Electronic Structure of Pristine and Solute‐Incorporated SrTiO3: II, Grain‐Boundary Geometry and Acceptor Doping

Abstract

Grain boundaries in ceramics have a major influence on various mechanical and electrical properties of the material system. Nonlinear electronic properties of electroceramics are directly linked with the grain‐boundary phenomena caused by the variations in the crystallography and the chemical environment, and consequent variations in the electronic structure of the grain boundaries. In this Part II of the three‐part report, the electronic structure of pristine and acceptor‐impurity‐incorporated 36.8° symmetric tilt σ5 grain boundary in SrTiO3 is investigated. A relaxed model of the atomic structure of this grain boundary derived by Ravikumar et al. using lattice statics simulations based on pair‐potential calculations has been used for electronic structure calculations. This model is a very good approximation to the true relaxed structure, because it accurately reveals some of the structural features observed experimentally. The methodology of one‐electron first‐principle cluster calculations discussed in Part I has been used to study the pristine titanium‐ and strontium‐centered grain‐boundary clusters. Clusters with a single acceptor impurity at the central titanium site also have been considered in order to investigate the effects of impurities at the grain boundaries. As in Part I, no additional local lattice relaxations have been considered for the impurity‐incorporated clusters. Calculations involve determination of the aspects of the electronic structure outlined in Part I. The influence of grain‐boundary crystallography on local electronic structure is evaluated in terms of variations in densities of states and spatial charge densities. The influence of the grain boundary on local charge transfer and on impurity‐induced changes in charge populations also is investigated. The role of impurity incorporation at the grain boundaries is discussed in comparison with impurity incorporation in the bulk. The 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 boundary 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 nature 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 implications on grain‐boundary segregation, space‐charge, and conductivity phenomena associated with the impurities.