The coupling between an electric field and mechanical stress was investigat
ed, focusing on the incommensurate (IC) phase of K2ZnCl4 with the ferroelec
tric commensurate (C) phase. The coupling by both external fields is derive
d theoretically by the Landau theory. Ln order to prove the theoretical res
ults, the electric field dependence of dielectric constants was studied und
er the mechanical stress of 0 Mpa, 1.5 MPa and 2.5 MPa. Under the stress of
2.5 Mpa, the electric field dependence of chi shows the anomalous behavior
from the IC phase, and it corresponds to the phase transition from the IC
phase to the pseudo-C phase. Such a behavior suggests the coupling between
the polarization and the strain in the IC phase.