Phase Boundary Behavior Of Quasi-ternary Relaxor Ferroelectrics PNN-PT-PA With Giant Piezoelectric Effect

A compositionally-driven morphotropic phase boundary（MPB）separating rhombohedral and tetragonal phase has been found in Pb（Zr,Ti）O₃ and Pb（Mg,Nb）O₃-Pb Ti O₃ with the change of Pb Ti O₃ content.The electrical parameters reach their maximum value near the MPB.Recently,researchers have found that the enhanced piezoelectric performance of Pb Ti O₃-Bi(Ni_1/2Zr_1/2)O₃and Bi(Ni_1/2Hf_1/2)O₃-Pb Ti O₃ is related to the tetragonal-pseudocubic phase boundary.The Pb(Nb_2/3Ni_1/3)O₃-Pb Ti O₃-Pb AO₃（PNN-PT-PA）system with the ultra-high piezoelectric coefficients have the tetragonal-pseudocubic（T-PC）and rhombohedral-pseudocubic（R-PC）phase boundaries.The relationship between structure and properties near these phase boundaries should attract attention.In this paper,the PNN-PT-PA systems were prepared using the solid-state reaction method,and the relationship between crystal structure and electrical properties near T-PC and R-PC phase boundaries was systematically studied.（1）The tetragonal distortion decreases and the crystal structure changes from the T phase to PC phase in the x PNN-（0.7-0.7x）PT-（0.3-0.3x）PA system with the increase of PNN content.The T-PC phase boundary is located around the composition with x=0.55.The relative permittivity（9520）after poling,piezoelectric coefficient（1264 p C/N）and inverse piezoelectric coefficient（1170 pm/V）reached their optimum values near the T-PC boundary.The relative permittivity becomes frequency-dependent（x=0.3,slope=-10,x=0.6,slope=-187）with the change of the phase structures,suggesting the increased frequency dispersion.The changes from diffused ferroelectrics（x≤0.35）to ferroelectric relaxors with the spontaneous ferroelectric-relaxor transition（x=0.45-0.5）,and to ferroelectric relaxors with the diffused ferroelectric-relaxor transition（x>0.5）were observed.Correspondingly,the temperature for the maximum value of the relative permittivity（T_m=262°C@1 k Hz for x=0.3;T_m=91°C@1 k Hz for x=0.6）and the depolarization temperature decrease.（2）Based on the above findings,the average and local structures,and electrical properties of the T-PC phase boundary composition（x=0.55）were investigated.The pseudocubic phase originates from the small tetragonal distortions and the local structure is also tetragonal according to the average and local structure analyses.The possibility of the presence of the rhombohedral phase was excluded by the fitting results of refined structures for X-ray diffraction,Raman spectra and convergent-beam electrondiffraction（CBED）.There is no field-induced phase transition near the T-PC phase boundary according to the structure analyses before and after poling.The finding suggests that the piezoelectric enhancement mechanism is related to the polarization extension.The low-temperature electrical behavior indicates an abrupt increase in the relative permittivity and planar electromechanical coupling factor k_p in the temperature range of-25°C to 25°C.（3）For the xPNN-（0.3-0.3x）PT-（0.7-0.7x）PA system,all compositions show a rhombohedral phase structure.The rhombohedral distortion decreases with the increasing PNN content.The relationship between dielectric behavior and temperature indicates a change from the ferroelectrics with the diffused phase transition to ferroelectric relaxors with the spontaneous ferroelectric-relaxor transition.The piezoelectric coefficients of the composition near the R-PC phase boundary are much lower than those of the T-PC phase boundary compositions with the similar depolarization temperature.