非中心对称晶体材料中关键功能基元对其晶格动力学行为的影响
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摘要
非对称中心结构晶体材料由于其在空间对称性上的特殊性从而在宏观上呈现出一系列物理现象,例如压电效应、铁电效应、倍频效应等。为了进一步扩展该类材料的实际应用,理论和实验工作者从事了大量相关的研究以改进材料的性能。但之前的研究工作大都集中在材料的电学、磁学、光学等方面,建立了相关性能和微观基元的构效关系,而对材料应用关系密切的晶格动力学性质则研究较少,对影响晶格动力学行为的关键功能基元了解不多。晶体材料晶格动力学行为的研究对解析晶体结构,了解材料的介电、热力学性质以及材料的结构稳定性和相变问题有重要的意义,因此目前急需要相关的理论工作来探寻材料众多晶格动力学行为的发生机制。本论文的研究工作正是基于这样的出发点,研究了几种非中心对称晶体材料中的关键功能基元和相关晶格动力学行为,通过基于第一性原理的计算和分析建立了两者之间的关系。主要研究内容和成果如下：
第一章绪论中首先介绍了本文的研究对象——非中心对称晶体材料及其相关的几类物理效应,包括压电、铁电、热释电、倍频效应等；其次详细介绍了晶体材料中几种晶格动力学相关性质,包括拉曼光谱、介电性质、热力学、相变等,从而提出了本文的科学问题：非中心对称晶体材料中微观功能基元作用于其宏观晶格动力学性质的具体机制,同时对论文的研究内容做了简单介绍。
第二章简要介绍了本文研究工作所需要的理论方法,首先介绍了第一性原理计算所依赖的密度泛函理论的理论框架；其次介绍了两种声子谱的计算方法：密度泛函微扰理论和冻声子方法,最后介绍了计算工作中用到的软件包。
第三章我们首先研究了非中心对称晶体材料的拉曼光谱行为,它对于鉴别晶体微观结构、组分以及自拉曼激光器的应用方面有重要的作用。我们选取了具有倍频效应的CdSiP2晶体,基于第一性原理计算了该晶体的不同几何配置下的偏振拉曼光谱,并同实验测量进行了对比。结果显示大多数拉曼峰的位置和峰强同实验有较好的一致,因此我们可以从理论上给出了每个拉曼峰所对应的振动基元的具体振动方式,包括化学键和原子的平动、拉伸运动、旋转运动等。理论计算的结果当中出现了同实验测量不一致的拉曼峰,分别是y(zz)y配置下的A1模式,频率为350.021cm-1,z(xy)z配置下的B2模式,频率为109.926cm-1以及b(aa)b配置下的B2模式,频率为481.412cm-1,我们发现这些模式主要来自P原子和Cd原子以及P-Cd键的振动,以此可以推断出该晶体中存在的缺陷类型。
第四章我们研究了非中心对称晶体材料的晶格振动对其介电性质的影响。非中心对称晶体材料大多是电介质材料,在介电领域有较广的应用,因此它们的电子性质和介电性质对于材料的应用至关重要。在本章中我们以Ag2CdGeS4为例,研究了它的晶格动力学性质和电子性质,探讨了晶格振动对材料介电性质影响的微观机制。Ag2CdGeS4作为Ⅰ2Ⅱ-Ⅳ-Ⅵ4型四元金刚石结构半导体材料的一个代表在太阳能和热电领域有广泛的应用,其阳离子的不同排布会产生材料三种不同的相,两个相属于Pna21空间群,一个相属于Pmn21空间群。材料内部原子位置的变化会引起振动性质的改变,通过声子和电子的耦合机制来改变材料的电子性质和介电性质,从而改变材料的应用。在本章中我们基于第一性原理研究了不同阳离子排布同材料物理化学性能的联系。电子性质的计算结果显示P型和Pmn21相中相似的阳离子排布方式产生了相同的能带带隙值,1.06eV；而对于Pna21的另一种结构,S型来说,由于它拥有和其他两种结构不同的阳离子排布方式,从而产生了较大的带隙值,1.30eV。在晶格动力学的计算部分中我们详细分析了三种结构中银离子和硫离子的玻恩有效电荷。在P型中,两个离子的玻恩有效电荷相较于S型更明显的偏离了它们的名义电荷,这意味着银离子的3d轨道和硫离子的2p轨道之间有着更强的共价成键形式,因此它在能带结构中价带顶的位置较S型有所提高,形成了更小的能带带隙。根据阳离子振动极化所计算出的红外光谱显示出了它们在低频区域的差异性,这可以用来帮助实验人员区分该体系的不同结构。
第五章研究了非中心对称晶体材料的热力学性质。晶体材料的晶格振动产生声子,它决定了材料众多热力学性质,如热容、振动熵、自由能等。但这些热力学性质都是基于声子间的简谐近似,而声子间的非谐作用则同材料的热膨胀、热导率有重要的联系。在本章工作中我们以几类具有倍频效应的红外二阶非线性光学晶体材料为研究对象,它们具有相同的四方黄铜矿晶体结构,其区别在于化学键强度的不同。我们通过计算它们的热膨胀系数和热导率,探讨了这些热力学性质相关的声子之间的非简谐行为,并建立了化学键这一功能基元影响材料热力学性质的具体机制。在第一部分工作中我们通过计算a轴和c轴的轴向格林耐森参数分别研究了CdSiP2和ZnGeP2的非简谐热力学各向异性,探讨了ABC2黄铜矿系列晶体材料中的热膨胀各向异性的产生机制。在计算轴向格林耐森参数的过程中,我们分别采用了德拜模型和晶格动力学方法,在德拜模型的计算过程中我们发现ZnGeP2晶体的剪切模量的随着晶格常数正常变化,对于CdSiP2晶体来说剪切模量的变化却表现出了异常行为,因此我们认为剪切模量对体系的非简谐各向异性行为起着关键的作用。在以声子频率为基础数据的晶格动力学的研究中,我们计算了轴向的模式格林耐森参数,不仅包括布里渊区的中心点,还涵盖了布里渊区的其他高对称波矢点。我们发现在两种化合物中存在两种软模分别在最低能量对称性为B1和B2模式中,在之前的体积模式格林耐森参数的计算中这两种软模并没有发现。在CdSiP2晶体中,软化模式的格林耐森参数的数值更大,意味着它在低温区域内存在着更强的热力学非简谐性,它在110K左右变为正值,而在ZnGeP2晶体中,格林耐森参数从负值到正值的转变温度为80K左右。在第二部分中我们通过计算声学支声子的格林耐森参数研究了CdSiP2和AgGaS2晶体中热导率差异较大的来源。结果显示,两种材料化学键强度差异造成了两者德拜温度的不同,从而会对热导率产生一定的影响,但造成两者热导率差异的主要原因是声子的非简谐作用。两个晶体声学支格林耐森参数大都为负值,说明声学支振动模式在整个布里渊区表现为软模,其中波矢量x点到r区域的格林耐森参数较小,说明体系在该区域存在更强的结构不稳定性。在两种体系声学支总格林耐森参数的对比中,CdSiP2晶体(-0.9)在绝对值上明显小于AgGaS2晶体(-2.6),由此产生了两者热导率上的巨大差异。
在第六章中我们研究了非中心对称材料的非本征态结构,目的在于建立点缺陷同非中心对称晶体材料声子态的影响。在之前的章节中,我们研究了非中心对称晶体材料中关键功能基元对一系列晶格动力学行为的影响,但它们都有一个共同的前提,那就是理论研究的材料在结构上都是完整无缺陷的。晶体在的实际生长过程中会出现很多的缺陷和杂质,完美的晶体结构是不存在的,而这些缺陷形式会对晶体的宏观性能产生重要的影响,因此了解这些缺陷形式作用于材料性能的机制十分重要。之前的研究大多集中在缺陷或杂质对材料的电子态影响上,建立了具体的缺陷形式同能带结构、光学性质变化的联系。但对于晶体材料中缺陷体对材料声子态的研究却十分匮乏,而它们对声子态的影响往往同材料的结构相变和热力学等行为关系密切。在本章中我们建立了含有一个Bi空位的BiFeO3超晶胞结构,基于第一性原理和冻声子法分别计算了完整晶体和含缺陷晶体的声子态性质,包括声子谱、声子态密度、热容等。结果显示,在声子谱高频区域光学支声子振动强度减弱,而在低频区域的布里渊区中心附近则发现声学支声子有明显的软化。通过声子分波态密度的分析我们得知在含缺陷的BiFeO3体系中,氧原子和铋原子各自的振动贡献有明显减弱,从而造成了声子谱中振动模式的改变。其中声学支的软化较为明显,它直接影响了材料在低温附近的热力学行为。通过低温区热容的研究发现,由于铋空位的存在,热容随温度的变化出现异常,使得光学支声子更早的参与到热容的贡献之中。我们的研究结果进一步加深了对BiFeO3材料非本征态的认识,对于研究BiFeO3材料在低温附近的结构稳定性和磁性相变机制提供了理论依据。
第七章总结了本论文研究的主要工作内容,提出了本工作的创新之处,并对相关工作进一步深入的研究做了展望。
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.
第一章绪论中首先介绍了本文的研究对象——非中心对称晶体材料及其相关的几类物理效应,包括压电、铁电、热释电、倍频效应等；其次详细介绍了晶体材料中几种晶格动力学相关性质,包括拉曼光谱、介电性质、热力学、相变等,从而提出了本文的科学问题：非中心对称晶体材料中微观功能基元作用于其宏观晶格动力学性质的具体机制,同时对论文的研究内容做了简单介绍。
第二章简要介绍了本文研究工作所需要的理论方法,首先介绍了第一性原理计算所依赖的密度泛函理论的理论框架；其次介绍了两种声子谱的计算方法：密度泛函微扰理论和冻声子方法,最后介绍了计算工作中用到的软件包。
第三章我们首先研究了非中心对称晶体材料的拉曼光谱行为,它对于鉴别晶体微观结构、组分以及自拉曼激光器的应用方面有重要的作用。我们选取了具有倍频效应的CdSiP2晶体,基于第一性原理计算了该晶体的不同几何配置下的偏振拉曼光谱,并同实验测量进行了对比。结果显示大多数拉曼峰的位置和峰强同实验有较好的一致,因此我们可以从理论上给出了每个拉曼峰所对应的振动基元的具体振动方式,包括化学键和原子的平动、拉伸运动、旋转运动等。理论计算的结果当中出现了同实验测量不一致的拉曼峰,分别是y(zz)y配置下的A1模式,频率为350.021cm-1,z(xy)z配置下的B2模式,频率为109.926cm-1以及b(aa)b配置下的B2模式,频率为481.412cm-1,我们发现这些模式主要来自P原子和Cd原子以及P-Cd键的振动,以此可以推断出该晶体中存在的缺陷类型。
第四章我们研究了非中心对称晶体材料的晶格振动对其介电性质的影响。非中心对称晶体材料大多是电介质材料,在介电领域有较广的应用,因此它们的电子性质和介电性质对于材料的应用至关重要。在本章中我们以Ag2CdGeS4为例,研究了它的晶格动力学性质和电子性质,探讨了晶格振动对材料介电性质影响的微观机制。Ag2CdGeS4作为Ⅰ2Ⅱ-Ⅳ-Ⅵ4型四元金刚石结构半导体材料的一个代表在太阳能和热电领域有广泛的应用,其阳离子的不同排布会产生材料三种不同的相,两个相属于Pna21空间群,一个相属于Pmn21空间群。材料内部原子位置的变化会引起振动性质的改变,通过声子和电子的耦合机制来改变材料的电子性质和介电性质,从而改变材料的应用。在本章中我们基于第一性原理研究了不同阳离子排布同材料物理化学性能的联系。电子性质的计算结果显示P型和Pmn21相中相似的阳离子排布方式产生了相同的能带带隙值,1.06eV；而对于Pna21的另一种结构,S型来说,由于它拥有和其他两种结构不同的阳离子排布方式,从而产生了较大的带隙值,1.30eV。在晶格动力学的计算部分中我们详细分析了三种结构中银离子和硫离子的玻恩有效电荷。在P型中,两个离子的玻恩有效电荷相较于S型更明显的偏离了它们的名义电荷,这意味着银离子的3d轨道和硫离子的2p轨道之间有着更强的共价成键形式,因此它在能带结构中价带顶的位置较S型有所提高,形成了更小的能带带隙。根据阳离子振动极化所计算出的红外光谱显示出了它们在低频区域的差异性,这可以用来帮助实验人员区分该体系的不同结构。
第五章研究了非中心对称晶体材料的热力学性质。晶体材料的晶格振动产生声子,它决定了材料众多热力学性质,如热容、振动熵、自由能等。但这些热力学性质都是基于声子间的简谐近似,而声子间的非谐作用则同材料的热膨胀、热导率有重要的联系。在本章工作中我们以几类具有倍频效应的红外二阶非线性光学晶体材料为研究对象,它们具有相同的四方黄铜矿晶体结构,其区别在于化学键强度的不同。我们通过计算它们的热膨胀系数和热导率,探讨了这些热力学性质相关的声子之间的非简谐行为,并建立了化学键这一功能基元影响材料热力学性质的具体机制。在第一部分工作中我们通过计算a轴和c轴的轴向格林耐森参数分别研究了CdSiP2和ZnGeP2的非简谐热力学各向异性,探讨了ABC2黄铜矿系列晶体材料中的热膨胀各向异性的产生机制。在计算轴向格林耐森参数的过程中,我们分别采用了德拜模型和晶格动力学方法,在德拜模型的计算过程中我们发现ZnGeP2晶体的剪切模量的随着晶格常数正常变化,对于CdSiP2晶体来说剪切模量的变化却表现出了异常行为,因此我们认为剪切模量对体系的非简谐各向异性行为起着关键的作用。在以声子频率为基础数据的晶格动力学的研究中,我们计算了轴向的模式格林耐森参数,不仅包括布里渊区的中心点,还涵盖了布里渊区的其他高对称波矢点。我们发现在两种化合物中存在两种软模分别在最低能量对称性为B1和B2模式中,在之前的体积模式格林耐森参数的计算中这两种软模并没有发现。在CdSiP2晶体中,软化模式的格林耐森参数的数值更大,意味着它在低温区域内存在着更强的热力学非简谐性,它在110K左右变为正值,而在ZnGeP2晶体中,格林耐森参数从负值到正值的转变温度为80K左右。在第二部分中我们通过计算声学支声子的格林耐森参数研究了CdSiP2和AgGaS2晶体中热导率差异较大的来源。结果显示,两种材料化学键强度差异造成了两者德拜温度的不同,从而会对热导率产生一定的影响,但造成两者热导率差异的主要原因是声子的非简谐作用。两个晶体声学支格林耐森参数大都为负值,说明声学支振动模式在整个布里渊区表现为软模,其中波矢量x点到r区域的格林耐森参数较小,说明体系在该区域存在更强的结构不稳定性。在两种体系声学支总格林耐森参数的对比中,CdSiP2晶体(-0.9)在绝对值上明显小于AgGaS2晶体(-2.6),由此产生了两者热导率上的巨大差异。
在第六章中我们研究了非中心对称材料的非本征态结构,目的在于建立点缺陷同非中心对称晶体材料声子态的影响。在之前的章节中,我们研究了非中心对称晶体材料中关键功能基元对一系列晶格动力学行为的影响,但它们都有一个共同的前提,那就是理论研究的材料在结构上都是完整无缺陷的。晶体在的实际生长过程中会出现很多的缺陷和杂质,完美的晶体结构是不存在的,而这些缺陷形式会对晶体的宏观性能产生重要的影响,因此了解这些缺陷形式作用于材料性能的机制十分重要。之前的研究大多集中在缺陷或杂质对材料的电子态影响上,建立了具体的缺陷形式同能带结构、光学性质变化的联系。但对于晶体材料中缺陷体对材料声子态的研究却十分匮乏,而它们对声子态的影响往往同材料的结构相变和热力学等行为关系密切。在本章中我们建立了含有一个Bi空位的BiFeO3超晶胞结构,基于第一性原理和冻声子法分别计算了完整晶体和含缺陷晶体的声子态性质,包括声子谱、声子态密度、热容等。结果显示,在声子谱高频区域光学支声子振动强度减弱,而在低频区域的布里渊区中心附近则发现声学支声子有明显的软化。通过声子分波态密度的分析我们得知在含缺陷的BiFeO3体系中,氧原子和铋原子各自的振动贡献有明显减弱,从而造成了声子谱中振动模式的改变。其中声学支的软化较为明显,它直接影响了材料在低温附近的热力学行为。通过低温区热容的研究发现,由于铋空位的存在,热容随温度的变化出现异常,使得光学支声子更早的参与到热容的贡献之中。我们的研究结果进一步加深了对BiFeO3材料非本征态的认识,对于研究BiFeO3材料在低温附近的结构稳定性和磁性相变机制提供了理论依据。
第七章总结了本论文研究的主要工作内容,提出了本工作的创新之处,并对相关工作进一步深入的研究做了展望。
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.
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