  In multiferroic BiFeO3 thin films grown on highly mismatched LaAlO3
substrates, we reveal the coexistence of two differently distorted polymorphs
that leads to striking features in the temperature dependence of the structural
and multiferroic properties. Notably, the highly distorted phase
quasi-concomitantly presents an abrupt structural change, transforms from a
hard to a soft ferroelectric and transitions from antiferromagnetic to
paramagnetic at 360+/-20 K. These coupled ferroic transitions just above room
temperature hold promises of giant piezoelectric, magnetoelectric and
piezomagnetic responses, with potential in many applications fields.
