  Theoretical {\em ab initio} and experimental methods were used to investigate
the $x$Bi(Zn,Ti)O$_3$-(1-$x$)PbTiO$_3$ (BZT-PT) solid solution. We find that
hybridization between Zn 4$p$ and O 2$p$ orbitals allows the formation of
short, covalent Zn-O bonds, enabling favorable coupling between A-site and
B-site displacements. This leads to large polarization, strong tetragonality
and an elevated ferroelectric to paraelectric phase transition temperature.
nhomogeneities in local structure near the 90$^\circ$ domain boundaries can be
deduced from the asymetric peak broadening in the neutron and x-ray diffraction
spectra. These extrinsic effects make the ferroelectric to paraelectric phase
transition diffuse in BZT-PT solid solutions.
