Effect of Li Substitution on the Structure and Dielectric Constant of Bi0.5(Na⁠1-xLix)0.5TiO⁠3 (0 ≤ x ≤ 0.20) Solid Solution Series


 Bismuth sodium titanate Bi0.5Na0.5TiO3 (BNT) is a lead-free piezoelectric ceramic material with high Curie temperature. The effect of substitution of the smaller ion Li+ for the larger ion Na+ in Bi0.5(Na⁠1−xLix)0.5TiO⁠3 (0 ≤ x ≤ 0.20) on the structure of BNT is studied using powder X-ray diffraction (XRD) and Raman spectroscopy. Rietveld refinement analysis of the powder XRD patterns showed that all the compositions formed under monoclinic Cc space group, with the lattice parameters showing minor changes above x > 0.08. Raman spectral parameters such as position and intensity of a peak also showed a similar trend in the same Li concentration range with increasing Li content. A corresponding change in the variation of the dielectric constant with increasing Li content is observed suggesting a close correlation between the local structure and properties of the different compositions in the Bi0.5(Na⁠1−xLix)0.5TiO⁠3 solid solution series.


Introduction
Bismuth sodium titanate, Bi 0.5 Na 0.5 TiO 3 (BNT) is a piezoelectric ceramic material with strong ferroelectric properties such as large remnant polarization (P r = 38 µC/cm 2 ) and high Curie temperature (T C = 320 o C) [1][2][3][4]. The main drawbacks of BNT for practical applications are its large coercive eld and high electrical conductivity. Several studies have been reported in the literature on the modi cations of BNT to overcome these drawbacks and to improve its piezoelectric properties [5][6][7][8][9][10]. Most of these studies are centered on the substitution of Bi-or Na-site of the perovskite lattice by other metal ions of comparable size, ionic character, etc. Duke et al studied the improvement of ionic conductivity in A-site lithium doped sodium bismuth titanate. The optimal doping level of 4 at% Li on the Bi site improves the ionic conductivity by one order of magnitude to ~ 7 × 10 − 3 S/cm at 600 °C. Comparison with other oxide-ion conductors indicated that Li-doped BNT materials are promising candidates for intermediate temperature SOFC applications [11]. Gao et al studied the electric eld and composition-dependent behavior of (Na 0.484 Bi 0.456 Ba 0.06 )Ti 0.97 Nb 0.03 O 3 -xLi + (x = 0, 0.005, 0.01, 0.015) ceramics and [12]. The introduction of Li cations enhanced the piezoelectric strain with the highest bipolar strain of 0.47% achieved at x = 0.01 at an applied electric eld of 70 kV/cm, and the corresponding normalized strain ( ) reached up to 671 pm/V. It has been reported that the dielectric and piezoelectric properties are enhanced when Na + is replaced by K + in Bi 0.5 (Na 1 − x K x ) 0.5 TiO 3 solid solution series (BNT-BKT) [13][14]. Lin et al studied Li and K co-substituted Bi 0.5 (Na 1 − x−y K y Li x ) 0.5 TiO 3 solid solution series and observed that a small amount of Li + substitution improved the sintering characteristics and the piezoelectric properties [15]. The properties of Li + and K + co-substituted BNT solid solution series have also been studied by other researchers [16][17][18].
There are few studies reported in the literature on the substitution of Na + by Li + in BNT. Lu et al synthesized Bi 0.5 (Na 1 − x Li x ) 0.5 TiO 3 solid solution series by the ceramic method and observed that the compositions in the range 0 ≤ x ≤ 0.20 have perovskite structure with rhombohedral symmetry [19]. The maximum piezoelectric constant, , and planar electromechanical coupling factor, , are obtained as K p 110 pC/N and 0.18 at x = 0.15, respectively. The un-substituted BNT has a lower piezoelectric coupling constant (80 pC/N) and planar electromechanical coupling factor (0.14). The mechanical quality factor, , was found to decrease with increasing Li + content. Said and Maaoui investigated the dielectric behavior of the Bi 0.5 (Na 1 − x Li x ) 0.5 TiO 3 solid solution series and found relaxor-type behavior with a diffuse phase transition for low Li + substitution in the range 0 ≤ x ≤ 0.20 [20]. The authors explained the relaxor behavior is due to the cation disorder in the A-site of the perovskite lattice.
Detailed structural studies on Bi 0.5 (Na 1 − x Li x ) 0. Structural analysis of the sintered samples was performed by powder X-ray diffraction (XRD) on a PANalytical X'pert PRO powder X-ray diffractometer. Raman spectra were recorded on a Lab RAM HR spectrometer (HORIBA JOBIN YVON, Model No. HR 800) and deconvoluted using the Origin software. The densities of the sintered pellets were measured by the Archimedes method. Dielectric constants of the sintered pellets were measured at room temperature, using an impedance bridge (Model 1608A, General Radio Company) at 1 kHz. The microstructures of the sintered samples were obtained using a scanning  whereas 'b' is slightly increased. The ionic radius of Li + is 0.92 Å and that of Na + is 1.18 Å, for 8-fold coordination [21]. When larger Na + is replaced by the smaller ion Li + , the unit cell volume is decreased (Fig. S1, ESM). The substitution of Li + causes contraction of the unit cell, and hence corresponding changes are seen in the lattice parameters. The difference between the magnitudes of the lattice parameters for x = 0 and 0.2 in the values of 'a' and 'c' is obtained as approximately 0.02 Å. Although the lattice parameter 'b' showed a small increase, the difference is very small (~ 0.006 Å). The small increase in the value of 'b' could be due to the reduced distortion or tilting of the TiO 6 octahedra in the unit cell on substitution of smaller Li + for larger Na + .
It is interesting to note that the rates of decrease of the lattice parameters 'a' and 'c' are different above and below x ≈ 0.08. A large decrease in the values of the lattice parameters is observed between x = 0.08 and 0.16, above which the lattice parameters remain almost constant. Also, c ≈ a 1/3 , for x > 0. 16. Similarly, the lattice parameter 'b' remains almost constant up to x ≈ 0.1 and after this concentration, a small increase is observed up to x = 0.16 and 'b' becomes almost constant again at higher Li + concentrations. Similar changes are observed in the variation of the unit cell volume also (Fig. S1, ESM). Since the changes in the lattice parameters are very small with increasing Li content, it may be concluded that there is no major structural phase transformation as observed in the case of (1-x)BNT-xBKT, where the structure is found to change from monoclinic to tetragonal at higher BKT content [23]. The small change in the variation of the lattice parameters around x ≈ 0.1 may be due to the local structural changes, arising from the inhomogeneous distribution of the substituted Li ions in the lattice for x < 0.1. However, onset of a structural phase transition cannot be ruled out in this compositional region, even though good ts are obtained in the Rietveld re nement analysis, using the monoclinic structure in the entire compositional region studied. The fact that the lattice parameters become almost constant for x > 0.16 and all three lattice parameters show a major change in the compositional region 0.08 ≤ x ≤ 0.16 suggest a probable structural phase transition. If this is true, the compositional region 0.08 ≤ x ≤ 0.16 is likely to belong to a morphotropic phase boundary (MPB) region, as observed in the case of (1-x)BNT-xBKT [23].
Raman spectra of all the studied compositions are recorded and the changes in the Raman spectra as a function of composition in the BNT − BLT series are studied to get more insights in to the local structural changes with Li substitution. Raman spectra of different compositions of the (1-x)BNT-xBLT series, after baseline correction, are shown in Fig. 3. Raman spectra of BNT is similar to that already reported in the literature [24][25][26][27][28][29]. As observed in the case of the BNT-BKT series [24][25], For the rst two peaks within this band, the positions of the peaks remain almost the same for x < 0.08 and x > 0.12, with a drastic increase between x = 0.08 and 0.12. Similar changes are observed in the case of the third peak also, except for a large drop for 0.08 ≤ x ≤ 0.12. Figure 6 shows  (Fig. S3, ESM). The total area and the areas under the rst two peaks are increased whereas the area of the third peak is decreased with increasing BLT concentration (Fig. S4, ESM). Even though the total area increased almost linearly with increasing BLT concentration, the changes in the area of the individual peaks showed different trends below and above x = 0.08. The area of peak 1 is independent of x for x < 0.08 and then increased with x, area of peak 2 increased up to x = 0.8 and then remained almost constant, and area of peak 3 decreased up to x = 0.08 and then remained almost constant at higher x.
Thus, the changes in the parameters of the Raman spectra for the bands in the 200-400 cm − 1 and 400-800 cm − 1 regions suggest minor changes in the Ti-O bond length and some associated changes in the distortion of the TiO 6 octahedra with partial substitution of Li + for Na + in Bi 0.5 Na 0.5 TiO 3 . Results from the XRD studies suggested minor structural changes above x = 0.08 in (1-x)BNT-xBKT and the Raman studies suggest that this is due to the minor changes in the coordination environment at the A-and B-sites in the perovskite lattice. Hence, corresponding changes in the ferroelectric polarization is expected which may lead to variations in the dielectric constant.
The dielectric constant of all the studied compositions of (1-x)BNT-xBLT, measured at 1 kHz, as a function of BLT concentration is shown in Fig. 7. The dielectric constant shows a small increase up to x = 0.08 which is shown in the inset of Fig. 7 and an abrupt change in the slope is observed above x = 0.08. For x ≥ 0.12, the dielectric constant remains almost constant. The maximum dielectric constant observed for the BNT-BLT compositions is 420. However this value is relatively small, when compared to the maximum dielectric constant observed for the BNT-BKT series which is 1800 [23]. For the BNT-BKT system, maximum dielectric constant is observed in the MPB region where the monoclinic and tetragonal phases co-exist. Such a phase transition and MPB region is not observed in the case of the BNT-BLT system. Also, the distortion of the monoclinic structure is relatively larger for the BNT-BKT system due to the larger size of the K + ions. The observed dielectric constants of the different compositions of the BNT-BLT ceramics are relatively low when compared to the previous literature reports [19][20]. Lu et al reported a dielectric constant 800 for the x = 0.20 composition in the BNT−BLT series [19]. This is mainly due to the difference in the synthesis methods, processing conditions, density, sintering temperature and the pressure used for the compaction of pellets. Rietveld re nement analysis of the XRD patterns suggested small structural distortion and local symmetry changes in the crystal lattice. Raman spectroscopy analysis supported the results obtained from the XRD studies. The changes in the Raman parameters with substitution followed the same trend as that observed in the case of the unit cell parameters. Variation of the structural as well as Raman spectral parameters and the dielectric constant showed similar changes with increasing Li concentration, suggesting a close correlation between the structure and properties of the compositions.   Variation of the dielectric constant of (1-x)BNT-xBLT as a function of x; inset showing zoomed graph for 0 ≤ x ≤ 0.1.