  Here, we report a combined experimental-theoretical study showing that
collective application of rare earth doping on A-site and epitaxial strain to
ferroelectric bismuth titanate does not lead to a very large c-axis
polarization as reported previously. Further first principles calculations
based on the examination of polarization tensor suggest that simultaneous Bi
and Ti site doping could result in moderate polarization along c-axis of
bismuth titanate which is typically a preferential axis of film growth and thus
enabling c axis oriented films to have appreciable polarization. This approach
could also be applicable to other ferroic oxides where one can correlate the
doping, epitaxial strain, and polarization to design materials compositions
resulting in epitaxial films grown along desired directions yielding
substantial polarization.
