Multi-field Manipulations On Phase Transitions Of Perovskite-type Ferroic Materials

The physical properties and potential applications of materials depend on their structural characteristics.So,understanding structural phase transition and its internal relationship with physical properties has been a core scientific problem in the field of materials science for a long time.With the development and the construction of new iron compounds and heterostructures,research in the field of material science has opened up some more promising areas and moved towards research based on perovskite-type structures.Perovskite-type oxides have wide applicability in crystal symmetry and the ability to combine various cations.This leads to rich physical properties such as ferroelectric,ferromagnetic,piezoelectric and magnetoelectric coupling,which makes it widely used in the fields of energy,medicine and microelectronics.In particular,antiferroelectric and relaxor ferroelectric materials have complex and tunable perovskite structures like antiferroelectric Na Nb O₃ and relaxor ferroelectric x Pb(In_1/2Nb_1/2)O₃-y Pb(Mg_1/3Nb_2/3)O₃-(1-x-y)Pb Ti O₃.Their high energy density,high breakdown field strength and other excellent dielectric properties play an important role in energy storage capacitors,electromechanical sensors and other electronic devices.With the emergence of new magnetic properties,the strong magnetoelectric effect contained in multiferroic Gd Fe O₃ provides an opportunity to develop spintronic devices or small magnetic-controlled memory.Besides,metal halide perovskite(C₆H₅C₂H₄NH₃)₂Pb I₄ constructed on the perovskite structure has attracted extensive attention because of its outstanding photoelectric properties.They have great potential for many electronic devices such as solar cells and light-emitting diodes and so on.Therefore,we systematically explore the structure and physical properties of the above perovskite-type oxides by the means of suspended matter spectroscopy and scanning probe technology to reveal their key scientific laws such as structural phase transition,lattice vibration and electronic band structure engineering in the thermal and stress field.This will deepen our knowledge and understanding of perovskite oxide system and provide a reliable scheme for the design and application of advanced multifunctional devices.The main contents of researches of this paper are as follows:(I)The phase diagrams of temperature-pressure structure for antiferroelectric(1-x)Na Nb O₃-x Ca Sn O₃(0?x?0.04)ceramics have been accurately described by Raman spectroscopy and spectroscopic ellipsometry(SE),as well as the first-principles calculations.The field-controlled structural phase transitions of antiferroelectric ceramics(1-x)Na Nb O₃-x Ca Sn O₃(0?x?0.04)in thermal and stress fields have been systematically investigated by Raman spectroscopy and SE.We firstly clarify structural order of phase transitions on Ca Sn O₃-modified Na Nb O₃ ceramics within the temperature range of 80-840 K by discussing the anomalies of lattice and phonon dynamics.The doping effect of Ca Sn O₃ on the P-R phase transition has been summarized from the decreased critical temperature from 660 to 580 K.In addition,the anomalous pressure with respect to phonon frequency at the stress field of 0-25 GPa also provides the evidence of structural transformations at 6.55 and 10.05 GPa.Upon increasing the Ca Sn O₃ content,phase transition moves to lower pressure range.Furthermore,first-order discontinuous characteristics of structure transition at 660 K is revealed between phases of antiferroelectric orthorhombic Pbma and Pmnm,which is simultaneously identified by thermal kinetics of electronic transitions from the analysis of SE data.Moreover,the first-principles calculations greatly assist the spectral discussion,not only clarifying the intrinsic mechanism of electronic transitions with temperature,but also exhibiting the probable expending trends of band gaps with pressure.This work would provide fundamental guidelines to explore phase transition of the broad Na Nb O₃-based crystalline family with non-contact optical methods.(II)The monoclinic heterophase coexistence in the ternary Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-Pb Ti O₃ single crystal with optimized composition were constructed to quantify the correlation between spontaneous nanopolarity and phase heterogeneity,attempting to understand the origin of the exceptional functionalities.By designing in-situ highresolution spectroscopic-microscopic technique,we have observed M_a and M_c heterophase mixtures spatially separated by the monoclinic heterophase boundary(MHB),which are responsible for the ferroelectric-dominated and relaxor-ferroelectric-dominated nanodomain structure,respectively.Internal energy mapping from optical soft mode dynamics reveals the inhomogeneous polarization and local symmetry on both sides of the MHB.Various molecular polarizabilities and localized octahedral distortions correlate directly with monoclinic regions and electromechanical contribution.Besides,field screening effect resulting from electric double layer in tip-surface system benefits the better understanding towards the central physical mechanism for the electrical probing by scanning probe microscopy(SPM)in a polar liquid medium.This work claries the heterogeneity between structure,energy and polar order,provides a new design freedom for advanced relaxor ferroelectrics and paves the way to the broad applications of in vivo or in operando electrical analysis on the nanoscale by SPM technique in conductive liquid environments.(III)The change of lattice symmetry during ferromagnetic transition Fe³⁺and octahedra distortion in the stress field of Gd Fe O₃ single crystal were uncovered with high-temperature and high-pressure Raman spectroscopy.The evolutions of phonons assiocaited with octahedral motion in the temperature range of 80-800 K reveals that Fe³⁺ion in Gd Fe O₃ has a transition from paramagnetic to antiferromagnetic ordering transition at N�el temperature T_N,Fe.The lattice dynamics and distortion with local structure rearrangement during ferromagnetic transition has been uncovered by quantifying the cross-polarized Raman spectra.In particular,we claim that the depolarization ratio can be quantified and used to accurately determine the ferromagnetic transition of Gd Fe O₃ and observe the evolution of lattice symmetry evolution at the same time.Furthermore,the pressure dependence of phononsin the range of 0-25.03 GPa indicates that antiphase tilt of Fe O₆ octahedra is more susceptible to the stress field than the in-phase tilt.For Gd Fe O₃lattice,Gd O₁₂ dodecahedra has worse compressibility than Fe O₆ octahedra.This work explores the physical mechanism of local structural symmetry,octahedra tilt,and phonon dynamics in Gd Fe O₃under the thermal and stress field,which can provide a basic view for a series of Gd Fe O₃-type perovskites and more RFe O₃ systems.(IV)The photoluminescence(PL)enhancement resulted from the self-trapped excitons in two-dimensional layered metal halide perovskite(C₆H₅C₂H₄NH₃)₂Pb I₄ single crystal under pressure were investigated by PL spectroscopy,UV-vis absorption spectroscopy and Raman scattering.The free excitons induced narrowband emission at 582 nm with small Stoke shift about 0.13 e V is observed at ambient condition.A significant phenomenon known as pressure-induced emission(PIE)was observed upon compression.The intensity of PL peaks is 8 times enhanced than the initial value as the pressure reached at 2.87 GPa.The self-trapped excitons generated owing to the strong electron-phonon coupling are resulted from the lattice distortion upon compression enhance intensity of PL peak.The anomalous evolution of bandgap experienced redshift-blueshift-redshift dynamics under pressure and had a shrinkage of about 24.3%,which could be due to the collaboration between compression effect and pressure-induced amorphization.The Raman scattering results under the pressure field also confirmed the distortion process of crystal structure.This work will provide a novel routine to design the photoelectric devices with high performance for potential applications based on the metal halide perovskite.