  Functional properties of ferroelectrics and their change with time depend
crucially on the defect structure. In particular, point defects and bias fields
induced by defect dipoles modify the field hysteresis and play an important
role in fatigue and aging. However, a full understanding on how order,
agglomeration and strength of defect dipoles affect phase stability and
functional properties is still lacking. To close these gaps in knowledge, we
screen these parameters by \textit{ab\ initio} based molecular dynamics
simulations with the effective Hamiltonian method for the prototypical
ferroelectric material (Ba,Sr)TiO$_3$. Our findings suggest that the
{\it{active surface area}} of the defects, rather than the defect concentration
is the decisive factor. For a fixed defect concentration, clustering reduces
the {\it{active surface area}} and thus the defect-induced changes of phase
stability and field hysteresis. Particularly planar agglomerates of defects
appear as promising route for the material design as their impact on the field
hysteresis can be controlled by the field direction and their impact on the
phase stability shows a cross-over with the strength of the defect dipoles. For
this agglomeration, we furthermore find that pinched field hysteresis, which is
beneficial, e.g. for energy storage, can be achieved for a wide range of defect
dipole strengths and thus is not too sensitive to the choice of dopants.
