  Single-crystal samples of LiCu2O2 with spin 1/2 Cu2+ chains of edge-sharing
CuO4 square planes (ribbon chains), have been characterized by X-ray
diffraction, thermogravimetric analysis, and magnetic measurements. Neither the
atomic deficiency nor the mixing of Cu and Li atoms has been found, indicating
that lattice defects conceived as a possible origin of the reported
multiferroic behavior can be excluded. Anomalies found in the data of specific
heat and neutron magnetic Bragg reflections show clear evidence that the system
exhibits successive magnetic transitions at TN1=24.5 K and TN2=22.8 K. Based on
the magnetic structures in the intermediate (TN2<T<TN1) and low temperature
(T<TN2) phases, determined by the combined studies of neutron scattering and
7Li-NMR measurements, we can consistently understand the fact that the
multiferroic properties are observed only below TN2 by considering existing
theories.
