Cu3Nb2O8: a multiferroic with chiral coupling to the crystal structure
  By combining bulk properties, neutron diffraction and non-resonant X-ray diffraction measurements, we demonstrate that the new multiferroic Cu$_3$Nb$_2$O$_8$ becomes polar simultaneously with the appearance of generalised helicoidal magnetic ordering. The electrical polarization is oriented perpendicularly to the common plane of rotation of the spins --- an observation that cannot be reconciled with the "conventional" theory developed for cycloidal multiferroics. Our results are consistent with coupling between a macroscopic structural rotation, which is allowed in the paramagnetic group, and magnetically-induced structural chirality. 
R. D. Johnson, Sunil Nair, L. C. Chapon, A. Bombardi, C. Vecchini, D.   Prabhakaran, A. T. Boothroyd and P. G. Radaelli
cond-mat.str-el