Computational Study On The High-pressure Stable Structure And Properties Of H₆Se Hydride And LaLi Alloys

The research of superconducting materials has always been the focus of the scientists.Since the discovery in the early 20th century,theoretical studies on superconductivity have two important directions:one is to reveal superconducting mechanisms;on the other hand,due to zero resistance and complete diamagnetic resistance of superconducting materials whose characteristics are extremely novel,people have been constantly discovering new superconducting materials,and are committed to using theoretical prediction to provide important information to experiments,so that newly discovered high-temperature superconducting materials gradually move toward room temperature superconductivity.The theoretical prediction of superconductors can provide guidance for experiments.In this thesis,the genetic algorithm?GA?combined with first principles calculations was used to predict structures of H₆Se and LaLi under high pressures,then their energy band structures and density of states were calculated by Cambridge Sequential Total Energy Package?CASTEP?,and finally,to investigate the superconductivity of the H₆Se structures,the electron-phonon coupling?EPC?parameter,Eliashberg spectral function?²F???and superconducting critical temperatures T_c were calculated using the Quantum Espresso package.The theoretically predicted high-pressure stable structures of H₆Se were H₆Se-Pmma and H₆Se-C2/m.The results showed that the structure of H₆Se-Pmma was the most stable structure at the pressure range of 50-270 GPa,and when the pressure was above270 GPa,H₆Se-C2/m became the most stable structure.In addition,the thermodynamic stabilities of H₃Se-Im-3m and H₆Se-C2/m were compared,and at above 475 GPa H₆Se-C2/m would be thermodynamically stable and exceed H₃Se-Im-3m and both structures H₆Se-C2/m and H₆Se-Pmma exhibited metal properties and had superconductivity.The results showed that the superconducting temperature of H₆Se-Pmma structure was 19-39 K at 50-270 GPa.H₆Se-C2/m had a T_c of 50-87 K at 265-300 GPa.The theoretically high-pressure stable structure of LaLi and its properties were also predicted.We found a stable structures with the symmetries of I4₁/amd at 125-300 GPa,in which the I4₁/amd structure was transformed by C2/m at 100 GPa,and I4₁/amd kept thermodynamic stabilities and dynamic stabilities.