First-principles study of the crystal and magnetic structures of multiferroic Cu2OCl2

Abstract

Recently, the discovery of multiferroicity in pyrochlore-like compound Cu2OCl2 has generated significant interest, and several studies have been performed in this area. This transition metal oxychloride is unique because the divalent copper atoms create an correlated insulator and the pyrochlore lattice tends to frustrate spins. From neutron powder diffraction measurements, an incommensurate magnetic order of the ordering vector emerges below the Néel temperature of 70 K. At this temperature or slightly above, ferroelectricity (FE) or antiferroelectricity, accompanying a lattice distortion, has been observed. Experimentally, some discrepancies remain. In this paper, we report our first-principles simulation results by evaluating the possible lattice and spin spiral states. We found that the Fddd structure is not more stable than , which is supported by our reexamination of the x-ray diffraction data. In addition, we find that after we include magnetism in the calculation, it predicts that the lattice with a helical (proper screw) spin structure is energetically more stable than other spin configurations. Our results indicate charge-order-driven FE that subsequently induces magnetism.