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Title: The atomistic modelling approach to designing new metal-boride superconductors
Author: Shah, Sheena
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2013
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A series of metal-boride compounds have been studied using density functional theory with the goal of discovering and designing new superconductors with the highest possible superconducting critical temperature (T c). The materials design process involves predicting crystal structures using an evolutionary algorithm followed. by the calculation of the Tc for any metallic structures using the Migdal-Eliashberg theory. The reasons for superconductivity are rationalised through examining the electronic and vibrational features of the system. Finally, the stability of metal-boride compounds is studied to find hints to speed up the superconductor design process. An ambient pressure~ ground state structure of oPlO-FeB4 is proposed as being ne of the rare iron-based BCS-type superconductors, with a Tc of 15 - 20 K. Unlike MgB2 where the Fermi level is composed of boron electronic states, here 2/3 of the states at the Fermi level originate from the iron atom and only 1/3 from the boron atom. An extensive study has been done to show that the structure is neither ferro- nor anti-ferromagneic. CrB4 is also predicted to form same oP10 structure as FeB4 but is not expected to be superconducting. Experiments have recently confirmed the predicted oPlO FeB4 and CrB4 structures, with superconductivity being observed in FeB4. The whole CazB1_z composition range has been studied at ambient and gigapascal pressures, proposing stable high pressure superconducting compounds for CaB, CaB2 and CaB6.12S ' The highest Tc predicted is 5 K for CaB at 30 GPa. The rest of the compounds have Tc's on the order of 1 K showing that whilst may be possible to predict very staple structures, this is not so good for discovering a superconductor with a high Tc. Staple high pressure forms of CaB4 and CaBa are also discussed where their pressure induced crystallographic phase transitions are explained through comparing with alkaline earth analogues. The stability of a particular boron network at any pressure is shown to be heavily dependent on the metallic ion size.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available