  Proximity to phase transitions (PTs) is frequently responsible for the
largest dielectric susceptibilities in ferroelectrics. The impracticality of
using temperature as a control parameter to reach those large responses has
motivated the design of solid solutions with phase boundaries between different
polar phases at temperatures (typically room temperature) significantly lower
than the paraelectric-ferroelectric critical temperature. The flat energy
landscapes close to these PTs give rise to polarization rotation under external
stimuli, being responsible for the best piezoelectrics so far and a their huge
market. But this approach requires complex chemistry to achieve
temperature-independent PT boundaries and often involves lead-containing
compounds. Here we report that such a bridging state is possible in thin films
of chemically simple materials such as BaTiO3. A coexistence of tetragonal,
orthorhombic and their bridging low-symmetry phases are shown to be responsible
for the continuous vertical polarization rotation, recreating a smear
in-transition state and leading to giant temperature-independent dielectric
response. These features are distinct from those of single crystals,
multi-domain crystals, ceramics or relaxor ferroelectrics, requiring a
different description. We believe that other materials can be engineered in a
similar way to form a class of ferroelectrics, in which MPB solid solutions are
also included, that we propose to coin as transitional ferroelectrics.
