  The direction of ferroelectric polarization is prescribed by the symmetry of
the crystal structure. Therefore, rotation of the polarization direction is
largely limited, despite the opportunity it offers in understanding important
dielectric phenomena such as piezoelectric response near the morphotropic phase
boundaries and practical applications such as ferroelectric memory. In this
study, we report the observation of continuous rotation of ferroelectric
polarization in order-disorder type LiNbO3 thin films. The spontaneous
polarization could be tilted from an out-of-plane to an in-plane direction in
the thin film by controlling the Li vacancy concentration within the hexagonal
lattice framework. Partial inclusion of monoclinic-like phase is attributed to
the breaking of macroscopic inversion symmetry along different directions and
the emergence of ferroelectric polarization along the in-plane direction.
