Toward structure-property correlations in electroceramics: The chemical environment and the electronic structure of grain boundaries in strontium titanate

Electroceramics exhibit macroscopic nonlinear electrical properties owing to the presence of grain boundaries (GBs) and the phenomena associated with them. The mission of the research was to determine aspects of GB structure towards structure-property correlations in SrTiO{dollar}sb3{dollar}, a functional electroceramic. As a first important step the atomic level GB geometric structure was previously determined using both experimental and theoretical methods. This dissertation presents subsequent investigations of the chemical environment and electronic structure of the GBs. The correlations between structural aspects and existing literature on GB properties are also attempted.; With balanced experimental and theoretical investigations of both pristine and impurity incorporated GBs, a consistent picture between the GB segregation and space-charge phenomena was established. Experimental studies involved determination of chemical environment of both bicrystal and polycrystalline GBs using X-ray emission spectroscopy (XES) and electron energy loss spectroscopy (EELS). The nature of segregation, and the aspects of GB electronic structure and electrical activity were determined in connection with impurity incorporation using EELS and electron holography. Studies involved determination of valence and of substitutional behavior of impurities, and direct imaging and quantification of the electrostatic potentials across the GBs. The double-Schottky potential-barrier heights, the local charge density and electrostatic stress field distributions associated with these barriers, and the point defect density at individual GBs were derived directly from the holography results.; Owing to the complexity of the research, only perfect crystal (PC) and previously determined bicrystal GB geometries were considered for theoretical investigations. A density-functional (DF) self-consistent-field (SCF) discrete-variational (DV) embedded cluster calculation method was employed to determine the electronic structure of both pristine and aliovalent impurity incorporated clusters of these geometries. The charge- and spin-densities of states (DOS), Mulliken charge- and spin-electron populations, and charge- and spin-density distributions were determined for different orbitals of the cluster systems. The valence, nature of bonding and charge transfer behavior were also determined for the atoms of the cluster systems. The electronic structure thus determined forms an important bridge between segregation and space-charge phenomena and offers an explanation for the electrical properties exhibited by the GBs.