table 1 show an increase in density up to a concentration of 1.0 at % of La, which decreases for higher concentrations, but in all cases the relative density is equal or    A similar effect of grain size reduction has also been observed in other ferroelectric systems in addition to BNT, such as in the case of BaTiO3 and PZT [25][26][27]. The effect of La doping in the A sites of the BNT perovskite structure, substituting Na and Bi, leads to a decrease in grain size, and for higher concentrations, it is expected also that it contributes to the appearance of an additional phases [25][26][27].  The samples with a low concentration of La (0≤ x ≤1.3% at) show a good correspondence with the rhombohedral phase. However, for samples with a higher concentration of La (3.3≤

X-ray diffraction results
x ≤6.7 %at ), the diffraction peaks (012) and (110) that are presented in the inset of Fig. 2 show an evolution that allows identifying the presence of a mixture of crystalline phases, rhombohedral (R3c) and cubic (Pm-3m group). Previous studies about La-doping into perovskite structures have suggested that La 3+ replaces preferably A-sites due to their close radii [25,27]. D. Shannon [30] reports the next ionic radius for Na + (rNa=1.39Å, CN=12), La 3+ (rLa=1.36Å, CN=12) and for Bi 3+ only gives ionic radii for coordination 8, Bi 3+ (rBi=1.17 CN=8). In coordination XII Blessington et al. [29] report segregation, as observed in previous work [20] In the evolution of the XRD patterns it was possible to observe a proportional shift in the main diffraction peak located in 2 region (32.1°-33.1°) with the La amount added into the BNT system, which suggests a distortion on the lattice. First, an increase in lattice parameter until La concentration of 1.0% at, and after that, change to a mixture of rhombohedral and cubic structures for higher La concentrations.
To describe that changes into lattice parameters, the diffraction patterns of pure BNT and Ladoped BNT samples were analyzed by profile fitting method using the Fullprof Suite program [33]. Fig. 4 shows the Rietveld refinement for sample of BNT doped with 1.0% at La, in this case, the adjustment was realized considering only the rhombohedral phase. X-ray diffraction of sintered samples to undoped BNT and BNT with low La concentration (≤ 1.3 at. %) were indexed as a single rhombohedral phase belonging R3c group. Nevertheless, for higher La concentration (≥3.3 at.%), it was necessary to use in addition to the rhombohedral phase a cubic phase for the adjustment to the diffraction patterns, indicating the coexistence between the rhombohedral phase (R3c group) and non-ferroelectric cubic phase (Pm-3m group ) [27].  Table 2 shows the adjust parameters such as, rhombohedral and cubic lattice parameter (aR, cR, ac), unit-cell volume (VolR, VolC), phase percentage concentration (% phase), and the goodness of fit (χ 2 ), which should be ≤ 1.5 for a good fit [33]. However, high backgrounds of X-ray diffraction patterns for all compositions contributed to reaching higher χ 2 values.
As observed in Table 2, low La 3+ concentrations ≤ 1.0 at. % into the BNT system led to an increase of the unit-cell volume, for 1.3 at% La is observed a slightly volume decrease, and for higher La concentration, there is observed strong structural changes, appear the cubic phase as the majority crystalline phase for 3.3% of La, and for 6.7% of La, the sample is practically constituted by cubic phase with 98.41 phase concentration. This particular lattice distortion behavior, also observed in Pr 3+ -doped BNT and La 3+ -doped PZT systems [26,34], has been related to generation of micro-strain and cation-vacancies respectively. Both mechanisms could happen when vacancies in A-sites are generated in order to neutralize the positive charge into the unit cell and a subsequent distortion into the lattice in order to stabilize the perovskite structure. As previously indicated, the ionic radii of Na 1+ , Bi 3+ , and La 3+ are similar. La can enter isovalently by Bi, and as a donor by Na with an excess of 2+ electrical charge. For very low concentrations of La, it is expected that initially the vacancies of the A site will be compensated, as the concentration of La increases, there will be a competition between substitution for Bi and Na. The substitution by Na will give rise to Na vacancies to compensate for the electrical charge excess of La. These will  Hence, higher La-concentrations could finally, lead to an equilibrium into A(Bi 3+ /Na + )-site substitution therefore a decrease in cation-vacancy concentration. Additionally, a constant shrinkage of a and c lattice parameters and unit-cell volume are related with a continuous evolution to higher cell symmetry for the BNT system [27,35] therefore a continuous transition from rhombohedral to cubic phase.

Raman Characterization
The Raman spectra at room temperature using an excitation line of argon ion laser at 488.0 nm for the set of sintered samples are showed in Fig. 5 with oxygen vacancy concentration [37].
The plot was normalized for the highest intense band corresponding to the second main band.
According to XRD results at room temperature, the BNT system belongs to a rhombohedral structure (space group R3c) which is observed until 1.3% of La, for higher La concentrations the cubic phase appears and also increase its phase concentration. Figure 5B shows 12 individual Raman modes obtained by deconvoluting of fitted Raman spectra within the measured region (100-1000 cm -1 ) using a Gaussian-Lorentzian function. The Raman active phonon modes expected for a rhombohedral R3c phase with A-site disorder (e.g. Bi/Na sites into BNT system) are 13, 4A1+9E, actives also in IR [29,[38][39][40][41][42][43]. On the other hand, the cubic paraelectric phase (Pm3m) has four optical modes 3F1u+1F2u where only 3 modes with F1u symmetry are IR active and F2u is a silent mode, in principle those modes are not Raman actives modes [38,39], nevertheless, the distortions in the lattice, impurities and vacancies can give place to Raman bands in the cubic phase.   Oxygen Vacancies

A)
broadening, which can be attributed to the coexistence of the rhombohedral and cubic phases, as well as disorder in the A sites, due to the random substitution of La in the Bi/Na sites [41]. Finally, the highest frequency band consists of three individual modes (at 784, 810, and 860 cm -1 ), which correspond to oxygen vacancies where an increase in its intensity is related with a higher vacancy concentration. The pure BNT show these bands due to losses of high volatile elements such as bismuth oxide [37]. However, the intensity of this band diminished when La was added, especially with large La concentration (≥ 3.3 at. %). This behavior indicate that oxygen vacancies are reduced with the introduction of La. It could be produced for vacancies in Bi 3+ -sites created during the high temperature steps, but the introduction of isovalent La 3+ ion, in this range of concentration, reduce the generation of oxygen vacancies.

Dielectric characterization
The temperature dependence of losses and permittivity in samples of (Bi0.5Na0.5)1-xLaxTiO3 system varying La-concentration of x= 0, 0.003, 0.007, 0.01, 0.013, 0.033, and 0.067 at four different frequencies (1kHz, 10kHz, 100kHz, and 1 MHz) are sown in Fig 7. The dielectric function shows small dispersion with frequency. Only two main peaks can be appreciated within temperature range from 25°C to 450°C. They are located around at 220°C and 350°C for undoped BNT which are belonging to a ferroelectric-antiferroelectric phase transition (TD) and an antiferroelectric-cubic phase transition (Tmax) respectively [9,44]. Tmax increase linearly with increasing the (see Fig. 8) La-concentration, reaching a maximum at 1.3 at% (Tmax≈365°C) and then drops to lower temperatures with higher concentrations (≥3.3 %at). Furthermore, TD, known as depolarization temperature, decreases slightly with La-concentration increment from 220°C for undoped BNT to 207°C for BNT 1.3at%, for sample with 3.3% of La it was not possible to determine TD, because the peak at Tmax was very broad and the maximum for TD was not distinguishable, see Fig.7. That temperature shift of TD towards lower temperature has a relation with increment of ionic radii of Lanthanides [45] where Lanthanum ionic radius belonging to a coordination number 12 (CN) has one of the biggest radii of them therefore, it could be expected to observe changes in that temperature with addition of Lanthanum. Additionally, dielectric curves show broader shape which are related with replacement of A sites (Bi/Na site) which leads to a local heterogeneity in cation distribution promoting a greater diffuse dielectric behavior [9,44,45]. For the highest La-concentration (6.7 at.%), a large TD shift towards low temperatures near 71°C was observed (Fig. 8). It could imply to have an antiferroelectric behavior at close room Temperature due to addition of Lanthanum [44,46,47].  lower La concentration, imply a reduction in electrical conductivity until 1.3% of La, and the interpretation of the reduction of oxygen vacancies is reinforced [48,49].

Hysteresis Loops
The hysteresis loops (P vs E) were measured at room temperature for all compositions. Fig.   9-a shows clearly that ferroelectric properties of BNT system were considerably affected by addition of La. Continuous change of ferroelectric loop behavior from a normal ferroelectric   Performing an analysis of the set of results presented in the BNT system doped with La, we found the following relevant aspects.
The synthesis of the BNT doped with La obtained from powders of the drying of a precursor solution of BNT using the acetic acid route as solvent of the acetates used and adding an encapsulation step of the samples in the sintering step, allowed us to obtain samples with higher relative densities than 94%. Even for the highest La concentration, it was not observed any segregation of the secondary phase. So, it is possible to study the effect of La in modifying the perovskite lattice of BNT and the evolution of its ferroelectric, dielectric, and pyroelectric properties. The introduction of La 3+ in the A sites acts as an iso-valent ion in the case of substitution by Bi 3+ , and as a donor when substituting Na +1 , in the latter case, will lead to the creation of vacancies of Na. The XRD results show a distortion in the lattice, with an increase in the volume of lattice until 1.3%, and for concentrations greater than or equal to 3.3% gives rise to the structural transition from rhombohedral to the cubic crystalline phase, also by the SEM micrographs is observed a reduction in the grain size with increasing In general, for La concentration greater than or equal to 3.3%, the ferroelectric and pyroelectric properties of BNT are degraded due to the promotion of the cubic phase, which increases for higher La concentrations. Nevertheless, the region of La concentration around 6.7% is also important because, implies the possibility to obtain BNT in the cubic stable phase, where its good dielectric properties can be useful in some devices like capacitors.

Conclusions
Stable solutions of pure BNT system and doped with lanthanum using concentrations from 0 to 6.7 at. % were possible to obtain by sol-gel method and the acetic acid route, using of La concentration is the appropriate doping region for applications such as heat flux sensors, and power radiation detectors due to its best obtained pyroelectric response. For higher concentrations, a transition from the rhombohedral to cubic phase is promoted, with the consequent degradation of the ferroelectric properties. Additional studies with concentrations of La around 6.7% are interesting because it is likely to induce a complete transformation to the stable cubic structure whose dielectric properties are of technological interest in capacitors.