| With the progress in density functional theory (DFT) and its numerical methods, DFT based first-principles calculation has become a routine method for condensed matter theory, quantum chemistry and material science. In this dissertation, we study the functional units and the structure-property relationships of some novel inorganic functional materials by using DFT method. The concerned materials include photoelectric functional crystals, superhard materials, metal oxide functional materials, and low-dimensional materials; the concerned properties include crystal structures, electronic, optical, and lattice dynamic properties; the concerned functional units include the cations, lone pair electrons, bonding characters, crystal symmetry, dimensionality, and pressure (electron, ion'chemical bonding'crystal structure'dimensionality'external factor). The main research contents and results list below:In the first chapter, we introduce the research ideas and significance on structure-property relationships based on functional units, as well as the basic concepts, physical and chemical features of several novel inorganic materials, and review their recent research progresses. On this basis, we briefly introduce the research ideas and concents of this dissertation.In chapter 2, we briefly introduce the basic theoretical framework and review its recent progress and application. Finding good approximation for exchange-correlation functional is one of the main targets in DFT research. With the development of functional research, DFT leads to more and more accurate results from the initial LDA, GGA to hybridization functional. On the basis, we introduce the density functional perturbation theory (DFPT) and several kinds of familiar computational softwares are summarized in brief.Starting from chapter 3, we begin to focus on the functional units, structures and properties of real materials from first principles. In chapter 3, we study two kinds of important photoelectric functional crystals-XInSe2(X=Cu, Ag, Li) and X2CdYS4 (X=Cu, Li; Y=Ge, Sn). We study their crystal structures, electronic, optical and lattice dynamic properties by using first principles method, aiming to discuss the influence of cations and crystal structures on their physical properties. The results indicate that the metal cations have great influence on the physical properties of XlnSe2 and X2CdYS4 crystals. The introduction of Li cation could dramatically improve their physical properties. For XInSe2 crystals, the band gap of LilnSe2 is larger than those of CuInSe2 and AgInSe2. The main reason is that the hybridization between Cu 3d/Ag 4d and Se 4p states enhances the valence band maximum (VBM) and reduces the band gap of CuInSe2 and AgInSe2. However, due to the absence of d electron of Li, the hybridization between In sp and Se 4p states is the main contribution of chemical bonding, which reduces the VBM and enhances of the conduction band minimum (CBM), and thus enlarges the band gap of LiInSe2. Also, the ionic character of Li-Se bond enlarges the LO/TO splitting of high-frequency polar modes. The vibration of Li makes the phonon frequency and Debye temperature larger, which may further enlarge the laser damage threshold and is benefit for its application in nonlinear optic devices. For X2CdYS4 crystals, X cations have dramatic influence on their physical properties whereas Y cations have little. When X is Li, the compound demonstrates excellent photoelectric properties. On the other hand, the difference in crystal structures makes the physical properties anisotropic. For LiInSe2, the 6-coordinated a-NaFeO2-type phase has smaller band gap and phonon frequency with large anisotropy, which is against the nonlinear applications. So the stableβ-NaFeO2-type LiInSe2 is the outstanding performance of the photoelectric function crystals.In chapter 4, we focus on one kind of important superhard materials, namely B-C-N compounds. They are not only the potential superhard materials, but also have wide application prospects in luminescent materials, semiconductor devices, sensors, light conversion materials, etc. due to their special optical, thermal, mechanical and electronic properties. In experiment, many kinds of B-C-N materials have been synthesized. However, the fine crystal structures of B-C-N compounds have not been confirmed and some basic scientific issues have not been made clear due to the complexity of their components and structures. In this chapter, we take BC2N and BC6N as examples to illustrate how to conform the fine crystal structure of B-C-N compounds by using first principles method. Also, we predict their electronic, optical, and lattice dynamic properties. We find that by simulating the infrared and Raman spectrum in the framework of DFPT, we can differentiate their fine crystal structures. The results indicate that the Raman spectrum of z-BC2N and t-BC2N has one strong Raman peak (B2 LO mode) at 1297 and 1313cm-1, respectively, corresponding to the experimental B2 LO peak at 1324.8cm-1 of Raman spectrum. Therefore, we could regard z-BC2N or t-BC2N as the fine crystal structure of BC2N. By using the same method, we find the infrared and Raman spectrum of two BC6N structures is quite different. We find that the crystal structures of BCeN can be differentiated by IR spectrum than Raman spectrum. Due to the absence of the experimental infrared and Raman spectrum of BC6N, our theoretical work will supply some prediction and guidance to experimental workers. On the other hand, via calculating the electronic, optical, and lattice dynamic properties of BC2N and BC6N, we find that the differences in their properties are arising from their different bonding types and bonding numbers. Also, we find that BC2N and BC6N may have excellent thermal properties. The heat capacities and Debye temperatures of BC6N are superior to those of BC2N.In chapter 5 we focus on the metal oxide materials TeO2 and SnO2. The application of metal oxide materials is wide. People hope to control the properties of materials by using the special structure itself, such as the lone pair electrons. Also, people would like to control their physical and chemical properties by some external methods so as to broadening their application fields. High pressure is one of the important methods. The changes of distance between atoms and electronic structures induced by pressure may lead to the changes of crystal structures, mechanical, optical, thermal, magnetism, and electric properties of the materials. Therefore, by using high pressure research method, we could find out some new phenomenon which could not be found in the room pressure. And we can further reveal some new rules, which make us better understand the interaction between the microscopic particles of materials. In this chapter, we study the influence of the lone pair electrons on the physical properties of TeO2, and the changes of structures, electronic, and optical properties of high pressure phases of SnO2 with pressure. The results indicate that Te 5s2 lone pair electrons have some contributions to the VBM of the DOS, indicating that the influence of the lone pair electrons on the electronic properties can not be ignored. The lone pair electrons make the space arrangement of TeO4 units distorted, forming a series of pores in the crystals. This is the main reason for the instability of TeO2 crystals. The special structures may broaden their applications in nonlinear optics, hydrogen storage, etc. Besides, the research confirms that TeO2 is one kind of materials with extreme high dielectric constant. The static dielectric constants ofα-、β-andγ-TeO2 are 22,27 and 21, respectively. Theβ-TeO2 shows larger optical anisotropy. On the other hand, with the increase of the pressure, SnO2 undergoes phase transition. The coordination number of high pressure phases is larger than that of the stable phase at room pressure. When the pressure increases, the charge transfer between Sn and O becomes larger, and the covalence of Sn-O bonds becomes stronger. The main differences of the density of states between high pressure SnO2 and stable one focus on the conduction bands. With the increase of the pressure, the hybridization between Sn 5s and O 2p states has been enhanced, the band gaps has been enlarged, and the optical properties curves has been blueshifted.The above three chapters concern bulk materials only. In chapter 6, we study the stability and electronic structures of some low-dimensional materials. Low-dimensional materials have some special characters different from the bulk, such as the small size effect, quantum confinement effect, etc. NWs and NRs all have been widely used in nanodevices. Although they are all one-dimensional materials, the confinement effects of the two are different. In this chapter, we study the Ag doping effect in ZnO NWs and NRs. We also discuss the influence of surface, edge, shapes of cross-section on the electronic structures of Ag doping models. The results indicate that Ag prefers to substitute the surface Zn atom in NWs and NRs. The substitutional Ag to O position and interstitial Ag are all instability. The lowest formation energy of Ag substitution in ZnO NRs is lower than that in ZnO NWs. For Ag-doped NW models, the electronic properties are directly related to the cross sections and surfaces. Ag substituting the Zn atom in the zigzag surface of triangular ZnO NWs is benefit for the p-type conduction of ZnO NWs. The increase of Ag concentration will damage the p-type conduction. For Ag-doped NR models, the electronic properties are directly related to the thickness and edge states of NRs. Ag substituting Zn atoms located at the middle of NRs is better for p-type doping. The hole mobility of Ag-doped bilayer ZnO NRs is not so good while we can improve it by increasing the Ag doping concentration.In chapter 7, we summarize the conclusions and innovative points of this dissertation, and preview the further studies. |
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