First Principles Studies of Complex Oxides for Energy Applications
thesisposted on 24.02.2014 by Alejandro F. Rebola
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
In this thesis I present the results and analyses of my first principles studies on two complex oxide materials with potential energy applications. In the first part, I concentrate on the misfit-layered Ca3Co4O9 (CCO), which stands out as one of the most promising thermoelectric materials for high temperature applications. Although CCO has been the subject of many experimental investigations, theoretical studies on CCO have been scarce and lagged behind experiment. This is mostly due to the complexity of the CCO structure, which is composed of two mutually incommensurate subsystems along one of the lattice directions. To model the non-periodic structure of this material, I follow an approach based on increasing-order “Fibonacci rational approximants”, which converge to the experimentally observed stoichiometry of CCO. Following this method and taking into account electron correlations, I show that the electronic and lattice properties of CCO can be determined from first principles, in good agreement with experimental findings. These results are then used in conjunction with the Boltzmann Transport Equation to calculate the thermal conductivity of CCO. In the second part of my work I focus on how to enhance the ionic conductivity of LaGaO3 (LGO), an excellent candidate to be used as a solid oxide electrolyte in future generations of lower temperature solid oxide fuel cells. Although LGO exhibits a very high ionic conductivity when doped with either Sr or Mg, one of the drawbacks of doping is that interactions between dopants and charged carriers often lead to a reduction in the conductivity. In order to overcome this limitation, I explore the idea of increasing the number of ionic carriers in LGO by the computational design of a heterostructure containing a negatively charged LGO//MgAl2O4 interface. From the theory of polar interfaces, it is known that such an interface requires some mechanism of charge compensation. In this case, this is accomplished by the spontaneous formation of oxygen vacancies in LGO. As a result, I show that by synthetizing such a polar LGO//MAO heterostructure it might be possible to enhance the ionic conductivity of LGO.