  The application of modern layer-by-layer growth techniques to
transition-metal oxide materials raises the possibility of creating new classes
of materials with rationally designed correlated electron properties. An
important step toward this goal is the demonstration that electronic structure
can be controlled by atomic composition. In compounds with partially occupied
transition-metal d shells, one important aspect of the electronic structure is
the relative occupancy of different d orbitals. Previous work has established
that strain and quantum confinement can be used to influence orbital occupancy.
In this paper we demonstrate a different modality for orbital control in
transition-metal oxide heterostructures, using density-functional band
calculations supplemented by a tight-binding analysis to show that the choice
of nontransition-metal counterion X in transition-metal oxide heterostructures
composed of alternating LaNiO3 and LaXO3 units strongly affects orbital
occupancy, changing the magnitude and in some cases the sign of the orbital
polarization.
