The Effect Of Key Functional Groups On The Lattice Dynamic Behavior In Several Non-centrosymmetric Crystal Materials

Due to its special spatial symmetry, non-centrosymmetric crystal materials can present a series of physical phenomenon, such as piezoelectric effect, ferroelectric effect, second-harmonic generation effect, and so on. Both theoretical and experimental researches have been implemented to improve the properties in order to enlarge the application of the materials. Most of the previous works have been concentrated on the electric, optic and magnetic properties of materials, and relative relationship between microscopic functional elements and macroscopic properties has also been established, whereas there are so limited researches on lattices dynamic properties which has close relation with material application, and the understanding of key functional elements of material structure that can influence such properties is scarce. The lattice dynamic behaviors of crystal materials is of significant in crystal structure identification, comprehending the dielectric and thermodynamic properties, material structure stability, phase transition, it is thus in urgent need of correlated theoretical study to explore the mechanism of lattice dynamic behaviors. To meet the requirement, the present thesis research several microscopic functional elements of non-centrosymmetric crystal materials and relative lattices dynamic behavior, and establish their relationship based on first-principal calculation and analysis. The main content and results are as follows:The first chapter firstly introduces our object of present study—non-centrosymmetric crystal materials and several physical effect including piezoelectricity, ferroelectricity, pyroelectricity and second-harmonic generation. Lattices dynamic properties of crystal materials are then introduced such as Raman spectroscopy, dielectric property, thermodynamic property, phase transition. Finally, we put forward our scientific problem:the specific mechanism of effect between microscopic functional elements and macroscopic lattice dynamic properties, and a brief introduction of the whole thesis is also made.In the second chapter, we introduce the main theoretical method used in present article. We give the frame of density functional theory which is the precondition of first-principle calculation, and introduce the two measures to calculated phonon dispersion:density functional perturbation theory and frozen phonon method. The software packages we employed in present research is also introduced.In the third chapter, we start to study the first lattice dynamic properties of non-centrosymmetric crystal materials—Raman spectroscopy which has important role in distinguishing crystal structure, constituent and self-Raman laser application. We choose CdSiP2single crystal material with second-harmonic generation, and calculate the the polarized Raman spectroscopy under different geometry configuration. The comparison between our theoretical and experimental work is given. The location and intensity of most Raman peak are accordance with the experiment results, thus we can give the specific vibrational pattern for each Raman-peak related vibrational group from theoretical point including the translation, stretching, rotation of chemical bonds and atomics. Our results also demonstrate several anomaly modes as A1mode with frequency350.021cm-1under y(zz)y,B2mode with109.926cm-1under z(xy)z, as well as B2mode with481.412cm-1under b(aa)b. We find that all the exceptional modes are related with the vibration of P, Cd atom and P-Cd bond, which can be a useful tool for us to deduce the specific defect patterns in CdSiP2single crystal material.In the fourth chapter we research the dielectric properties of non-centrosymmetric crystal material influenced by lattice vibration. Most of the non-centrosymmetric crystal materials are dielectric substance, and the electronic and dielectric properties are essential to their application. In this chapter we choose Ag2CdGeS4as our object, and study the electronic and lattice dynamic properties to explore the microscopic mechanism of dielectric properties influenced by lattice vibration. As a representation of I2-II-IV-VI4type quaternary diamonds-like semiconductor material, Ag2CdGeS4has shown itself a widespread application in the area of solar-cell and thermoelectric material. There are three phase in this material due to different cation arrangement, two phase in Pna21(S type and P type) and one phase in Pmn21. The vibrational properties can be altered as the atomic position changes, thus it can change the application of material by changing its electric and dielectric properties by electron-phonon coupling. In present work, we study the relationship of physicochemical property and cation ordering. The results of electronic property show that the similar cation ordering in P type and Pmn21phase produce similar band gap,1.06eV. For another phase of Pna21, the S type, it demonstrates larger band gap,1.30eV due to different cation arrangement. In the section of lattice dynamic calculation we detail edly analyze the Born effective charge of silver and sulfur atom in three phase. In P type, the Born effective charges of the two atom more seriously deviate from their nominal charge compare to S type, which means that it exists stronger covalent bonding type between3d orbital of silver atom and2p orbital of sulfur resulting in higher position of valence-band maximum in P type, and the narrower band gap is formed. The infrared spectroscopy calculated by the vibrational polarization of cation demonstrate themselves the discrepancy in the low frequency region, which can help experimenter to distinguish the microscopic crystal structure from different phase.In the fifth chapter we research the thermodynamic properties of non-centrosymmetric crystal material. The lattice vibration can produce phonon which determines many thermodynamic properties such as heat capacity, vibrational entropy, and free energy. All of the properties are calculated based on the harmonic approximation among phonons, however phonon anharmonic interaction is the foundation of material thermal expansion and thermal conductivity. In present work, we study several infrared nonlinear optical crystal materials with second-harmonic generation effect. They are ABC2type ternary tetragonal chalcopyrite structure material, and the chemical bonding is the main functional group among these materials. We discuss the anharmonic behavior among phonons by calculating thermal expansion coefficient and thermal conductivity, and the specific mechanism of thermal properties influenced by chemical bonding has been established. In the first part, we research the anisotropic behavior of anharmonic thermal properties, and discuss the mechanism of anisotropy of thermal expansivity among ABC2type chalcopyrite crystal materials by calculating a-and c-axial gruneisen constant of CdSiP2and ZnGeP2. We adopt the Debye model and lattice dynamic method in calculating axial gruneisen constant. In the process of Debye model calculation, we find that the change of shear modulus reveals normal variation for ZnGeP2and abnormal behavior for CdSiP2. The role of shear modulus is thus considered of importance in anharmonic anisotropic behavior. In the section of lattice dynamic research based on phonon, we calculate the axial gruneisen parameter include not only the center point, but also other high-symmetric wave vector points of the first Brillouin zone. The results show two new soft modes as B1and B2with the lowest energy, which demonstrate as normal modes in previous study of volume-dependent mode gruneisen constant. For CdSiP2, the gruneisen constant of soft modes manifest larger magnitude than ZnGeP2meaning it exists stronger thermal anharmonicity in low temperature range. The values of soft-mode gruneisen constant turn positive at11OK for CdSiP2, and at80K for ZnGeP2. In the second part we research the source of thermal conductivity of CdSiP2and AgGaS2single crystal by calculating the gruneisen parameter of acoustic phonon. The results show that the main mechanism of thermal conductivity stem from the phonon anharmonicity, although the different Debye temperature aroused by different chemical bond type can also affect thermal conductivity. Both gruneisen constants of acoustic phonon of two materials are negative stating the soft modes of acoustic phonon in the whole Brillouin zone, and the value of gruneisen constants exhibit lower magnitude from X to Γ meaning that materials would demonstrate unstable structure in this area. For the two compounds, the magnitude of CdSiP2(-0.9) is obviously smaller than that of AgGaS2(-2.6), which give rise of the wide difference of thermal conductivity of the two materials.In the sixth chapter, the non-eigenstate structure of non-centrosymmetric crystal materials was researched to establish the relationship of point defect and the phonon state of non-centrosymmetric crystal materials. All of our study in the previous chapters is based on the perfect crystal structure; however there are different types of defects and impurities in the process of crystal growth, and they can produce sever influence on the macroscopic properties of crystal. Most of previous researches were concentrated on the electronic state of materials influenced by the defect and impurity, and the relationship between different types of defect and electronic and optic properties has been found. The knowledge of phonon state influenced by the defect is essential to thermodynamic and phase transition of crystal materials, nevertheless the relative research is still scarce. In this chapter we build the supercell crystal structure of BiFeO3with one Bi vacancy, and calculate the phonon properties including phonon dispersion, phonon density of state, heat capacity of both perfect crystal and crystal with defect on account of first principle theory and frozen phonon method. Our study reveals that the vibrational intensity of optical phonon branch weakens in high phonon frequency region, and the acoustic phonon branch apparently soften in the low-frequency region near the center of Brillouin zone. The oxygen and bismuth atomic vibration of defect system is observed weaken by the analysis of phonon density of state causing the change of the phonon dispersion. The soft of acoustic phonon is apparent, which can directly influence the thermodynamic behavior in the low temperature region. Due to the Bi vacancy the heat capacity reveals abnormal behavior followed with temperature resulting that the optical phonon is earlierly participated in the contribution to heat capacity. Our research deepens the understanding of non-eigenstate of BiFeO3, and provides the theoretical foundation to study the structural stability and magnetic phase transition in low temperature area.The seventh chapter summarizes the main content of present article, and proposes the innovation. The further research of this scholar field is also expected.