| Exploring the room temperature superconductors have always been the focus of the condensed matter physics. However, no matter the early discovered copper oxide superconductors in the 80s of the last century or the recently proposed iron based superconductors, they are far away from the real room temperature supcerconductors, due to the fact that their superconducting mechanism is remain elusive, there are many problems to be widely used and the superconducting transition temperature (Tc) are relatively low. Therefore, it is necessary to search for the superconductors with relatively high Tc in the traditional suconductors, whose superconducting mechanism is well known.In 2001, Japanese scientists Jun Nagmatsu et al. discovered that MgB2 with simplely layered structure is a very good superconductor, whose Tc reaches 40 K (much higher than 23 K of Nb3Ge discovered in 1973). This new discovery immediately aroused a tremendous stir in the field of superconductors. Further studies suggest that the superconducting mechanism of MgB2 can be reasonably characterized by the well known Bardeen-Cooper-Schrieffer theory, thus MgB2 is a typical traditional superconductor. As a simple compound, MgB2 has many potential advantages. It is very cheap, easily synthesized, and of prospects to be widely used. Therefore, the discovery of superconducting MgB2 strongly stimulates scientists'interest to search for the high Tc superconductors within the simple compounds. It is well known that 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 an insulator at ambient conditions and thus it is impossible to be a superconductor. It is well-known that high pressure can effectively reduce the band gap and make an insulator transform to a metal. Therefore, searching for high pressure metallic phase of solid hydrogen became a topic of high pressure science. However, experiment did not find the metallization of hydrogen even at the pressures up to ~ 300 GPa, above which experiment remains a great challenge. Recently, it has been proposed by Ashcroft et al. that in group IVa hydrides (CH4, SiH4, GeH4 and SnH4) hydrogen has already undergone the chemical precompression exterted by heavy elements and once impelled by further external pressure it can easily enter a metallic phase. In addition, there are again eight electrons per formular unit in group IVa hydrides, which is similar to the case of MgB2. Therefore, hydrogen-rich compounds are considered as good candidates to search for high Tc superconductors within the reach of the experimental diamond anvil cell. The major work of this thesis is to explore the high-pressure structure and superconductivity of hydrogen-rich compounds such as CH4, GeH4, SnH4, GaH3 and transition metal hdyrides, and the details are as follows:1. CH4 has a very complex and poorly understood phase diagram at low pressure. Under high pressure, it is expected that CH4 becomes chemically unstable and undergoes a chemical dissociation. Experiments conducted at high temperatures suggested that CH4 decomposes into diamond and hydrogen at 10 ~ 50 GPa, while theoretical studies proposed that CH4 polymerizes into heavier hydrocarbons (below 100 GPa) and dissociates fully to form diamond only at pressures above 300 GPa. However, room-temperature compression experiments on CH4 did not reveal its dissociation up to ~ 200 GPa. To give the low-pressure structures and the specific decomposition pressure of CH4 under high pressure, resolve the controversy over its decomposition product, and explore the possibility of its superconductivity under pressure, here we use an ab initio evolutionary algorithm USPEX to explore the behavior of CH4 at high pressures (? 20 GPa) and low temperatures. We found that CH4 is stable in the molecular structure at low pressure and we predicted three novel insulating molecular phases with P212121, Pnma, and Cmcm space groups. Remarkably, under high pressure, methane becomes unstable and dissociates into ethane (C2H6) at 95 GPa, butane (C4H10) at 158 GPa and further carbon (diamond) and hydrogen above 287 GPa at zero temperature. Our computed pressure-temperature phase diagram by quasiharmonic approximation suggested that temperature lead to the observations of the unusually low formation pressure of diamond at high temperature and the failure of experimental observation of dissociation of CH4 at room temperature. Due to the fact that CH4 will dissociate under high pressure and its low-pressure structures remain insulating up to very high pressures, it is impossible for this compound to be a metal at pressures reachable with current static high pressure techniques.2. Recently, there is no report on the high-pressure structures of GeH4, SnH4, and GaH3 in experiment. For GeH4, earlier theoretical studies simply borrowed the crystal structures of SiH4 as educated guesses based on structures known for other materials. There is a possibility that hitherto unexpected structures are stable instead. For SnH4, using simulated annealing and geometry optimization, John et al. predicted that the high-pressure phase of SnH4 has P6/mmm symmetry. Regretably, the crystal structure prediction method they used needs an initial structure. Therefore, if the initial structure is far away from the global best structure, it will easily fall into the local best structure. Moreover, their predicted P6/mmm structure is completely composed of"H2"units, which suggests a strong trend to decompose, indicating its unstability. Therefore, there is a large possibility for SnH4 to be stable in other structures under pressure. For GaH3, there is no any theoretical study on its high-pressure structures. Therefore, under high pressure, what structures GeH4, SnH4, and GaH3 will be stable in, what form H will exist in, whether these structures can be metalized and if metalized, whether they are of superconductivity, how many Tc are and what the superconducting mechanisms are. With these questions, through genetic methodology and particle swarm optimization combined with ab initio total energy computation, we extensively explored the high-pressure structures of GeH4, SnH4, and GaH3 under high pressure and obtained the following resulsts: (1) We found that GeH4 is unstable to decompose to Ge and H2 below 196GPa; SnH4 is unstable to decompose to Sn and H2 below 96GPa; GaH3 is unstable to decompose to Ga and H2 below 160GPa; (2) Under high pressure, GeH4 is stable in a monoclinic and layered C2/c structure; SnH4 is stable in orthorhombic Ama2 and hexagonal P63/mmc, which both contain hexagonal layers of Sn atoms; GaH3 is stable in cubic Pm3n, which is the same as the structure observed in AlH3. In this structure, Ga atoms locate at the body centered cubic lattice point and H atoms occupy the interstitial sites. (3) Surprisingly, the high-pressure structures of GeH4 and SnH4 both contain novel"H2"units, while GaH3 contains the ionic H atoms. (4) Study of the electronic band structures and density of states suggests that the high-pressure structures of GeH4, SnH4, and GaH3 are all metallic. Electron-phonon coupling calculations show that C2/c - GeH4 has a high Tc of 64 K at 220 GPa; P63/mmc - SnH4 has a high Tc of 62 K at 200 GPa; Pm3n - GaH3 has a much higher Tc , which reaches 86 K at 160 GPa; Our study suggest that GeH4, SnH4, and GaH3 are all very good superconductors under high pressure. (5) Further studies suggest that the novel"H2"units in GeH4 and SnH4 and the ionic H atoms in GaH3 make very important contribution to the respective high Tc.3. Due to the fact that the noble metals have comparable or larger electronegativity than hydrogen, noble metals do not form hydrides with hydrogen at ambient conditions. High pressure is able to produce a steep increase of chemical potential of hydrogen, whose reaction with noble metals might become feasible. Earlier experiments have indicated that high pressure might make it possible to synthesize RhH, PdH, and PtH. This quests us to perform thorough search on noble metal hydrides under high pressure. In addition, the pure noble metals remain remarkably stable under even extremly high pressure, whether the introduction of hydrogen into the noble metals can induce the lattice changes? To fulfill the task, here, we use a combination of particle swarm optimization technique on crystal structure prediction and first-principles calculations based on density functional theory to explore the reactivity of hydrogen with noble metals under high pressure. We demonstrated that except for Au, noble metals can commonly form monohydrides under high pressure even at zero temperature. We find universal high pressure structure features of these monohydrides---the structures either are stable or eventually transform into closed packed fcc or hcp with hydrogen occupying the octahedral interstitial sites of the metal sublattices. Of particular interests by introduction of hydrogen are the high pressure structural changes of these noble metals which, however, remain remarkably stable for pure elements. Our research highlights the key role of pressure played in the formation of these novel metal hydrides.4. Recent synchrotron x-ray diffraction (XRD) studies discovered that TiH2 will stable in the fcc structure at ambient conditions. Above 0.6 GPa, fcc will transform to I4/mmm, which will be stable up to at least 90 GPa without other phase transition within this pressure range. However, by comparing the experimental XRD patterns and the simulated XRD data of the I4/mmm structure, we found that the simulated XRD patterns of the I4/mmm structure can not produce all the experimental diffraction peaks at 90 GPa, especially the diffraction peak located between 16 oand 17 o. Thus, there may be other phase transition below 90 GPa. Transition metal Nb holds the record for the highest Tc (9.3 K) of an element at normal pressure. Moreover, many compounds of Nb such as Nb3Ge (23 K), NbC (11 K) and NbN (16 K) have the much higher Tc in the respective same kind of compounds. Recent theoretical studies suggested that Li and Na can react with more hydrogen to form hydrogen-rich compounds under high pressure. Therefore, if Nb can react with more hydrogen to form hydrogen-rich compounds and metallize under high pressure, it will have a large possibility to be a good superconductor with a high Tc. Here, using our newly developed particle swarm optimization method for crystal structure prediction, we extensively explored the structures of TiH2 and the most stable composition for Nb-H system under high pressure, and obtained the following results: (1) We predicted a phase transition from I4/mmm to P4/nmm at 63 GPa. Study of its thermodynamical and dynamical stability and comparing with the experimental X ray diffraction patterns, unit cell parameters suggest that TiH2 has transformed from I4/mmm to P4/nmm at 90 GPa, which revise the recent experimental conclusion. (2) It is found that NbH2 will always have the lowest formation enthalpy below 500 GPa, which implies that it is the most stable composition in the Nb-H system. Interestingly, the formation enthalpy of NbH3 is gradually close to that of NbH2 under pressure. Therefore, it is probable to synthesize metastable NbH3 under high pressure. Through Allen-Dynes modified McMillan equation, we found that NbH3 is a very good superconductor.
|
PDF Full Text Request