Observation of local non-centrosymmetry in weakly biferroic YCrO ₃

Using high-resolution neutron powder diffraction we have determined the average and local structure of YCrO₃ in order to explain the recently reported properties of YCrO₃. Unlike conventional ferroelectric systems, YCrO₃ is reported to possess a centrosymmetric crystal structure which is inconsistent with the development of weak electric polarization above the dielectric transition around 440K because it requires atomic off-centring. We have characterized the different length scales existent in YCrO₃ and found that, although the average crystallographic structure above and below the dielectric transition YCrO₃ is orthorhombic and centrosymmetric (Pnma), in the ferroelectric-like state it is locally non-centrosymmetric and Cr is displaced about 0.01 from its position along the z direction. We conclude that the local character of the Cr off-centring and the small value of the displacement observed could explain the weak ferroelectric-like polarization. This new concept of 'local non-centrosymmetry' might be of great importance for the understanding of unusual properties of other multifunctional materials as well.