Proper ferroelectric transition in the multiferroic YMnO₃

Compounds presenting coexistence of ferroelectricity and magnetism have interesting properties allowing the manipulation of electric and magnetic moments by magnetic and electric fields, respectively. These compounds are called multiferroics. While a lot of effort has been put into the search and design of new multiferroics, the nature of the mechanism of ferroelectricity and thus the nature of the coupling between the different degrees of freedom is still not well understood [1]. Among the multiferroics, the hexagonal (h-)RMnO3 are particularly interesting due the high polarization at room temperature (PS∼5.5 C/cm2). The h-RMnO3 exhibit TFE∼1000 K and TN∼100 K. We have studied above room temperature the parent compound YMnO3 of h-RMnO3 family. We observed for the first time using single-crystal high resolution synchrotron data, powder neutron diffraction, and dilatometry two phase transitions in YMnO3 associated with the ferroelectric transition and a tripling of the unit cell [2,3]. Using a complete group theoretical analysis and structure calculations, we identify the transition from a centrosymmetric to a ferroelectric state for the hexagonal RMnO3 as the succession of two paraelectric phases namely P63/mmc (high temperature) and P63/mcm (intermediate temperature) and one ferroelectric phase with P63cm symmetry (room temperature) [2,4]. We prove that this family of compounds is not improper ferroelectrics but proper ferroelectrics in agreement with the magnitude of the polarization. The polarization can be described by a correlated zigzag tilting of the MnO5 polyhedra due to the displacements of the apical oxygens instead of the basal plane oxygens.