Comparison of strain and ferroelectric properties of La,Nb and Li,Nb co-modified BNKT–ST ceramics

A comparison between the influence of concurrent A and B-site acceptor-donor (Li,Nb) and donor-donor (La,Nb) doping in Pb-free Bi 0.5 (Na 0.84 0.16 ) 0.5 on structure, ferroelectric and high voltage induced strain were made. In this work, Lithium was chosen as acceptor and Lanthanum was chosen as donor dopant on A-site and Nb 5+ as a B-site donor. Both modifier impurities promoted a phase evolution from the coexistent of two phase structures (rhombohedral-tetragonal) into a more symmetric single pseudocubic structure. Nonetheless, (La,Nb) donor-donor doping induced the phase transition at 1 mol%, while (Li,Nb) acceptor-donor doping at 2 mol%. Interestingly, acceptor-donor (Li,Nb) co-substitution results in the broadening of ergodic relaxor and ferroelectric (ER-FE) mixed phase boundary which is favourable for multidevice applications. This comparative study show that the A-site vacancies play a substantial role to induce the phase transformation from nonergodic-ergodic relaxor (NR-ER) and the improvement in their high voltage induced strain.


Introduction
The emphasis of piezoelectric study during the last decade was given to expansion of lead-free alternative piezoelectric ceramics owing to the environmental issues and laws enacted on the Pbbased (PZT and its solid solutions) piezoceramics [1][2][3]. Bismuth (Bi 3+ )-based perovskites are the lead-free ceramics which have drawn a huge consideration due to similarity in electronic structure with Pb 2+ . Bismuth sodium titanate (BNT) ceramic has tempted special interest owing to its environmental friendliness, and good piezoelectric performance. Nonetheless, the problematic poling, low resistivity and high recorded coercive field are the downsides of unmodified BNT [2][3][4]. Various elemental alterations in pure BNT have been prepared to improve the desired electromechanical properties [5][6][7][8][9]. Among BNT-based binary systems, BNKT is the highly tempting system with relatively better electromechanical performance and morphotrophic phase boundary (MPB) likewise PZT ceramics.
Previous studies on BNKT-based piezoelectric ceramics found that the cation doping induces the hardening and softening effect which can improve the piezoelectric and ferroelectric properties [10][11][12][13][14][15]. Soft donor doping disrupts the ferroelectric (FE) order by generation of A-site vacancies and increase the dielectric, piezoelectric and electromechanical behavior [16,17]. On the contrary, the hardening effect due to acceptor doping may generate oxygen vacancies and decreasing dielectric constant, loss, and enhancing coercive field and mechanical quality factor [16,17] Then, the mixture of powders was put at ball-milling again for 24h and completely dried in oven.
Uniaxial pressing was used to produce disc-shaped samples (diameter of 10mm) at 98 MPa. These disc samples were annealed at the temperature of 1160°C with the dwell time of 2h.
Samples were coated by paste of silver and put in furnace at 650°C for the dwell time of 0.5h to form permanent electrodes. Poling process was done in bath of silicon oil by applying dc field of 3-4 kV/mm. X-ray diffraction (XRD, X'pert MPD 3040, Philips, The Netherlands) was utilized in this study to characterize crystal structure analysis. Polarization vs. electric field hysteresis (P-E) was analyzed by utilizing ferroelectric tester (Manufactured by Precision LC, Radiant Technologies Inc., Albuquerque, NM). Piezo strain property was recorded by utilizing contact-type sensor for displacement measurement (Millitron; Model 140).
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 (P r ), coercive field (E c ) and maximum polarization (P m ) for La,Nb and Li, Nb co-modified BNKT-ST piezoceramics at ambient temperature (RT) were presented in Fig. 2 and