| dcterms.abstract | Materials exhibit new physical and chemical properties under pressure. Discovering the new high-pressure structures, especially those which can be preserved as metastable structures at ambient pressure, are very instructive for practical applications. Compared with high-pressure experiments, theoretically investigating materials' behavior under high pressure is more efficient and less expensive. One motivation of this thesis is to discover new structures under high pressure using USPEX method and to evaluate the stabilities of high-pressure phases at ambient pressure. Another motivation is to calculate and analyze the electronic and mechanical properties of the new high-pressure structures at ambient pressure. With these motivations, we have studied the Hf-O, Zr-O and Hf-N systems under high pressure. In the Hf-O system, new compounds Hf5O2, Hf3O2, HfO and HfO3 are discovered to be thermodynamically stable at certain pressure ranges. Two new high-pressure phases are found for Hf2O: one with space group Pnnm and anti-CaCl2-type structure, another with space group I 41/amd. Pnnm-HfO3 shows an interesting structure, simultaneously containing oxide O2- and peroxide [O-O]2- anions. Remarkably, it is P?62m-HfO rather than OII-HfO2 that exhibits the highest mechanical characteristics among Hf-O compounds. Pnnm-Hf2O, Imm2-Hf5O2, P?31m-Hf2O and P?4m2-Hf2O3 phases also show superior mechanical properties. Theoretically these phases can become metastable phases at ambient pressure. Within 120 GPa, Zr-O compounds and Hf-O compounds share many similarities, but four new phases Cmmm-Zr3O, R?3c-Zr3O2, Pmma-ZrO and Fe2P-type ZrO2 (P?62m) appear in the Zr-O system. Actually Pmma-HfO and Fe2P-type HfO2 appear in the Hf-O system at higher pressure (above 120 GPa) due to lanthanide contraction. OII-ZrO2 (Pnma) transforms into Fe2P-type ZrO2 at 102 GPa. Meanwhile, Fe2P-type ZrO2 and P?62m-ZrO have similar structures based on !-Zr. However, the calculated Vickers hardness of Fe2P-type ZrO2 (5.6 GPa) is inferior to that of P?62m-ZrO (14.1 GPa). The hardness of P?62m-ZrO (14.1 GPa) is lower than that of P?62m-HfO (16.1 GPa) and P?62m-TiO (16.6 GPa). On the whole, Zr-O compounds exhibit lower hardnesses and bulk moduli than that of Hf-O compounds. In the Hf-N system, metallic P63/mmc-HfN (calculated Vickers hardness 23.8 GPa) is found to be more energetically favorable than NaCl-type HfN at zero and high pressure. Moreover, NaCl-type HfN actually undergoes a phase transition to P63/mmc-HfN below 670 K at ambient pressure. Novel HfN10, which simultaneously has in?nite armchair-like polymeric N-chains and N2 molecules in its crystal structure, is discovered to be stable at moderate pressure above 23 GPa and can be preserved as a metastable phase at ambient pressure. At ambient conditions (298 K, 0 GPa), the gravimetric energy densities and the volumetric energy densities of HfN10 are 2.8 kJ/g and 14.1 kJ/cm3, respectively. | |
