  Domain walls in ferroelectrics exhibit a plethora of phases and
functionalities not found in the bulk. The interplay of electrostatic,
chemical, topological, and distortive inhomogeneities at the walls can be so
complex, however, that this obstructs their technological performance. In
tetragonal ferroelectrics like PbZrxTi1-xO3, for example, the desired
functional 180{\deg} domain walls within out-of-plane-polarized c-domains are
interspersed by in-plane-polarized a-domains and the associated network of
domain walls remains challenging to analyze. Here we use a combination of STEM
and optical second harmonic generation (SHG) to determine the relation between
strain, film thickness, local electric fields and the resulting domain and
domain-wall structures across the entire thickness of a set of PZT films. We
quantify the distribution of a-domains in the c-domain matrix of the films.
Using locally applied electric fields we control the a/c distribution and
induce the technologically preferable 180{\deg} domain walls. We find that
these voltage induced walls are tilted and exhibit a mixed Ising-N\'eel type
transverse rotation of polarization across the wall with a specific nonlinear
optical response.
