Stability of complex host-guest structure and superconductivity in the alkali and alkaline earth metals under high pressure

Abstract

This thesis investigates the existence of host-guest structure under high pressure in elemental metals such as Mg, Li, Ca, Sc and As. The new stable structure of magnesium (Mg) is predicted by ab initio random structure searching (AIRSS) technique under high pressure. The I4/mmm and Pnma structures have been predicted to give the normal metallic state at the pressure above 160 GPa. Moreover, for Li, the AIRSS technique is used to identify the host-guest (HG) of lithium (Li). The distribution of electrons between the host-host atoms has also been investigated by electron localization function. The strongly localized electron of π bond has led to the stability of the HG structure under high pressure. The calculation result from Ca in the metallic phase has led to the discovery of high-Tc superconductivity, with a Tc of 25.2 K at 160 GPa. The Tc of this monoclinic Cc structure is in good agreement with experiment. In addition, the explanation of Sc-II revealed that the 3cH and the 4cG structures coexist which form the HG structure with CH/CG ratio of 4/3 at 70 GPa. The AIRSS technique predicts the high pressure structure of Sc-III to be P41212 and also reveals Tc of 8.36 K which is in good agreement with the experimental result. Furthermore, the body-centered tetragonal (bct) structure in arsenic (As) has been predicted using AIRSS technique at 150 GPa. The calculation suggests transition sequence from the simple cubic structure transforms to the host-guest structure at 41 GPa and then to the bct structure at 81 GPa. The spectral function of bct structure is higher than those of the bcc structure. This result is also reported to be Tc 4.2 K at 150 GPa.