  The specific properties of incommensurate multiferroic phases resulting from
the coupling of order-parameter replicates are worked out using the
illustrative example of iron vanadate. The dephasing between the
order-parameter copies induces an additional broken symmetry phase
corresponding to the lowest symmetry of the system and varies critically at the
transition to the multiferroic phase. It reflects the temperature dependence of
the angle between paired spins in the antiferromagnetic spiral structure.
Expressing the transition order-parameters in terms of spin-density waves
allows showing that isotropic exchange interactions contribute to the
stabilization of the ferroelectric phase.
