  Epitaxial strain, layer confinement and inversion symmetry breaking have
emerged as powerful new approaches to control the electronic and atomic-scale
structural properties in complex metal oxides. Nickelate heterostructures,
based on RENiO$_3$, where RE is a trivalent rare-earth cation, have been shown
to be relevant model systems since the orbital occupancy, degeneracy, and,
consequently, the electronic/magnetic properties can be altered as a function
of epitaxial strain, layer thickness and superlattice structure. One such
recent example is the tri-component LaTiO$_3$-LaNiO$_3$-LaAlO$_3$ superlattice,
which exhibits charge transfer and orbital polarization as the result of its
interfacial dipole electric field. A crucial step towards control of these
parameters for future electronic and magnetic device applications is to develop
an understanding of both the magnitude and range of the octahedral network's
response towards interfacial strain and electric fields. An approach that
provides atomic-scale resolution and sensitivity towards the local octahedral
distortions and orbital occupancy is therefore required. Here, we employ
atomic-resolution imaging coupled with electron spectroscopies and first
principles theory to examine the role of interfacial charge transfer and
symmetry breaking in a tricomponent nickelate superlattice system. We find that
nearly complete charge transfer occurs between the LaTiO$_3$ and LaNiO$_3$
layers, resulting in a Ni$^{2+}$ valence state. We further demonstrate that
this charge transfer is highly localized with a range of about 1 unit cell,
within the LaNiO$_3$ layers. The results presented here provide important
feedback to synthesis efforts aimed at stabilizing new electronic phases that
are not accessible by conventional bulk or epitaxial film approaches.
