We have studied the structural phase transition of multiferroic YMnO3​ from
first principles. Using group-theoretical analysis and first-principles density
functional calculations of the total energy and phonons, we perform a
systematic study of the energy surface around the prototypic phase. We find a
single instability at the zone-boundary which couples strongly to the
polarization. This coupling is the mechanism that allows multiferroicity in
this class of materials. Our results imply that YMnO3​ is an improper
ferroelectric. We suggest further experiments to clarify this point.Comment: published version, PRB (rapid comm), slight change in presentatio