Symmetry changes at the ferroelectric transition in the multiferroic   YMnO3
  We have identified, for the first time, the change in symmetry at the ferroelectric transition TFE near 1023K of the ferroelectromagnet YMnO3. This transition takes place 300K below the transition to the centrosymmetric state at TIP. Single crystal synchrotron diffraction coupled to a group theoretical analysis show that the paraelectric intermediate phase between TIP and TFE has P63/mcm symmetry. This proves that YMnO3 is a proper ferroelectric and not an improper ferroelectric, as suggested by a previous group theoretical assignment. The origin of the ferroelectricity is caused by a correlated tilting of the MnO5 polyhedra along the (100), (110) and (010) directions 
Gwilherm Nenert, Yang Ren, Harold T. Stokes, and Thomas T. M. Palstra