First-principles Study Of High-pressure Properties Of New Functional Materials

With the development of science and technology, many new functional materials emerged. For its extensive use, new functional materials have attracted intensively attention in recent years. It is well known that pressure is an important parameter to tune physical properties. High-pressure research on new functional materials is now helping us to better understand and improve the physical properties of materials and providing useful information in the design and synthesis of new materials. In the present study, high-pressure properties of several new functional materials are investigated by using first-principles calculation method. The whole thesis is divided into eight chapters.In chapter 1, the background and significance of the high-pressure research on new functional materials are briefly introduced, and then the recent research progresses of several new functional materials studied in this thesis are reviewed.In chapter 2, a brief introduction to the first-principles calculation methods based on plane wave functions and pseudopotential is given firstly. Then the phase transition theory are introduced, such as the classification of phase transition, the features of pressure-induced phase transition and the methods for crystal structure prediction of high-pressure phases.In chapter 3, the pressure induced structural transition of NaBH₄ from?-NaBH₄ (P421c) to?-NaBH₄ (BaSO₄-type, Pnma) is investigated. The BaSO₄-type structure of high-pressure phase is testified theoretically for the first time. The calculated transition pressure is 9.66 GPa and agrees reasonably well with the experimental results (⁸.9 GPa). Our results correctly predict the experimental observed phase transition from?-NaBH₄ to?-NaBH₄ and demonstrate that this high-pressure transition may occur at low temperature. The poor agreement between previous theoretically prediction and experimental results have been settled in a certain extend.In chapter 4, the phase transitions, electronic structures and optical properties of Mg₂X (X=C, Si, Ge, Sn) under high pressure are investigated. The calculated results demonstrate that Mg₂X (X=C, Si, Ge, Sn) undergo two pressure-induced phase transitions from the anti-fluorite to anti-cotunnite and then from the anti-cotunnite to the Ni₂In-type structures. For Mg₂Si, Mg₂Ge and Mg₂Sn, the two high-pressure phase transitions are first-order. While, for Mg₂C, the previous phase transition is first-order and the later is second-order. When approaching the phase transition, the changes of lattice parameters of the anti-cotunnite Mg₂X (X=C, Si, Ge, Sn) show noticeable nonlinearities. This can be considered as a precursor of the phase transition. The electronic structure calculations show that the band gaps of Mg₂C become broader with the increase of the pressure. But for Mg₂Si, Mg₂Ge and Mg₂Sn, the reverse is true. The results show that they have become metallic at high pressure. Finally, the imaginary and real parts of the dielectric function for different structures Mg₂X (X=C, Si, Ge, Sn) are calculated. The results show that the optical properties of Mg₂X (X=C, Si, Ge, Sn) change drastically with increasing pressure.In chapter 5, the pressure effects on the structural stabilities and electronic properties of CaMgX (X=Si, Ge, Sn) are discussed. Our results successfully predict a continuous phase transition from pnma to Ni₂In-type structure for CaMgX (X=Si, Ge, Sn). In addition, we discuss the electronic structures of both the pnma and Ni₂In-type CaMgX (X=Si, Ge, Sn). At ambient pressure, the pnma structure CaMgSi and CaMgGe display semimetal behaviors and the pnma structure CaMgSn displays metal behaviors. At high pressure, a semimetal to metal electronic transition is found for CaMgSi and CaMgGe. While, for CaMgSn, the electronic structures are found to be quite insensitive to pressure, the significant changes are the bands become broader at high pressure.In chapter 6, a theoretical investigation on the structural stabilities and electronic properties of TiS₂ under high pressures has been performed. The results show that TiS₂ undergoes a first-order pressure-induced phase transition from its 1T-type structure to cotunnite-type structure. The calculated transition pressure 16.20 GPa agrees quite well with the experimental finding (20.7 GPa). Compared with 1T-type structure, the cotunnite-type high-pressure phase has a more compact structure with a large bulk modulus. In addition, we discussed the electronic structures of TiS₂. Our results suggest that the structural phase transition of TiS₂ from 1T-type to cotunnite-type structure at high pressure is followed by a semimetal to metal electronic transition. In chapter 7, high-pressure behaviors of BeF₂ are investigated theoretically. The results demonstrate that the sequence of the pressure-induced phase transitions of BeF₂ under 50 GPa is from the?-quartz, to coesite, rutile, and?-PbO2-type structures. Moreover, the electronic properties of different crystal structures BeF₂ are compared. The results show that the electronic structures of BeF₂ are fairly insensitive to the particular crystal structures, which determined mainly by the BeF4 tetrahedron (or BeF6 octahedra).In chapter 8, the contents of this dissertation are summarized and future directions of research are given.