  Piezoresponse force microscopy (PFM) is a powerful characterization technique
to readily image and manipulate ferroelectrics domains. PFM gives insight into
the strength of local piezoelectric coupling as well as polarization direction
through PFM amplitude and phase, respectively. Converting measured arbitrary
units to physical material parameters, however, remains a challenge. While much
effort has been spent on quantifying the PFM amplitude signal, little attention
has been given to the PFM phase and it is often arbitrarily adjusted to fit
expectations or processed as recorded. This is problematic when investigating
materials with unknown or potentially negative sign of the probed effective
electrostrictive coefficient or strong frequency dispersion of
electromechanical responses since assumptions about the phase cannot be
reliably made. The PFM phase can, however, provide important information on the
polarization orientation and the sign of the electrostrictive coefficient. Most
notably, the orientation of the PFM hysteresis loop is determined by the PFM
phase. Moreover, when presenting PFM data as a combined signal, the resulting
response can be artificially lowered or asymmetric if the phase data has not
been correctly processed. Here, we demonstrate a path to identify the phase
offset required to extract correct meaning from PFM phase data. We explore
different sources of phase offsets including the experimental setup,
instrumental contributions, and data analysis. We discuss the physical working
principles of PFM and develop a strategy to extract physical meaning from the
PFM phase. The proposed procedures are verified on two materials with positive
and negative piezoelectric coefficients.
