The effects of composition, temperature and hydrostatic pressure on phase transition behaviors in (Pb1-1.5xLax)(Zr0.8Ti0.2)O3 ceramics

In this work, (Pb 1 − 1.5x La x )(Zr 0.80 Ti 0.20 )O 3 (abbreviated as PLZT, x = 0.01, 0.03, 0.04, 0.06, 0.07) ceramics are designed on the base of chemical composition modication and prepared by solid-state reaction. The effect of composition, temperature, and hydrostatic pressure on ferroelectric-antiferroelectric (FE-AFE) phase transition is investigated. It is obtained that phase transition from ferroelectric rhombohedral phase to antiferroelectric tetragonal phase as a function of La 3+ doping content, especially, the PLZT ceramics of x = 0.04, 0.06, and 0.07 are the coexistence of FE-AFE phase. It is also found the FE-AFE phase transition driven by increased temperature in poled PLZT ceramics (x = 0.04, 0.06). Furthermore, static charges density (P r ) of PLZT (x = 0.04, 0.06) are decreased from 29.11 µC/cm 2 and 31.52 µC/cm 2 to 19.76 {\mu }C/cm 2 , 6.45 {\mu }C/cm 2 under 400 MPa hydrostatic pressure due to the pressure-induced FE-AFE phase transition. The depolarization rates are 32.12% and 79.54%, respectively. Meanwhile, the phase diagram of (Pb 1 − 1.5x La x )(Zr 0.80 Ti 0.20 )O 3 ceramics is acquired roughly. These results provide guidance for the engineering application of (Pb 1 − 1.5x La x )(Zr 0.80 Ti 0.20 )O 3 ceramics.

Based on reported studies, modi cation with dopants has been an acknowledged and effective approach to optimize the performance of PZT ceramics [7][8][9][10][11][12]. In particular, compositions near phase boundary have been concentrated on more interest over the past few decades, because of that phase transition of polar state and nonpolar state involving to the generation/release electric polarization have a wide range of applications [7,[13][14][15]. Pb(Zr,Sn,Ti)O 3 , (Pb,Nb)(Zr,Sn,Ti)O 3 , and (Pb,La)(Zr,Sn,Ti)O 3 ceramics obtained through modi cation of PZT ceramics with Sn and La are all promising ferroelectric materials, extensively studied and applied [7,8,16].
Cross-sectional microstructure was observed using a TM 3000 Tabletop Microscope (Hitachi, Tokyo, Japan). The dielectric constant ( ) and tangent loss ( ) were tested by using a LCR meter (Model E4980; Agilent, Palo Alto, CA, USA). Polarization-electric eld (P-E) hysteresis loops and depolarization behaviors under hydrostatic pressure were measured using an aixACCT TF 2000 Analyzer FE measuring by system (aix ACCT Co., Aachen, Germany) with home-made hydrostatic loading apparatus [29].

Structure properties
As shown in Fig. 1, the perovskite structure with second phases PbO 2 (PDF#50-1430) is observed in the XRD patterns of PLZT ceramics. The peak is shifted slightly to higher 2 with the increased La content, indicating decrease of lattice constant and smaller unit cell volume, attributed to oxygen octahedral ℃ ℃ ℃ ℃ ϵ r t a n δ θ distortion causing by the replacement of La 3+ (1.36 Å) for Pb 2+ (1.49 Å) at A-sites. Detailed XRD patterns in 2 plotted in Fig. 1b and Fig. 1c, it can be observed the peak splitting in the (111) re ections in the PLZT ceramics of x = 0.01, 0.03, indicating a predominantly rhombohedral distortion, that is, the nature of FE R phase. With increased La content (x = 0.04, 0.06, 0.07), the peak splitting are observed in the (200) re ections and the (111) re ections are gradually transformed to single peak, indicating the phase switching to tetragonal distortion, that is, the coexistence of FE R phase and AFE T phase [30]. The result reveals that the occurrence of FE R -AFE T phase transition is driven by increased La content.
The cross-sectional SEM images of fresh samples sintered at 1300 °C are shown in Fig. 2. With increased La content, the grain size is decreased signi cantly. In addition, these cross-sections are fractured along intergranular direction under external force when La content x = 0.01, 0.03 and 0.04; whereas crosssections for x = 0.06 and 0.07 are fractured along trans-granular direction under external force.

Dielectric properties
Temperature-dependent relative dielectric constant ( ) and dielectric loss ( ) of PLZT ceramics as a function of temperature at different frequencies are displayed in Fig. 3a

Ferroelectric properties
The P-E hysteresis loops of all as-sintered specimens from 30 to 170 can be observed in Fig. 4 to further con rmed temperature-induced phase transition shown in Fig. 3. As shown in Fig. 4a, b, wellshaped P-E hysteresis loops show that these components are stable FE R independent of temperature, where 170 is less than T C shown in Fig. 3a and b. PLZT ceramics of x = 0.04, 0.06 are transformed from well-shaped P-E loops to double hysteresis loops and then to slim P-E loops, indicating transition of FE phase to AFE phase and then to PE phase [32,33]. Phase transformation from AFE to PE with La content x = 0.07 is exhibited in Fig. 4e. Moreover, the phase transition temperature is decreased from θ ϵ r t a n δ to 90 with increased La content corresponding to Fig. 3f, in which defects produced by the aliovalent substitution and the long range order of ferroelectric domains destroyed are bene cial to the inversion of domain wall [34]. P r and coercive eld E C are decreased remarkably and the loops are much slimmer with further increased temperature and La content [1]. In the Fig. 4f and Fig. 3f, it can be observed excellent temperature stability for P r of samples of x = 0.01, 0.03; while there is a sharp decrease in P r for samples of x = 0.04, 0.06, proving temperature dominates FE-AFE transition. In a way, there is temperature-induced structural phase transition and depolarization behaviors in the PLZT ceramics of x = 0.04 and 0.06.

Depolarization under hydrostatic pressure
PLZT(x = 0.04, 0.06) ceramics with higher P r and near the phase boundary between FE and AFE phase are chosen to study the depolarization under hydrostatic pressure. These samples were polarized at 2.0 kV/mm for 15 min at room temperature in an silicone oil bath, and then placed for 24 h after polarization to obtain stable P r . P-E hysteresis loops and I-E loops of PLZT(x = 0.04, 0.06) ceramics at electric eld of 4 kV/mm and under the hydrostatic pressure increasing from 0 MPa to 400 MPa are shown in Fig. 5. Well-shaped P-E hysteresis loops with the P r decreased from 29.11 µC/cm 2 to 19.76 µC/cm 2 are shown in Fig. 5(a1). It's also observed that the maximum pressure used in the experiment do not reach the critical depolarization pressure. What's more, E C is decreased from 1.09 to 0.94 kV/mm with increased pressure to 400 MPa, indicating that a predominant FE ordering gradually decreases. As shown in Fig. 5(a2), with increased hydrostatic pressure, the current peak gradually slows down and the polarization is gradually released in the form of current under additional eld. Clearly, the P r of sample x = 0.06 is decreased from 31.52 C/cm 2 to 6.45 C/cm 2 , as shown in the Fig. 5(b1). It's exhibited in Fig. 5(b2) that the single current peak is transformed to double current peak with the transition from FE R phase to AFE T phase under the effect of hydrostatic pressure, which is similar to temperature-induced phase transition shown in Fig. 3 and Fig. 4. According to soft mode theory, hydrostatic pressure with spherical symmetry increases the interactions between adjacent cations and anions more rapidly than it increases long-range Coulomb forces, increasing the AFE phase stability, and releasing stored charges in a very short period of time [30].
To further evaluate pressure-induced depolarization behaviors, the remnant polarization of poled PLZT (x = 0.04, 0.06) are shown in Fig. 6(a). It can be observed that the P r of x = 0.04 is decreased steadily under hydrostatic pressure, and the P r of x = 0.06 is decreased rapidly after hydrostatic pressure of 250 MPa.
Based on the above results, a simple phase diagram of (Pb 1 − 1.5x La x )(Zr 0.8 Ti 0.2 )O 3 ceramics is proposed.
The phase diagram of PLZT ceramics for x = 0.01, 0.03, 0.04, 0.06, 0.07 are summarized in Fig. 6(b), determined by the temperature of T C corresponding to the maximum values of dielectric constant and dielectric loss in the Fig. 3