Origin of ferroelectricity in orthorhombic LuFeO3

We demonstrate that small but finite ferroelectric polarization (similar to 0.01 mu C cm(2)) emerges in orthorhombic LuFeO3 (Pnma) at T-N (similar to 600 K) because of commensurate (k = 0) and collinear magnetic structure. The synchrotron x-ray and neutron diffraction data suggest that the polarization could originate from enhanced bond covalency together with subtle contribution from the lattice. The theoretical calculations indicate enhancement of bond covalency as well as the possibility of structural transition to the polar Pna2(1) phase below T-N. The Pna2(1) phase, in fact, is found to be energetically favorable below T-N in orthorhombic LuFeO3 (albeit with very small energy difference) than in isostructural and nonferroelectric LaFeO3 or NdFeO3. Application of electric field induces finite piezostriction in LuFeO3 via electrostriction resulting in clear domain contrast in piezoresponse force microscopy images.