Observation of local non-centrosymmetry in weakly biferroic YCrO<SUB>3</SUB>

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 440 K 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 &#197; 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