Studies On Structures And Properties Of Hexagnal And Rhombohedral Ordered Solid Solutions

Hexagonal and rhombohedral solid solution materials are candidates for many potential valuable applications. The studies on these materials aim at probing into the internal relation of properties and structures and predicting new materials, as well as providing theoretical supports for their actual applications and technical researches.We have completed the studies on the structures and properties of hexagonal and rhombohedral solid solution materials using the first principles method based on the density functional theory (DFT) combined with the concentration wave theory (CWT). The typical results of the dissertation are as follows:Using the CWT, we have determined 7 ordered structure types of simple rhombohedral lattice, and applied them to predict the structures of corundum type cation ordered solid solutions, which are supported by experimental results reported. For LixTiS2, stage-2 LixTiS2, AgxTiS2, and AgxTiTe2 series, we have constructed crystal cell models according to the CWT, and made studies on their structures and properties using materials modeling and simulating software package Materials Studio 4.0.In a range of 0≤x≤1, we have completed the geometry optimizations and total energy calculations for the 4 whole series with DFT-LDA scheme. For every series, the obtained lattice parameters assort well with each other and accord with experiment results, and especially, c0 of Li1/4TiS2 system is better than the results of analogous work in the literature. We have investigated the effects of intercalates staging in LixTiS2 systems, and found the complementary relation of the additional expansion and the compression of spacing between sandwiches, and presented reasonable explanations.Using formation energy to estimate the stability of ordered structures, we have predicted the structure types of stage-2 ordered Li1/4TiS2, ordered Ag3/4TiTe2, and other crystal systems. We have also proposed a qualitative relation of ionic conductivity and formation energy, and estimated that the order-disorder transition temperature and the ionic activation energy of AgxTiS2 are higher than those of LixTiS2, and the ionic conductivity of LixTiS2 is better than that of AgxTiS2, which are supported by experimental results reported.By virtue of DFT-GGA scheme, we have studied the optical properties of AgxTiTe2 series. The obtained reflectance spectra are in good accordance with the experimental results reported, and the absorption spectra show that AgxTiTe2 systems may be a new kind of photoelectronic materials for their good absorptive capacity for absorbing sunlight in a large range.