Relationship Between Phase Structure And Electrical Properties Of (Bi,Pb)(Ti,B)O₃ Ferroelectric Relaxors

The crystal symmetry transitions usually occur due to the change of the compositions in perovskite solid solution systems.The morphotropic phase boundary is referred to the boundary in a phase diagram that separating two different macroscopic symmetries.The coexistence of different symmetric structures is usually found near MPB,along with the dramatical changes in electrical properties.Recently,double-site disordered ferroelectric relaxors(Bi,Pb)(Ti,B)O₃(B=Mg²⁺,Ni²⁺,Zn²⁺,Zr⁴⁺,Hf⁴⁺,Nb⁴⁺…)have attracted much attention among the low-lead containing piezoelectric materials.The BZN–PT,BNH–PT and BF–45PT:La systems exhibit high electrical coefficients and large electrostrain.Unlike typical relaxor ferroelectrics,these three systems have tetragonal-pseudocubic phase boundary.And the piezoelectric enhancement mechanisms are not yet clear.In this paper,the above three systems were prepared using the solid-state reaction method,and the relationship between crystal structure and electrical properties was systematically studied.The tetragonal-pseudocubic phase boundary is around x=0.4 for x BZN–(1-x)PT ceramics.The composition x=0.41 has the highest relative permittivity(1549)and piezoelectric coefficient(442 p C/N).The temperature dependent dielectric data suggest that compositions with x?0.38has a spontaneous transition between the ferroelectric and relaxor states(T_F-R),while the pseudocubic compositions exhibit a relaxor-type dielectric anomaly.There is a large temperature difference between the freezing temperature(T_f)and the Burn temperature(T_B)in the pseudocubic phase compositions.For x=0.41,the temperature difference between T_f and T_B is close to 300 K(T_f=?485 K,T_B=?774 K).A further analysis showed that the composition x=0.41 near the phase boundray exhibits the following characteristics.(1)The coexistence of nanodomains with PNRs was demonstrated by bright-field TEM.(2)The crystal structure shows obvious tetragonal distortion after poling,but the dielectric behavior does not change significantly,and the frequency dependence of the relative permittivity after polarization is still obvious.(3)A low-temperature dielectric anomalies occurs at approximately 135 K that might be related to the origin of the frequency dispersion.(4)The planar electromechanical coupling constant(k_p)varies significantly with temperature at low temperatures.When the temperature increases from 93 K to 303 K,kp increases from 0.15 to 0.42.The finding indicates an intrinsic relationship between the piezoelectric and dielectric behavior.(5)The temperature dependence of the elastic modulus after poling was more sensitive than that of the dielectric behavior.The elastic modulus in the poled specimen displayed a significant anomaly near the depolarization temperature.(6)The normalized piezoelectric coefficient increases with increasing temperature.(7)The first-loop electrostrain versus electric field relationship shows that the field change behavior becomes more pronounced as x increases.A phase transition from the tetragonal symmetry to the pseudocubic symmetry is discovered near x=0.38 for x BNH–(1-x)PT ceramics.The composition near the phase boundary(x=0.39)has the largest relative permittivity(1940)and piezoelectric coefficient(456 p C/N).The temperature dependence of the relative permittivity indicates two dielectric anomalies at x=0.36and 0.37,and the dielectric anomaly in the low temperature range is associated with T_F-R.For the pseudocubic compositions,the relaxor-type dielectric anomalies are observed.The optimum composition x=0.39 shows the following characteristics in terms of structure and properties.(1)TEM reveals the coexistence of nanodomains consisting of local tetragonal distortions with PNRs in the unpoled state.The[100]axis in the poled state shows an ordered rotation of the oxygen octahedron.(2)Polarization induces a tetragonal-pseudocubic transition on the global scale,which irreversibly increases the coherence length.(3)Relaxor-like dielectric anomalies were observed(T_f=?453 K,T_B=?765 K).The relative permittivity after poling displays obvious frequency dispersion in the ferroelectric state.(4)The low temperature dielectric anomaly occurs at?155 K,which might be related to the origin of the dispersion.The k_p also changes considerably after heating,increasing from 0.16 at 93 K to 0.42 at 300 K.The finding indicates an intrinsic relationship between dielectric and piezoelectric behaviours.(5)The temperature dependence of the modulus of elasticity after poling is more sensitive than the dielectric behavior.Near the depolarization temperature,the modulus of elasticity in the poled specimen exhibits significant anomalies.(6)The normalized piezoelectric coefficient increases due to heating above room temperature,with a maximum of 1100 pm/V at 413 K.(7)The minimum strain on the bipolar strain loop's negative side of the first cycle is positive,confirming the electric field-induced pseudocubic to tetragonal phase transition.The tetragonal-pseudocubic phase boundary is around y=0.29 in the BF–45PT:Lasystems.The phase boundary composition(y=0.30)has a maximum relative permittivity of 1755.Temperature dependence of dielectric data show the presence of T_F-R for the y=0.27 composition.The phase boundary composition y=0.30 exhibits the following characteristics:(1)A dielectric anomaly was observed around the transition temperature between relaxor and ferroelectric-like states in the poled spcimen.(2)There was a large difference in elasticity versus temperature curves for unpoled and poled spcimens.(3)Large piezoelectric coefficient(d₃₃,280 p C/N),high remnant polarization(P_r,35.6?C/cm~2)and high maximum polarization(P_m,41.2?C/cm~2)were observed.(4)The normalized piezoelectric coefficient increases with temperature.(5)The thin double ferroelectric hysteresis loops occur during the ferroelectric-relaxor transition.(6)The first-cycle electrostrain loops show that the field change behavior becomes more pronounced as y increases.