  The origins of enhanced piezoelectric coupling along nonpolar
crystallographic directions in ferroelectric BaTiO3 are investigated using in
situ neutron spectroscopy. It is observed that an electric field applied away
from the equilibrium polarization direction causes changes in the phonon
spectra that lead to an increase in the interaction between the transverse
acoustic and transverse optic branches (TA-TO) near the Brillouin zone center.
This provides a direct lattice dynamics mechanism for enhanced
electromechanical coupling, and could act as a guide for designing improved
piezoelectric materials.
