Electronic Structure of Pristine and Solute‐Incorporated SrTiO3: I, Perfect‐Crystal Geometry and Acceptor Doping

Abstract

In Part I of this three‐part report, various aspects of the electronic structure of pristine and acceptor‐impurity‐incorporated perfect‐crystal SrTiO3 were investigated. An embedded‐cluster approach was used to perform first‐principles, self‐consistent‐field calculations. Within the framework of the local‐density functional approximation, one‐electron equations were evaluated using a discrete variational method to study pristine titanium‐ and strontium‐centered clusters of perfect‐crystal SrTiO3. Clusters with a single acceptor impurity substitution at the central titanium site also were considered. However, the relaxation of the atomic structure resulting from the incorporation of these single impurities were not considered in the calculations. Calculations involved determination of the densities of states and spin densities of states of the cluster atoms near the Fermi energy. Mulliken charge populations of atomic orbitals, valence of individual atoms, and nature of bonds between the atoms also were determined. Spatial distribution of charge density and spin density of valence band orbitals, which were included in the variational space, were determined for pristine and impurity‐centered clusters. The influence of impurity substitution at a titanium site on the local electronic structure was evaluated in terms of the variations in the densities of states and in the spatial distribution of charge densities. The role of local charge transfer and impurity‐induced changes in Mulliken populations also were investigated in connection with the electronic activity of SrTiO3. Possible mechanisms of electrical conductivity were studied. Cluster calculations revealed a mixed ionic‐covalent nature of the Ti‐O bond and purely ionic nature of the Sr‐O bond for the perfect‐crystal geometry of SrTiO3. The optical bandgap and the densities of states calculated using the cluster method were in good agreement with previous theoretical and experimental investigations. The methodology correctly predicted the acceptor nature and the trends therein of the transition‐metal impurities at the titanium sites. For pristine and impurity‐incorporated clusters, the densities of states, the charge‐ and spin‐density distributions, and the Mulliken charge population consistently converged to the same results, providing information regarding the electronic behavior of the SrTiO3.