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

The symmetry of the occupied electronic bands of crystalline films of vinylidene fluoride (70%) with trifluoroethylene (30%) and the vibrational dipole modes are identified using angle resolved photoemission and high resolution 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 reorientation and a surface Brillouin zone doubling across the surface transition, thus these results strongly support a first order transition with a uniaxial polarization direction in these crystalline ferroelectric polymer films. There is compelling evidence that this surface phase transition is an improper ferroelectric transition and discontinuous.