Electronic structure of pristine apd solute-incorporated SrTiO3: II, grain-boundary geometry and acceptor doping

Grain boundaries in ceramics have a major influence on various mechanical a nd electrical properties of the material system. Nonlinear electronic prope rties of electroceramics are directly linked with the grain-boundary phenom ena caused by the variations in the crystallography and the chemical enviro nment, and consequent variations in the electronic structure of the grain b oundaries. In this Part II of the three-part report, the electronic structu re of pristine and acceptor-impurity-incorporated 36.8 degrees symmetric ti lt Sigma 5 grain boundary in SrTiO3 is investigated. A relaxed model of the atomic structure of this grain boundary derived by Ravikumar et al using l attice statics simulations based on pair-potential calculations has been us ed for electronic-structure calculations. This model is a very good approxi mation to the true relaxed structure, because it accurately reveals some of the structural features observed experimentally. The methodology of one-el ectron first-principle cluster calculations discussed in Part I has been us ed to study the pristine titanium- and strontium-centered grain-boundary cl usters. Clusters with a single acceptor impurity at the central titanium si te also have been considered in order to investigate the effects of impurit ies at the grain boundaries. As in Part I, no additional local lattice rela xations have been considered for the impurity-incorporated clusters. Calcul ations involve determination of the aspects of the electronic structure out lined in Part I. The influence of grain-boundary crystallography on local e lectronic structure is evaluated in terms of variations in densities of sta tes and spatial charge densities. The influence of the grain boundary on lo cal charge transfer and on impurity-induced changes in charge populations a lso is investigated. The role of impurity incorporation at the grain bounda ries-is discussed in comparison with impurity incorporation in the bulk. Th e calculations reveal an increased covalence in the nature of the Ti-O and Sr-O bonds at the SrTiO3 grain boundary. The optical bandgap at the grain b oundary is reduced because of the broadening of the O 2p valence and Ti 3d conduction states toward the Fermi level. An enhancement in the acceptor na ture of transition-metal impurities is observed at the grain boundary with an increase in the degree of association between the impurity ions and the oxygen neighbors. The variations in the electronic structure resulting from the variations in the crystallography at the grain boundary have implicati ons on grain-boundary segregation, space-charge, and conductivity phenomena associated with the impurities.