XRD Phase analysis were conducted for all sintered specimens of different concentrations, as shown in Figure 1. For unmodified BNKT–ST (x,y = 0.00), the coexistent tetragonal-rhombohedral phases, shown by splitting of peaks at ~40° and ~46°, related to rhombohedral of (111)/(1ī1) peaks and for tetragonal symmetry of (002)/(200) peaks, respectively [18,19]. For both La-Nb and Li-Nb co-doped BNKT–ST piezoceramics, as the doping concentration raised, the splitting of peaks in XRD at around ~40° and ~46° fused into single pseudocubic phase reflection of (111) and (200) peaks, respectively. In case of La-Nb co-doping, the structural phase transition was observed at x,y = 0.010, while Li-Nb co-doped system showed phase transition at x,y = 0.020. This kind of phenomenon was observed previously in BNKT-based piezoceramics modified with Hf [10], Zr [11] and Nb [13].
The comparison of high voltage polarization induced hysteresis (P–E) and related composition dependence of remnant polarization (Pr), coercive field (Ec) and maximum polarization (Pm) for La,Nb and Li, Nb co-modified BNKT-ST piezoceramics at ambient temperature (RT) were presented in Fig. 2 and Fig. 3 respectively. For unmodified composition (x,y = 0.00), well-saturated hysteresis with large Pr ~26 μC/cm2 and Ec ~36 kV/cm values confirm the of E-field dependent irreversible evolution to normal-FE phase from nonergodic relaxor (NR) nano-domain state, good agreement with the relatively abrupt peak of current density (J) displayed in the J–E graphs at E = Ec = 3.1 kV/mm. The x,y = 0.00 composition due to its small local random fields between polar nano-regions (PNRs) is predominantly NR state and converted rather easily from the randomly oriented frozen nano-domain state into a ordered state of stable long-range FE (NR-FE phase transition) by applying relatively low critical E-fields slightly less than Ec and saturated at ~50 kV/cm.
In case of La, Nb co-modification, 1 mol% of La and Nb co-modification partially replace the cations of A and B-site of the host BNKT–ST piezoceramics respectively, due to their comparable ionic sizes. La+3 will act as A-site donor [20] and Nb+5 [13] as B-site donor, and likely to produce A-site vacancies. The produced vacancies may enhance the local random field and break down the long-range FE order by enhancing local random fields along with four current peaks. Consequently, the reversible transition from FE to ergodic relaxor (ER) state occurs evident by the pinching of corresponding P–E loop [21–23]. On the other hand, 1 mol % of Li, Nb co-substitution results in increase of Pr and decrease in Ec with improved ferroelectricity. The role of Nb as a B-site donor may produce the A-site vacancies similar to La, Nb doped ceramic. However Li+ is expected to enter the A-site (due to its comparable ionic size to Bi, Na, K and Sr) may generate oxygen-vacancies which can be responsible for stabilization of FE order by domain pinning and delay the reversible FE-ER phase transition up to 2 mol% of Li,Nb, as confirmed from relatively sharp corresponding J–E curves. The comparison shows that 1mol % Li (acceptor), Nb (donor) doping results in improvement of FE state while 1mol% La (donor), Nb (donor) doping significantly disturb the FE order. Furthermore, the compositions (Li, Nb 0.020 and 0.030), shows almost similar pinching, indicating the broad boundary of strain-generating FE-ER phase transition which is interesting from practical applications point of view. On the contrary, a narrow compositional FE-ER phase boundary was observed for La, Nb 0.010 composition. From these observations, it is suggested that A and B-site concurrent donor doping is more effective to destabilize the long-range FE order rather than the simultaneous A-site and B-site acceptor-donor dopent.
A noticeable difference between the two modifier elements La,Nb and Li,Nb was also observed in their effects on the bipolar strain (S–E) loops. Figure 4 displays the strain versus electric field hysteresis curves of La,Nb- and Li,Nb-modified BNKT–ST piezoceramics measured at room temperature. An un-modified BNKT–ST demonstrated a typically butterfly-shaped curve, which has been detected in ferroelectrics [24,25]. The definite value of negative strain (Sneg) at both +Ec and –Ec suggests the existence of FE domains, whose orientations switched when the external applied field is reversed. Similar to polarization P–E loops, 1mol % La, Nb composition drastically affect the FE long-range order evident by vanishing of Sneg. For this composition, the comparable free energy of ER and FE states destabilize FE state under un-biased conditions and the inter-conversion between induced FE domains and nano-domains occur with a comparatively large electric-field-induced strain response and almost zero Sneg. On the other hand, 1mol.% Li, Nb concurrent acceptor-donor doping maintained its FE order with the enhancement of Sneg and maximum strain (Smax) due to the flexible response of domains re-orientation to the external electric field. The Sneg is symbolized by the difference between lowest strain points and the zero-field strain of the butterfly loop and the Smax corresponds to the strain from zero-field to the maximum electric field (Emax) [21]. From strain measurement data, both Sneg and Smax were calculated and plotted as a function of dopant contents in Fig. 5. With further donor-donor (La,Nb) co-addition, system transform to a typical ER state. On the contrary, acceptor-donor (Li,Nb) co-substitution results in the delay of onset of ER state and broadened the ER-FE mixed phase boundary which is favourable for industrial applications.
Fig. 6 shows the comparative values of energy storage density (W), energy storage efficiency (η) and low energy loss density (Wloss) of both compositions. These are the three important ligands of dielectrics for energy storage applications and determined from P–E loop by using following equations (see Equations 1 and 2 in the Supplementary Files)
It can be realized from Fig. 6 that a relatively high energy storage density of 0.38 J/cm3, lower energy loss density (0.14 J/cm3) along with good efficiency (60%) were observed for La, Nb system at x = 0.020. While for Li, Nb system, the values of energy storage density, loss and efficiency were 0.38 J/cm3, 0.29 J/cm3 and 25% respectively. These results suggest that the La, Nb system showed greater energy density properties. The observed energy density values obtained for La, Nb system are comparable to previous results [26–28]. The higher energy density response could be related to the relatively higher Pmax and small Pr and Ec values.