  LiCu_2O_2 single crystals were studied in the temperature range 300-1100 K by
means of heating-cooling curves of differential thermal analysis (DTA),
thermogravimetry (TG), X-ray powder diffraction and electrical measurements. A
reversible first-order phase transition between orthorhombic and tetragonal
phases was found to take place at 993 K. At the transition, a peak is observed
in the DTA curves, as well as jumps of the unit cell parameters and electrical
resistivity. Considering the crystal structure of LiCu_2O_2 and the entropy
change associated with the phase transition, it is concluded that the phase
transition is related to processes of order-disorder of the Cu2+ and Li+
cations onto their crystallographic positions.
