Study On The Strong-Field Electrical Properties Of Lead-Based Medium/High-Entropy Ferroelectrics With Perovskite Structure

Lead based perovskite ferroelectrics are widely used in tunable microwave devices,energy storage capacitors and solid-state refrigerators due to their functional properties such as high dielectric tunability,energy storage and electrocaloric effect.With the development of miniaturization and integration of devices,the traditional domain engineering strategies such as constructing quasi-homotype phase boundary and controlling grain size have been shown to be insufficient for the modification and optimization of electrical properties.In contrast,it can take full advantage of the comprehensive factors such as ferroelectric activity and radius of atoms for the design of local polarization configuration which is expected to significantly improve the flexibility of the applied electric field response,and greatly improve the comprehensive electrical performance when applying the concept of high entropy to perovskite ferroelectrics.In this paper,lead-based medium/high-entropy ferroelectrics with perovskite structure（including ceramics and thin films）were studied by optimizing ceramic sintering process,controlling preferred orientation of thin films and adjusting annealing process,the research results were showed as follows:（1）The Pb(Sc_0.25In_0.25Nb_0.25Ta_0.25)O₃（PSINT）medium-entropy ferroelectric ceramics were successfully synthesized by a spark plasma sintering（SPS）technology.It is found that the ceramic samples prepared by the two-step sintering process have larger grain size and higher densification degree than those prepared by the one-step sintering process.The latter shows larger electrocaloric（EC）effect（40 k V/cm,（35）T～0.85 K）and higher coefficient of EC effect（～0.021K·cm/k V）and wider effective EC refrigeration temperature area（～120 K）at room temperature.Large EC effect is associated with high oxygen vacancy content.The XPS results show that there are more oxygen vacancies in the ceramics prepared by the two-step sintering method,and there are more defect dipoles formed in the corresponding lattice.The polarization of the dipoles increases the total entropy of the system,so the ceramic prepared by the two-step sintering method shows a larger EC effect.The wide working temperature range of the EC effect can be attributed to the large dielectric relaxation dispersion caused by the mesentropic disordered polar nanoscale（PNRs）structure constructed by the B-heteroelements（Sc,In,Nb and Ta）.（2）The Pb(Sc_0.25In_0.25Nb_0.25Ta_0.25)O₃（PSINT）medium-entropy ferroelectric thin film deposited on La Ni O₃/Si（111）（La Ni O₃ for short）,La Ni O₃/Pt（111）/Ti O_x/Si O₂/Si（La Ni O₃/Pt for short）and Pt（111）/Ti O_x/Si O₂/Si（Pt for short）substrates were prepared by sol-gel method.A high dielectric tunability（η～50%）and a high dielectric tunability strength（η/E～0.12%cm/k V）was achieved at a low electric field（E）of～407 k V/cm in the PSINT thin film on Pt.Increase the applied electric field,a higher dielectric tunability(η_max～65%)was achieved at～679 k V/cm.This is mainly attributed to the lower oxygen vacancy content of PSINT films on Pt results in weaker domain pinning effect of oxygen vacancy domains,which can effectively increase the polarization contribution of ionic polarization of the lattice,the redirection of PNRs and the motion of domain walls.The films also showed high temperature stability over a range of～200 K（223-423 K）with a medianηfluctuation of only 4.8%.In addition to the highη,a large EC effect（（35）T～15 K）were also obtained in the film on Pt at～1947 k V/cm.（3）The Pb(Sc_0.25In_0.25Nb_0.25Ta_0.25)O₃（PSINT）medium-entropy ferroelectric thin film deposited on Pt were annealed at different times.The effects of annealing times on the energy storage and electrocaloric performance of PSINT films were studied.After annealing,the breakdown field strength of the films was significantly improved,which increaseed from 677 k V/cm（before annealing）to1261 k V/cm（annealing for 15 h）,and the corresponding energy storage density increased from 5.7 J/cm³ to 12.6 J/cm³.The electrocaloric effect increases first and then decreases with increasing the annealing time.When the annealing time is 10 h,the maximum of the electrocaloric effect（（35）T～14.1 K）was achieved at a low electric field（705 k V/cm）,which is 3.2 times of the as-prepared sample（（35）T～4.3 K）.With the extension of annealing time,cracks and holes appear in the films due to the serious volatilization of Pb,and the performance of electrocaloric effect decreases.It is found that the electrocaloric effect of the film increases first and then decreases mainly due to the change of B-site PNRs order and microstructure of films after annealing.（4）The Pb(Li_0.2Sc_0.2In_0.2Nb_0.2Ta_0.2)O₃（PLSINT）high-entropy ferroelectric thin film on the ITO/Glass（ITO for short）,Pt,La Ni O₃/Pt and p-type Ga N（Ga N for short）substrates were synthesized by sol-gel method.A high dielectric tunability（η～75.1%）was achieved at a low electric field of～557 k V/cm in the PLSINT thin film deposited on ITO substrate.An excellent thermal stability with a broad working temperature from 223 K to 423 K was also obtained.An ultra-large electrocaloric effect（（35）T～21.5 K）was achieved at a low electric field（786k V/cm）with ultra-wide working temperature range（～200 K）was achieved in PLSINT thin film on Pt substrate.The maximum polarization intensity of the film on Pt increases when increase test temperature,which indicates that more dipoles（electric dipoles and defect dipoles）are turned over with the applied electric field.And the dipole entropy increases which result in a high adiabatic temperature change.At the same time,the local fluctuation caused by the high entropy component limits the phase transition and widens the working temperature region.Compared with the PSINT medium-entropy thin film,the electrocaloric effect of PLSINT high-entropy thin film increases sharply at the same electric film,indicating that high-entropy component design is a simple and effective way to improve the electrocaloric effect.Different from the large electrocaloric effect of PLSINT thin film on Pt,the films on Ga N showed a large negative electrocaloric effect（～-8.1 K）with a wide working temperature range（～95 K）at 575 k V/cm.This is mainly attributed to the fact that the orthogonal phase induced at the interface between the PLSINT thin film and Ga N substrate can be transformed into tripartite phase at the applied of electric field.（5）The Pb(Sc_1/6In_1/6Nb_1/6Ta_1/6Fe_1/6Zn_1/6)O₃（PSINTFZ）high-entropy ferroelectric thin film on La Ni O₃/Pt and Pt substrates were prepared by sol-gel method.An ultra-high dielectric tunability（η～89.2%）and a high temperature stability was achieved at a very low electric field of～503 k V/cm in the PSINTFZ thin film on Pt.Theηof PSINTFZ high-entropy thin film deposited on Pt substrate increased 43.92%compared with the PSINT medium-entropy films（407 k V/cm,η～50%）.However,the dielectric tunability of the films on La Ni O₃/Pt is lower（η～41.07%）at 314 k V/cm.The excellent dielectric tunability performance of PSINTFZ thin film on Pt is mainly attributed to the low oxygen vacancy pinning effect effectively increases the polarization contribution of intrinsic lattice ionic,the redirection of PNRs,the domain wall motion.And the introduction of the concept of high entropy results in a large phase transition dispersion.In contrast,the high oxygen vacancy content of the films deposited on La Ni O₃/Pt leads to the dielectric breakdown at strong field and poor dielectric tunability performance.