Structure Design Of Several Typical Hard Or Superconducting Functional Materials At High Pressure

High pressure can effectively change the interatomic distances and the electrons overlap between two adjacent atoms. As a result, the crystal and electronic structures will be changed forming a series of new high pressure phases with novel physical and chemical properties. Moreover, high pressure can also lower the kinetic energy thus promoting the formation of many new compounds that cannot be formed at normal conditions. The advent of these new high pressure phases and compounds provides us a wide area to find new hard or superconducting functional materials.As a functional material, hard materials have been widely used in national defense. aerospace, geology and mining, machinofacture and modern cutting-edge science. Hard materials usually refer to those with Vickers hardness higher than 20 GPa. while those above 40 GPa are classified as superhard materials. Analysis has shown that high valence electron density and strong and directional covalent bonding are the necessary conditions for hard materials. Recently, a series of fantastic borides. carbides and nitrides of transition metals have been synthesized at high pressures and temperatures, such as OsC, OsB2, OsN2, IrN2,WB4. It is noteworthy that in these new compounds, the transition metals can provide high valence electron density while light elements can form strong and directional covalent bonds. As a result, compounds formed by transition metals and light elements became a new family to search hard materials outside the traditional B-C-N-O systems. There still have a lot of problems in the study of these new systems, of which the most important and fundamental one is the determination of their crystal structures. It is still a difficult for experiment to determine their structure due to the large mass difference between transition metals and light elements. Therefore, to resolve their structures theoretically is imperatively. One of the major works of our thesis is to resolve the crystal structures of various typical compounds in this area, and the details as follows:(1) OsN2 and IrN2 are two new compounds recently synthesized by experiments at～50 GPa and 2000 K. Both compounds are regarded as potential hard materials due to their relatively large bulk modulus. However, their crystal structures remain elusive. In addition, it is highly possible that the synthesized structures are metastable since these compounds are both synthesized under high pressures. This release a question about what crystal structures OsN2 and IrN2 adopt at ambient and higher pressures. The absence of basic structural information prevents our deep understanding of these fantastic compounds. Therefore, we have extensively predicted the crystal structures of OsN2 and IrN2 by using first-principle calculations and crystal structure prediction technique. We have successfully resolved the synthesized structures of OsN2 and IrN2. Moreover, we have firstly promoted their ground-state and ultrahigh pressure structures. Our result has perfected the high pressure phase sequence of OsN2 and IrN2, which provides basic information for studying their properties. In addition, we have also extended our study to the yet synthesized RuN2 and RhN2. We have predicted the possible synthesized conditions of RuN2 and RhN2. which can provide important guidance for experimental synthesis.(2) The excellent mechanical properties of OsN2 imply that OsN could also be a good candidate for hard material. However, OsN has not been synthesized yet in experiment and its crystal structure remains open. By using first-principle calculations and crystal structure prediction technique, we have predicted two orthorhombic structures for OsN which are energetically more stable than previous guessed structures. Our results revealed that OsN is indeed a hard material and can be synthesized at high pressure. We have successfully designed a hard material theoretically and provided a possible synthesized condition.(3) OsC has been synthesized about fifty years ago. However, the crystal structure of OsC remained unsolved. Phonon calculations have revealed that the previous proposed structures are all dynamical unstable. By using "frozen-phonon" technique, we have predicted a more reasonable structure for OsC. Our results have clarified the long-term controversy on the crystal structure of OsC.Due to the special properties, superconductor has been widely used in electric transportation, transportation, information transfer and various energy areas. Since the discovery of superconductivity, the research of superconductors with high superconducting temperature (Tc) is one of major targets of scientific study. Base on the BCS theory. Tc is proportional to the Debye temperature, while Debye temperature is inversely proportional to the mass of a material. Therefore, it is suggested that the lightest element hydrogen should possess an extremely high Tc. However, solid hydrogen is insulator at ambient conditions and is impossible to be a superconductor. It is well-known that high pressure can effectively reduce the band gap and realize insulator to metal transformation. Therefore, the search for high pressure metal phase of hydrogen became a topic of high pressure study. However, the experimental metallization is yet achieved even at the pressures up to～300 GPa, above which experiment remains a great difficulty. Subsequently, Ashcroft proposed that hydrogen-rich compounds would also present high superconducting critical temperatures, while becoming metallic at lower pressures due to the chemical precompression of hydrogen. Therefore, hydrogen-rich compounds are considered as good candidates to search for high Tc superconductors. Although a lot of achievements have been made in this area, there are still many problems. Among these problems, the most fundamental one is the difficulty on the determination of their crystal structure. The experimental structure determination remains a major challenge since x-ray diffraction data cannot provide the structural information of hydrogen due to its relatively small scatter factor. Unfortunately, the absence of structure information has prevented the metallization and superconductivity studies of these hydrogen-rich compounds. Another major work of our thesis is to predict the crystal structure of two typical hydrogen-rich compounds SiH4(H2)2 and YH3. and the details as follows:(1) SiH4(H2)2, composed by two full-shell molecules SiH4 and H2. is a new type of hydrogen-rich compound which can only synthesized at high pressures. The structure of SiH4(H2)2 contain abundant molecular H2. According to the previous studies on hydrogen-rich compounds, we have found that the "H2" unit is mainly responsible for the large Tc. Therefore, we naturally guess that SiH4(H2)2 could also possess extremely high Tc when metallization. However, the crystal structure of SiH4(H2)2 is still an open question. By using first-principles calculations and structure prediction technique, we have solved the crystal structure of the SiH4(H2)2 and surprisingly found that molecular H2 in SiH4(H2)2 are orientationally disordered at low pressures. Moreover, the results at high pressure revealed that SiH4(H2)2 transformed to a superconductor with an extremely high Tc of 98-107 K at 250 GPa. Analysis suggests that the direct interactions between H2 and SiH4 molecules at high pressure play the major role on the high superconductivity of SiH4(H2)2, while the contribution from H2 vibrons is minor. The current study will inevitably stimulate the future experiments on SiH4(H2)2 and have important implication on other high pressure H2-containing compounds, e.g., H2O-H2, CH4-H2, NH3BH3-H2, Ar-H2, and Xe-H2.(2) As a potential high temperature superconductor, YH3 have also received much attention. Recent theoretical studies have shown that YH3 transforms to a superconductor at 17.7 GPa with a Tc of～40 K and undergoes a mysterious superconductor-metal-superconductor transition under pressure. YH3 possess the lowest reported pressure for hydrogen-rich materials to date. What structures YH3 adopts at higher pressures are valuable research subject. By using first-principles calculations and structure prediction technique, we have predicted two new structures for YH3 at high pressures, which provide basic information for further exploration of its superconductivity.