Hidden order in hexagonal RMnO3 multiferroics (R = Dy-Lu, In, Y, and Sc)

Hexagonal RMnO3 manganites are improper ferroelectrics in which the electric polarization is a by-product of the tripling of the unit cell. In YMnO3, there is a second transition at similar to 920 K whose nature remains unexplained. We argue that this transition can be seen as a sort of hidden order in which a residual symmetry displayed by the trimerization order parameter is spontaneously broken. This additional order gives rise to 12 structural domains instead of six, and structural domain boundaries that can be either ferroelectric or nonferroelectric domain walls. We also suggest a generic P6(3)cm <-> P3c1 <-> P (3) over bar c1 phase diagram, fully realized in InMnO3.