Phase Transition And Phonon Thermodynamics In Giant Piezoelectric Mn Doped Potassium Sodium Niobate Crystals

Lead-free ferroelectric perovskite K_0.5Na_0.5Nb O₃（KNN）,as a representative oxide with excellent dielectric and piezoelectric properties,has attracted extensive attention in recent years.As a new generation of lead-free piezoelectric ceramics,KNN has excellent properties such as high electromechanical coupling coefficient and quality factor.Moreover,the research on environment-friendly material system like KNN is helpful to replace the lead based ceramic lead zirconate titanate in the market,which accords with the basic national policy of sustainable development.In the existing studies,modified KNN with piezoelectric coefficients comparable to those of lead based materials has been successfully prepared.However,the difficulty is that the coexistence region of KNN’s polycrystalline phase transformation is an inclined region dependent on temperature change.Compared with the nearly vertical phase coexistence region of the PZT,the high piezoelectric coefficient at the phase boundary of KNN is controlled by temperature.This greatly limits the use of environmentally friendly piezoelectric materials.In order to solve the current application problems,it is necessary to have a deeper understanding of the phase structure details of materials.In this paper,the temperature-dependent structure evolution of Manganese（Mn）-doped K_0.5Na_0.5Nb O₃-LiBiO₃single crystal has been systematically studied.It shows an ultrahigh voltage electric coefficient of about 1050p C/N^-1and excellent dielectric/ferroelectric properties.By demonstrating phonon thermodynamics related to all raman activity modes over a wide temperature range of 80-800K.The nonpolarized and polarized scattering properties reflecting various phonon modes and structures are described in detail.It is proved that the symmetry difference,symmetry break and symmetry rearrangement in molecular vibration are related to multiphase coexistence and discontinuity of first-order phase transition.The complete phase transition sequence and shift of phase transition point were observed in KNN-LiBiO₃crystals with different Mn contents by lattice dynamics method.The purpose of this work is to reveal the details of the good ferroelectric/dielectric properties,structure and phonon thermodynamics in KNN systems,and to understand the first-order phase transition mechanism of KNN systems under the influence of doping.Raman spectroscopy has unique advantages in detecting the details of molecular structure and tiny lattice distortion.Due to its nondestructive and high sensitivity,it can be used for further physical structure analysis under variable temperature and pressure conditions combined with other analytical methods.Therefore,in this study,raman scattering technology was mainly used to explore the structural phase transition and phonon thermodynamics of KNN-LiBiO₃single crystal with excellent performance grown by the new process.The piezoelectric constant d₃₃of 0.375%Mn doped KNNLB single crystal is about1050p C/N.The core contents and innovations of this paper are as follows:（1）The continuous phase transition process from rhombohedral（R）to orthorhombic（O）to tetragonal（T）to cubic（C）and the influence of different doping concentration on phase transition point were summarized by taking advantage of polarization Raman scattering spectrum sensitivity to molecular/lattice scale analysis of phase structure and symmetry details.（2）By analyzing the preparation method and ferroelectric/dielectric properties,it is found that Mn ions occupy A and B positions in ABO₃lattice with+2 and+4 valences,which effectively reduces the leakage current.The use of annealing process improves the atomic order degree of A and B positions in the lattice,and reduces the total internal energy of the lattice.The annealing atmosphere of oxygen makes lattice oxidation and inhibits the generation of holes,which further improves ferroelectric performance.（3）Through the analysis of lattice symmetry,it is found that with the increase of Mn content,the depolarization ratio of KNN-LiBiO₃crystal increases significantly in tetragonal phase,and the lattice structure tends to a linear configuration with low symmetry,proving that the incorporation of Mn will enhance the tetragonal property of the crystal.This work provides a systematic physical explanation for the performance optimization and improvement of lead-free KNN based ferroelectrics from the lattice structure level.