Photoemission band symmetries and dipole active modes of crystalline filmsof vinylidene fluoride (70%) with trifluoroethylene (30%) across the ferroelectric transition(s)

The symmetry of the occupied electronic bands of crystalline films of vinyl idene fluoride (70%) with trifluoroethylene (30%) and the vibrational dipol e modes are identified using angle resolved photoemission and high resoluti on electron energy loss respectively. The symmetry of the occupied valence bands is preserved across the surface 'ferroelectric' phase transition. The temperature dependent changes in the electronic structure occur primarily in the conduction band and are accompanied by a surface dipole reorientatio n and a surface Brillouin zone doubling across the surface transition, thus these results strongly support a first order transition with a uniaxial po larization direction in these crystalline ferroelectric polymer films. Ther e is compelling evidence that this surface phase transition is an improper ferroelectric transition and discontinuous.