Influence of magnetic on ferroelectric ordering in LuMnO3

We have studied the influence of antiferromagnetic ordering on the local dielectric moments of the MnO5 and LuO7 polyhedra by measuring neutron powder-diffraction patterns of LuMnO3 at temperatures near T-N. We show that the coupling is weak, because the magnetic exchange coupling is predominantly in the ab plane of the MnO5 trigonal bipyramids, and the electric dipole moments, originating in the LuO7 polyhedra, are oriented along the hexagonal c axis. Anomalies in the dielectric properties near T-N are thus caused by the geometric constraints between the MnO5 and the LuO7 polyhedra.